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[tor-commits] [tor/maint-0.3.5] Merge remote-tracking branch 'tor-github/pr/848' into maint-0.3.5



commit 3287cae68c8e6f2f72967bf6fb07e601792ecd2c
Merge: a57e168ca 669ec6432
Author: teor <teor@xxxxxxxxxxxxxx>
Date:   Mon Aug 12 09:38:46 2019 +1000

    Merge remote-tracking branch 'tor-github/pr/848' into maint-0.3.5

 changes/cid1444119               | 3 +++
 src/feature/dircommon/consdiff.c | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --cc src/feature/dircommon/consdiff.c
index 785205cb6,000000000..d0f7594ce
mode 100644,000000..100644
--- a/src/feature/dircommon/consdiff.c
+++ b/src/feature/dircommon/consdiff.c
@@@ -1,1414 -1,0 +1,1414 @@@
 +/* Copyright (c) 2014, Daniel Martí
 + * Copyright (c) 2014-2019, The Tor Project, Inc. */
 +/* See LICENSE for licensing information */
 +
 +/**
 + * \file consdiff.c
 + * \brief Consensus diff implementation, including both the generation and the
 + * application of diffs in a minimal ed format.
 + *
 + * The consensus diff application is done in consdiff_apply_diff, which relies
 + * on apply_ed_diff for the main ed diff part and on some digest helper
 + * functions to check the digest hashes found in the consensus diff header.
 + *
 + * The consensus diff generation is more complex. consdiff_gen_diff generates
 + * it, relying on gen_ed_diff to generate the ed diff and some digest helper
 + * functions to generate the digest hashes.
 + *
 + * gen_ed_diff is the tricky bit. In it simplest form, it will take quadratic
 + * time and linear space to generate an ed diff given two smartlists. As shown
 + * in its comment section, calling calc_changes on the entire two consensuses
 + * will calculate what is to be added and what is to be deleted in the diff.
 + * Its comment section briefly explains how it works.
 + *
 + * In our case specific to consensuses, we take advantage of the fact that
 + * consensuses list routers sorted by their identities. We use that
 + * information to avoid running calc_changes on the whole smartlists.
 + * gen_ed_diff will navigate through the two consensuses identity by identity
 + * and will send small couples of slices to calc_changes, keeping the running
 + * time near-linear. This is explained in more detail in the gen_ed_diff
 + * comments.
 + *
 + * The allocation strategy tries to save time and memory by avoiding needless
 + * copies.  Instead of actually splitting the inputs into separate strings, we
 + * allocate cdline_t objects, each of which represents a line in the original
 + * object or in the output.  We use memarea_t allocators to manage the
 + * temporary memory we use when generating or applying diffs.
 + **/
 +
 +#define CONSDIFF_PRIVATE
 +
 +#include "core/or/or.h"
 +#include "feature/dircommon/consdiff.h"
 +#include "lib/memarea/memarea.h"
 +#include "feature/dirparse/ns_parse.h"
 +
 +static const char* ns_diff_version = "network-status-diff-version 1";
 +static const char* hash_token = "hash";
 +
 +static char *consensus_join_lines(const smartlist_t *inp);
 +
 +/** Return true iff a and b have the same contents. */
 +STATIC int
 +lines_eq(const cdline_t *a, const cdline_t *b)
 +{
 +  return a->len == b->len && fast_memeq(a->s, b->s, a->len);
 +}
 +
 +/** Return true iff a has the same contents as the nul-terminated string b. */
 +STATIC int
 +line_str_eq(const cdline_t *a, const char *b)
 +{
 +  const size_t len = strlen(b);
 +  tor_assert(len <= UINT32_MAX);
 +  cdline_t bline = { b, (uint32_t)len };
 +  return lines_eq(a, &bline);
 +}
 +
 +/** Return true iff a begins with the same contents as the nul-terminated
 + * string b. */
 +static int
 +line_starts_with_str(const cdline_t *a, const char *b)
 +{
 +  const size_t len = strlen(b);
 +  tor_assert(len <= UINT32_MAX);
 +  return a->len >= len && fast_memeq(a->s, b, len);
 +}
 +
 +/** Return a new cdline_t holding as its contents the nul-terminated
 + * string s. Use the provided memory area for storage. */
 +static cdline_t *
 +cdline_linecpy(memarea_t *area, const char *s)
 +{
 +  size_t len = strlen(s);
 +  const char *ss = memarea_memdup(area, s, len);
 +  cdline_t *line = memarea_alloc(area, sizeof(cdline_t));
 +  line->s = ss;
 +  line->len = (uint32_t)len;
 +  return line;
 +}
 +
 +/** Add a cdline_t to <b>lst</b> holding as its contents the nul-terminated
 + * string s.  Use the provided memory area for storage. */
 +STATIC void
 +smartlist_add_linecpy(smartlist_t *lst, memarea_t *area, const char *s)
 +{
 +  smartlist_add(lst, cdline_linecpy(area, s));
 +}
 +
 +/** Compute the digest of <b>cons</b>, and store the result in
 + * <b>digest_out</b>. Return 0 on success, -1 on failure. */
 +/* This is a separate, mockable function so that we can override it when
 + * fuzzing. */
 +MOCK_IMPL(STATIC int,
 +consensus_compute_digest,(const char *cons,
 +                          consensus_digest_t *digest_out))
 +{
 +  int r = crypto_digest256((char*)digest_out->sha3_256,
 +                           cons, strlen(cons), DIGEST_SHA3_256);
 +  return r;
 +}
 +
 +/** Compute the digest-as-signed of <b>cons</b>, and store the result in
 + * <b>digest_out</b>. Return 0 on success, -1 on failure. */
 +/* This is a separate, mockable function so that we can override it when
 + * fuzzing. */
 +MOCK_IMPL(STATIC int,
 +consensus_compute_digest_as_signed,(const char *cons,
 +                                    consensus_digest_t *digest_out))
 +{
 +  return router_get_networkstatus_v3_sha3_as_signed(digest_out->sha3_256,
 +                                                    cons);
 +}
 +
 +/** Return true iff <b>d1</b> and <b>d2</b> contain the same digest */
 +/* This is a separate, mockable function so that we can override it when
 + * fuzzing. */
 +MOCK_IMPL(STATIC int,
 +consensus_digest_eq,(const uint8_t *d1,
 +                     const uint8_t *d2))
 +{
 +  return fast_memeq(d1, d2, DIGEST256_LEN);
 +}
 +
 +/** Create (allocate) a new slice from a smartlist. Assumes that the start
 + * and the end indexes are within the bounds of the initial smartlist. The end
 + * element is not part of the resulting slice. If end is -1, the slice is to
 + * reach the end of the smartlist.
 + */
 +STATIC smartlist_slice_t *
 +smartlist_slice(const smartlist_t *list, int start, int end)
 +{
 +  int list_len = smartlist_len(list);
 +  tor_assert(start >= 0);
 +  tor_assert(start <= list_len);
 +  if (end == -1) {
 +    end = list_len;
 +  }
 +  tor_assert(start <= end);
 +
 +  smartlist_slice_t *slice = tor_malloc(sizeof(smartlist_slice_t));
 +  slice->list = list;
 +  slice->offset = start;
 +  slice->len = end - start;
 +  return slice;
 +}
 +
 +/** Helper: Compute the longest common subsequence lengths for the two slices.
 + * Used as part of the diff generation to find the column at which to split
 + * slice2 while still having the optimal solution.
 + * If direction is -1, the navigation is reversed. Otherwise it must be 1.
 + * The length of the resulting integer array is that of the second slice plus
 + * one.
 + */
 +STATIC int *
 +lcs_lengths(const smartlist_slice_t *slice1, const smartlist_slice_t *slice2,
 +            int direction)
 +{
 +  size_t a_size = sizeof(int) * (slice2->len+1);
 +
 +  /* Resulting lcs lengths. */
 +  int *result = tor_malloc_zero(a_size);
 +  /* Copy of the lcs lengths from the last iteration. */
 +  int *prev = tor_malloc(a_size);
 +
 +  tor_assert(direction == 1 || direction == -1);
 +
 +  int si = slice1->offset;
 +  if (direction == -1) {
 +    si += (slice1->len-1);
 +  }
 +
 +  for (int i = 0; i < slice1->len; ++i, si+=direction) {
 +
 +    const cdline_t *line1 = smartlist_get(slice1->list, si);
 +    /* Store the last results. */
 +    memcpy(prev, result, a_size);
 +
 +    int sj = slice2->offset;
 +    if (direction == -1) {
 +      sj += (slice2->len-1);
 +    }
 +
 +    for (int j = 0; j < slice2->len; ++j, sj+=direction) {
 +
 +      const cdline_t *line2 = smartlist_get(slice2->list, sj);
 +      if (lines_eq(line1, line2)) {
 +        /* If the lines are equal, the lcs is one line longer. */
 +        result[j + 1] = prev[j] + 1;
 +      } else {
 +        /* If not, see what lcs parent path is longer. */
 +        result[j + 1] = MAX(result[j], prev[j + 1]);
 +      }
 +    }
 +  }
 +  tor_free(prev);
 +  return result;
 +}
 +
 +/** Helper: Trim any number of lines that are equally at the start or the end
 + * of both slices.
 + */
 +STATIC void
 +trim_slices(smartlist_slice_t *slice1, smartlist_slice_t *slice2)
 +{
 +  while (slice1->len>0 && slice2->len>0) {
 +    const cdline_t *line1 = smartlist_get(slice1->list, slice1->offset);
 +    const cdline_t *line2 = smartlist_get(slice2->list, slice2->offset);
 +    if (!lines_eq(line1, line2)) {
 +      break;
 +    }
 +    slice1->offset++; slice1->len--;
 +    slice2->offset++; slice2->len--;
 +  }
 +
 +  int i1 = (slice1->offset+slice1->len)-1;
 +  int i2 = (slice2->offset+slice2->len)-1;
 +
 +  while (slice1->len>0 && slice2->len>0) {
 +    const cdline_t *line1 = smartlist_get(slice1->list, i1);
 +    const cdline_t *line2 = smartlist_get(slice2->list, i2);
 +    if (!lines_eq(line1, line2)) {
 +      break;
 +    }
 +    i1--;
 +    slice1->len--;
 +    i2--;
 +    slice2->len--;
 +  }
 +}
 +
 +/** Like smartlist_string_pos, but uses a cdline_t, and is restricted to the
 + * bounds of the slice.
 + */
 +STATIC int
 +smartlist_slice_string_pos(const smartlist_slice_t *slice,
 +                           const cdline_t *string)
 +{
 +  int end = slice->offset + slice->len;
 +  for (int i = slice->offset; i < end; ++i) {
 +    const cdline_t *el = smartlist_get(slice->list, i);
 +    if (lines_eq(el, string)) {
 +      return i;
 +    }
 +  }
 +  return -1;
 +}
 +
 +/** Helper: Set all the appropriate changed booleans to true. The first slice
 + * must be of length 0 or 1. All the lines of slice1 and slice2 which are not
 + * present in the other slice will be set to changed in their bool array.
 + * The two changed bool arrays are passed in the same order as the slices.
 + */
 +STATIC void
 +set_changed(bitarray_t *changed1, bitarray_t *changed2,
 +            const smartlist_slice_t *slice1, const smartlist_slice_t *slice2)
 +{
 +  int toskip = -1;
 +  tor_assert(slice1->len == 0 || slice1->len == 1);
 +
 +  if (slice1->len == 1) {
 +    const cdline_t *line_common = smartlist_get(slice1->list, slice1->offset);
 +    toskip = smartlist_slice_string_pos(slice2, line_common);
 +    if (toskip == -1) {
 +      bitarray_set(changed1, slice1->offset);
 +    }
 +  }
 +  int end = slice2->offset + slice2->len;
 +  for (int i = slice2->offset; i < end; ++i) {
 +    if (i != toskip) {
 +      bitarray_set(changed2, i);
 +    }
 +  }
 +}
 +
 +/*
 + * Helper: Given that slice1 has been split by half into top and bot, we want
 + * to fetch the column at which to split slice2 so that we are still on track
 + * to the optimal diff solution, i.e. the shortest one. We use lcs_lengths
 + * since the shortest diff is just another way to say the longest common
 + * subsequence.
 + */
 +static int
 +optimal_column_to_split(const smartlist_slice_t *top,
 +                        const smartlist_slice_t *bot,
 +                        const smartlist_slice_t *slice2)
 +{
 +  int *lens_top = lcs_lengths(top, slice2, 1);
 +  int *lens_bot = lcs_lengths(bot, slice2, -1);
 +  int column=0, max_sum=-1;
 +
 +  for (int i = 0; i < slice2->len+1; ++i) {
 +    int sum = lens_top[i] + lens_bot[slice2->len-i];
 +    if (sum > max_sum) {
 +      column = i;
 +      max_sum = sum;
 +    }
 +  }
 +  tor_free(lens_top);
 +  tor_free(lens_bot);
 +
 +  return column;
 +}
 +
 +/**
 + * Helper: Figure out what elements are new or gone on the second smartlist
 + * relative to the first smartlist, and store the booleans in the bitarrays.
 + * True on the first bitarray means the element is gone, true on the second
 + * bitarray means it's new.
 + *
 + * In its base case, either of the smartlists is of length <= 1 and we can
 + * quickly see what elements are new or are gone. In the other case, we will
 + * split one smartlist by half and we'll use optimal_column_to_split to find
 + * the optimal column at which to split the second smartlist so that we are
 + * finding the smallest diff possible.
 + */
 +STATIC void
 +calc_changes(smartlist_slice_t *slice1,
 +             smartlist_slice_t *slice2,
 +             bitarray_t *changed1, bitarray_t *changed2)
 +{
 +  trim_slices(slice1, slice2);
 +
 +  if (slice1->len <= 1) {
 +    set_changed(changed1, changed2, slice1, slice2);
 +
 +  } else if (slice2->len <= 1) {
 +    set_changed(changed2, changed1, slice2, slice1);
 +
 +  /* Keep on splitting the slices in two. */
 +  } else {
 +    smartlist_slice_t *top, *bot, *left, *right;
 +
 +    /* Split the first slice in half. */
 +    int mid = slice1->len/2;
 +    top = smartlist_slice(slice1->list, slice1->offset, slice1->offset+mid);
 +    bot = smartlist_slice(slice1->list, slice1->offset+mid,
 +        slice1->offset+slice1->len);
 +
 +    /* Split the second slice by the optimal column. */
 +    int mid2 = optimal_column_to_split(top, bot, slice2);
 +    left = smartlist_slice(slice2->list, slice2->offset, slice2->offset+mid2);
 +    right = smartlist_slice(slice2->list, slice2->offset+mid2,
 +        slice2->offset+slice2->len);
 +
 +    calc_changes(top, left, changed1, changed2);
 +    calc_changes(bot, right, changed1, changed2);
 +    tor_free(top);
 +    tor_free(bot);
 +    tor_free(left);
 +    tor_free(right);
 +  }
 +}
 +
 +/* This table is from crypto.c. The SP and PAD defines are different. */
 +#define NOT_VALID_BASE64 255
 +#define X NOT_VALID_BASE64
 +#define SP NOT_VALID_BASE64
 +#define PAD NOT_VALID_BASE64
 +static const uint8_t base64_compare_table[256] = {
 +  X, X, X, X, X, X, X, X, X, SP, SP, SP, X, SP, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +  SP, X, X, X, X, X, X, X, X, X, X, 62, X, X, X, 63,
 +  52, 53, 54, 55, 56, 57, 58, 59, 60, 61, X, X, X, PAD, X, X,
 +  X, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
 +  15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, X, X, X, X, X,
 +  X, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
 +  41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, X, X, X, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +  X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
 +};
 +
 +/** Helper: Get the identity hash from a router line, assuming that the line
 + * at least appears to be a router line and thus starts with "r ".
 + *
 + * If an identity hash is found, store it (without decoding it) in
 + * <b>hash_out</b>, and return 0.  On failure, return -1.
 + */
 +STATIC int
 +get_id_hash(const cdline_t *line, cdline_t *hash_out)
 +{
 +  if (line->len < 2)
 +    return -1;
 +
 +  /* Skip the router name. */
 +  const char *hash = memchr(line->s + 2, ' ', line->len - 2);
 +  if (!hash) {
 +    return -1;
 +  }
 +
 +  hash++;
 +  const char *hash_end = hash;
 +  /* Stop when the first non-base64 character is found. Use unsigned chars to
 +   * avoid negative indexes causing crashes.
 +   */
 +  while (base64_compare_table[*((unsigned char*)hash_end)]
 +           != NOT_VALID_BASE64 &&
 +         hash_end < line->s + line->len) {
 +    hash_end++;
 +  }
 +
 +  /* Empty hash. */
 +  if (hash_end == hash) {
 +    return -1;
 +  }
 +
 +  hash_out->s = hash;
 +  /* Always true because lines are limited to this length */
 +  tor_assert(hash_end >= hash);
 +  tor_assert((size_t)(hash_end - hash) <= UINT32_MAX);
 +  hash_out->len = (uint32_t)(hash_end - hash);
 +
 +  return 0;
 +}
 +
 +/** Helper: Check that a line is a valid router entry. We must at least be
 + * able to fetch a proper identity hash from it for it to be valid.
 + */
 +STATIC int
 +is_valid_router_entry(const cdline_t *line)
 +{
 +  if (line->len < 2 || fast_memneq(line->s, "r ", 2))
 +    return 0;
 +  cdline_t tmp;
 +  return (get_id_hash(line, &tmp) == 0);
 +}
 +
 +/** Helper: Find the next router line starting at the current position.
 + * Assumes that cur is lower than the length of the smartlist, i.e. it is a
 + * line within the bounds of the consensus. The only exception is when we
 + * don't want to skip the first line, in which case cur will be -1.
 + */
 +STATIC int
 +next_router(const smartlist_t *cons, int cur)
 +{
 +  int len = smartlist_len(cons);
 +  tor_assert(cur >= -1 && cur < len);
 +
 +  if (++cur >= len) {
 +    return len;
 +  }
 +
 +  const cdline_t *line = smartlist_get(cons, cur);
 +  while (!is_valid_router_entry(line)) {
 +    if (++cur >= len) {
 +      return len;
 +    }
 +    line = smartlist_get(cons, cur);
 +  }
 +  return cur;
 +}
 +
 +/** Helper: compare two base64-encoded identity hashes, which may be of
 + * different lengths. Comparison ends when the first non-base64 char is found.
 + */
 +STATIC int
 +base64cmp(const cdline_t *hash1, const cdline_t *hash2)
 +{
 +  /* NULL is always lower, useful for last_hash which starts at NULL. */
 +  if (!hash1->s && !hash2->s) {
 +    return 0;
 +  }
 +  if (!hash1->s) {
 +    return -1;
 +  }
 +  if (!hash2->s) {
 +    return 1;
 +  }
 +
 +  /* Don't index with a char; char may be signed. */
 +  const unsigned char *a = (unsigned char*)hash1->s;
 +  const unsigned char *b = (unsigned char*)hash2->s;
 +  const unsigned char *a_end = a + hash1->len;
 +  const unsigned char *b_end = b + hash2->len;
 +  while (1) {
 +    uint8_t av = base64_compare_table[*a];
 +    uint8_t bv = base64_compare_table[*b];
 +    if (av == NOT_VALID_BASE64) {
 +      if (bv == NOT_VALID_BASE64) {
 +        /* Both ended with exactly the same characters. */
 +        return 0;
 +      } else {
 +        /* hash2 goes on longer than hash1 and thus hash1 is lower. */
 +        return -1;
 +      }
 +    } else if (bv == NOT_VALID_BASE64) {
 +      /* hash1 goes on longer than hash2 and thus hash1 is greater. */
 +      return 1;
 +    } else if (av < bv) {
 +      /* The first difference shows that hash1 is lower. */
 +      return -1;
 +    } else if (av > bv) {
 +      /* The first difference shows that hash1 is greater. */
 +      return 1;
 +    } else {
 +      a++;
 +      b++;
 +      if (a == a_end) {
 +        if (b == b_end) {
 +          return 0;
 +        } else {
 +          return -1;
 +        }
 +      } else if (b == b_end) {
 +        return 1;
 +      }
 +    }
 +  }
 +}
 +
 +/** Structure used to remember the previous and current identity hash of
 + * the "r " lines in a consensus, to enforce well-ordering. */
 +typedef struct router_id_iterator_t {
 +  cdline_t last_hash;
 +  cdline_t hash;
 +} router_id_iterator_t;
 +
 +/**
 + * Initializer for a router_id_iterator_t.
 + */
 +#define ROUTER_ID_ITERATOR_INIT { { NULL, 0 }, { NULL, 0 } }
 +
 +/** Given an index *<b>idxp</b> into the consensus at <b>cons</b>, advance
 + * the index to the next router line ("r ...") in the consensus, or to
 + * an index one after the end of the list if there is no such line.
 + *
 + * Use <b>iter</b> to record the hash of the found router line, if any,
 + * and to enforce ordering on the hashes.  If the hashes are mis-ordered,
 + * return -1.  Else, return 0.
 + **/
 +static int
 +find_next_router_line(const smartlist_t *cons,
 +                      const char *consname,
 +                      int *idxp,
 +                      router_id_iterator_t *iter)
 +{
 +  *idxp = next_router(cons, *idxp);
 +  if (*idxp < smartlist_len(cons)) {
 +    memcpy(&iter->last_hash, &iter->hash, sizeof(cdline_t));
 +    if (get_id_hash(smartlist_get(cons, *idxp), &iter->hash) < 0 ||
 +        base64cmp(&iter->hash, &iter->last_hash) <= 0) {
 +      log_warn(LD_CONSDIFF, "Refusing to generate consensus diff because "
 +               "the %s consensus doesn't have its router entries sorted "
 +               "properly.", consname);
 +      return -1;
 +    }
 +  }
 +  return 0;
 +}
 +
 +/** Line-prefix indicating the beginning of the signatures section that we
 + * intend to delete. */
 +#define START_OF_SIGNATURES_SECTION "directory-signature "
 +
 +/** Pre-process a consensus in <b>cons</b> (represented as a list of cdline_t)
 + * to remove the signatures from it.  If the footer is removed, return a
 + * cdline_t containing a delete command to delete the footer, allocated in
 + * <b>area</>.  If no footer is removed, return NULL.
 + *
 + * We remove the signatures here because they are not themselves signed, and
 + * as such there might be different encodings for them.
 + */
 +static cdline_t *
 +preprocess_consensus(memarea_t *area,
 +                     smartlist_t *cons)
 +{
 +  int idx;
 +  int dirsig_idx = -1;
 +  for (idx = 0; idx < smartlist_len(cons); ++idx) {
 +    cdline_t *line = smartlist_get(cons, idx);
 +    if (line_starts_with_str(line, START_OF_SIGNATURES_SECTION)) {
 +      dirsig_idx = idx;
 +      break;
 +    }
 +  }
 +  if (dirsig_idx >= 0) {
 +    char buf[64];
 +    while (smartlist_len(cons) > dirsig_idx)
 +      smartlist_del(cons, dirsig_idx);
 +    tor_snprintf(buf, sizeof(buf), "%d,$d", dirsig_idx+1);
 +    return cdline_linecpy(area, buf);
 +  } else {
 +    return NULL;
 +  }
 +}
 +
 +/** Generate an ed diff as a smartlist from two consensuses, also given as
 + * smartlists. Will return NULL if the diff could not be generated, which can
 + * happen if any lines the script had to add matched "." or if the routers
 + * were not properly ordered.
 + *
 + * All cdline_t objects in the resulting object are either references to lines
 + * in one of the inputs, or are newly allocated lines in the provided memarea.
 + *
 + * This implementation is consensus-specific. To generate an ed diff for any
 + * given input in quadratic time, you can replace all the code until the
 + * navigation in reverse order with the following:
 + *
 + *   int len1 = smartlist_len(cons1);
 + *   int len2 = smartlist_len(cons2);
 + *   bitarray_t *changed1 = bitarray_init_zero(len1);
 + *   bitarray_t *changed2 = bitarray_init_zero(len2);
 + *   cons1_sl = smartlist_slice(cons1, 0, -1);
 + *   cons2_sl = smartlist_slice(cons2, 0, -1);
 + *   calc_changes(cons1_sl, cons2_sl, changed1, changed2);
 + */
 +STATIC smartlist_t *
 +gen_ed_diff(const smartlist_t *cons1_orig, const smartlist_t *cons2,
 +            memarea_t *area)
 +{
 +  smartlist_t *cons1 = smartlist_new();
 +  smartlist_add_all(cons1, cons1_orig);
 +  cdline_t *remove_trailer = preprocess_consensus(area, cons1);
 +
 +  int len1 = smartlist_len(cons1);
 +  int len2 = smartlist_len(cons2);
 +  smartlist_t *result = smartlist_new();
 +
 +  if (remove_trailer) {
 +    /* There's a delete-the-trailer line at the end, so add it here. */
 +    smartlist_add(result, remove_trailer);
 +  }
 +
 +  /* Initialize the changed bitarrays to zero, so that calc_changes only needs
 +   * to set the ones that matter and leave the rest untouched.
 +   */
 +  bitarray_t *changed1 = bitarray_init_zero(len1);
 +  bitarray_t *changed2 = bitarray_init_zero(len2);
 +  int i1=-1, i2=-1;
 +  int start1=0, start2=0;
 +
 +  /* To check that hashes are ordered properly */
 +  router_id_iterator_t iter1 = ROUTER_ID_ITERATOR_INIT;
 +  router_id_iterator_t iter2 = ROUTER_ID_ITERATOR_INIT;
 +
 +  /* i1 and i2 are initialized at the first line of each consensus. They never
 +   * reach past len1 and len2 respectively, since next_router doesn't let that
 +   * happen. i1 and i2 are advanced by at least one line at each iteration as
 +   * long as they have not yet reached len1 and len2, so the loop is
 +   * guaranteed to end, and each pair of (i1,i2) will be inspected at most
 +   * once.
 +   */
 +  while (i1 < len1 || i2 < len2) {
 +
 +    /* Advance each of the two navigation positions by one router entry if not
 +     * yet at the end.
 +     */
 +    if (i1 < len1) {
 +      if (find_next_router_line(cons1, "base", &i1, &iter1) < 0) {
 +        goto error_cleanup;
 +      }
 +    }
 +
 +    if (i2 < len2) {
 +      if (find_next_router_line(cons2, "target", &i2, &iter2) < 0) {
 +        goto error_cleanup;
 +      }
 +    }
 +
 +    /* If we have reached the end of both consensuses, there is no need to
 +     * compare hashes anymore, since this is the last iteration.
 +     */
 +    if (i1 < len1 || i2 < len2) {
 +
 +      /* Keep on advancing the lower (by identity hash sorting) position until
 +       * we have two matching positions. The only other possible outcome is
 +       * that a lower position reaches the end of the consensus before it can
 +       * reach a hash that is no longer the lower one. Since there will always
 +       * be a lower hash for as long as the loop runs, one of the two indexes
 +       * will always be incremented, thus assuring that the loop must end
 +       * after a finite number of iterations. If that cannot be because said
 +       * consensus has already reached the end, both are extended to their
 +       * respecting ends since we are done.
 +       */
 +      int cmp = base64cmp(&iter1.hash, &iter2.hash);
 +      while (cmp != 0) {
 +        if (i1 < len1 && cmp < 0) {
 +          if (find_next_router_line(cons1, "base", &i1, &iter1) < 0) {
 +            goto error_cleanup;
 +          }
 +          if (i1 == len1) {
 +            /* We finished the first consensus, so grab all the remaining
 +             * lines of the second consensus and finish up.
 +             */
 +            i2 = len2;
 +            break;
 +          }
 +        } else if (i2 < len2 && cmp > 0) {
 +          if (find_next_router_line(cons2, "target", &i2, &iter2) < 0) {
 +            goto error_cleanup;
 +          }
 +          if (i2 == len2) {
 +            /* We finished the second consensus, so grab all the remaining
 +             * lines of the first consensus and finish up.
 +             */
 +            i1 = len1;
 +            break;
 +          }
 +        } else {
 +          i1 = len1;
 +          i2 = len2;
 +          break;
 +        }
 +        cmp = base64cmp(&iter1.hash, &iter2.hash);
 +      }
 +    }
 +
 +    /* Make slices out of these chunks (up to the common router entry) and
 +     * calculate the changes for them.
 +     * Error if any of the two slices are longer than 10K lines. That should
 +     * never happen with any pair of real consensuses. Feeding more than 10K
 +     * lines to calc_changes would be very slow anyway.
 +     */
 +#define MAX_LINE_COUNT (10000)
 +    if (i1-start1 > MAX_LINE_COUNT || i2-start2 > MAX_LINE_COUNT) {
 +      log_warn(LD_CONSDIFF, "Refusing to generate consensus diff because "
 +          "we found too few common router ids.");
 +      goto error_cleanup;
 +    }
 +
 +    smartlist_slice_t *cons1_sl = smartlist_slice(cons1, start1, i1);
 +    smartlist_slice_t *cons2_sl = smartlist_slice(cons2, start2, i2);
 +    calc_changes(cons1_sl, cons2_sl, changed1, changed2);
 +    tor_free(cons1_sl);
 +    tor_free(cons2_sl);
 +    start1 = i1, start2 = i2;
 +  }
 +
 +  /* Navigate the changes in reverse order and generate one ed command for
 +   * each chunk of changes.
 +   */
 +  i1=len1-1, i2=len2-1;
 +  char buf[128];
 +  while (i1 >= 0 || i2 >= 0) {
 +
 +    int start1x, start2x, end1, end2, added, deleted;
 +
 +    /* We are at a point were no changed bools are true, so just keep going. */
 +    if (!(i1 >= 0 && bitarray_is_set(changed1, i1)) &&
 +        !(i2 >= 0 && bitarray_is_set(changed2, i2))) {
 +      if (i1 >= 0) {
 +        i1--;
 +      }
 +      if (i2 >= 0) {
 +        i2--;
 +      }
 +      continue;
 +    }
 +
 +    end1 = i1, end2 = i2;
 +
 +    /* Grab all contiguous changed lines */
 +    while (i1 >= 0 && bitarray_is_set(changed1, i1)) {
 +      i1--;
 +    }
 +    while (i2 >= 0 && bitarray_is_set(changed2, i2)) {
 +      i2--;
 +    }
 +
 +    start1x = i1+1, start2x = i2+1;
 +    added = end2-i2, deleted = end1-i1;
 +
 +    if (added == 0) {
 +      if (deleted == 1) {
 +        tor_snprintf(buf, sizeof(buf), "%id", start1x+1);
 +        smartlist_add_linecpy(result, area, buf);
 +      } else {
 +        tor_snprintf(buf, sizeof(buf), "%i,%id", start1x+1, start1x+deleted);
 +        smartlist_add_linecpy(result, area, buf);
 +      }
 +    } else {
 +      int i;
 +      if (deleted == 0) {
 +        tor_snprintf(buf, sizeof(buf), "%ia", start1x);
 +        smartlist_add_linecpy(result, area, buf);
 +      } else if (deleted == 1) {
 +        tor_snprintf(buf, sizeof(buf), "%ic", start1x+1);
 +        smartlist_add_linecpy(result, area, buf);
 +      } else {
 +        tor_snprintf(buf, sizeof(buf), "%i,%ic", start1x+1, start1x+deleted);
 +        smartlist_add_linecpy(result, area, buf);
 +      }
 +
 +      for (i = start2x; i <= end2; ++i) {
 +        cdline_t *line = smartlist_get(cons2, i);
 +        if (line_str_eq(line, ".")) {
 +          log_warn(LD_CONSDIFF, "Cannot generate consensus diff because "
 +              "one of the lines to be added is \".\".");
 +          goto error_cleanup;
 +        }
 +        smartlist_add(result, line);
 +      }
 +      smartlist_add_linecpy(result, area, ".");
 +    }
 +  }
 +
 +  smartlist_free(cons1);
 +  bitarray_free(changed1);
 +  bitarray_free(changed2);
 +
 +  return result;
 +
 + error_cleanup:
 +
 +  smartlist_free(cons1);
 +  bitarray_free(changed1);
 +  bitarray_free(changed2);
 +
 +  smartlist_free(result);
 +
 +  return NULL;
 +}
 +
 +/* Helper: Read a base-10 number between 0 and INT32_MAX from <b>s</b> and
 + * store it in <b>num_out</b>.  Advance <b>s</b> to the characer immediately
 + * after the number.  Return 0 on success, -1 on failure. */
 +static int
 +get_linenum(const char **s, int *num_out)
 +{
 +  int ok;
 +  char *next;
 +  if (!TOR_ISDIGIT(**s)) {
 +    return -1;
 +  }
 +  *num_out = (int) tor_parse_long(*s, 10, 0, INT32_MAX, &ok, &next);
 +  if (ok && next) {
 +    *s = next;
 +    return 0;
 +  } else {
 +    return -1;
 +  }
 +}
 +
 +/** Apply the ed diff, starting at <b>diff_starting_line</b>, to the consensus
 + * and return a new consensus, also as a line-based smartlist. Will return
 + * NULL if the ed diff is not properly formatted.
 + *
 + * All cdline_t objects in the resulting object are references to lines
 + * in one of the inputs; nothing is copied.
 + */
 +STATIC smartlist_t *
 +apply_ed_diff(const smartlist_t *cons1, const smartlist_t *diff,
 +              int diff_starting_line)
 +{
 +  int diff_len = smartlist_len(diff);
 +  int j = smartlist_len(cons1);
 +  smartlist_t *cons2 = smartlist_new();
 +
 +  for (int i=diff_starting_line; i<diff_len; ++i) {
 +    const cdline_t *diff_cdline = smartlist_get(diff, i);
 +    char diff_line[128];
 +
 +    if (diff_cdline->len > sizeof(diff_line) - 1) {
 +      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +               "an ed command was far too long");
 +      goto error_cleanup;
 +    }
 +    /* Copy the line to make it nul-terminated. */
 +    memcpy(diff_line, diff_cdline->s, diff_cdline->len);
 +    diff_line[diff_cdline->len] = 0;
 +    const char *ptr = diff_line;
 +    int start = 0, end = 0;
 +    int had_range = 0;
 +    int end_was_eof = 0;
 +    if (get_linenum(&ptr, &start) < 0) {
 +      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +               "an ed command was missing a line number.");
 +      goto error_cleanup;
 +    }
 +    if (*ptr == ',') {
 +      /* Two-item range */
 +      had_range = 1;
 +      ++ptr;
 +      if (*ptr == '$') {
 +        end_was_eof = 1;
 +        end = smartlist_len(cons1);
 +        ++ptr;
 +      } else if (get_linenum(&ptr, &end) < 0) {
 +        log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +                 "an ed command was missing a range end line number.");
 +        goto error_cleanup;
 +      }
 +      /* Incoherent range. */
 +      if (end <= start) {
 +        log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +                 "an invalid range was found in an ed command.");
 +        goto error_cleanup;
 +      }
 +    } else {
 +      /* We'll take <n1> as <n1>,<n1> for simplicity. */
 +      end = start;
 +    }
 +
 +    if (end > j) {
 +      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +          "its commands are not properly sorted in reverse order.");
 +      goto error_cleanup;
 +    }
 +
 +    if (*ptr == '\0') {
 +      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +               "a line with no ed command was found");
 +      goto error_cleanup;
 +    }
 +
 +    if (*(ptr+1) != '\0') {
 +      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +          "an ed command longer than one char was found.");
 +      goto error_cleanup;
 +    }
 +
 +    char action = *ptr;
 +
 +    switch (action) {
 +      case 'a':
 +      case 'c':
 +      case 'd':
 +        break;
 +      default:
 +        log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +            "an unrecognised ed command was found.");
 +        goto error_cleanup;
 +    }
 +
 +    /** $ is not allowed with non-d actions. */
 +    if (end_was_eof && action != 'd') {
 +      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +               "it wanted to use $ with a command other than delete");
 +      goto error_cleanup;
 +    }
 +
 +    /* 'a' commands are not allowed to have ranges. */
 +    if (had_range && action == 'a') {
 +      log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +          "it wanted to add lines after a range.");
 +      goto error_cleanup;
 +    }
 +
 +    /* Add unchanged lines. */
 +    for (; j && j > end; --j) {
 +      cdline_t *cons_line = smartlist_get(cons1, j-1);
 +      smartlist_add(cons2, cons_line);
 +    }
 +
 +    /* Ignore removed lines. */
 +    if (action == 'c' || action == 'd') {
 +      while (--j >= start) {
 +        /* Skip line */
 +      }
 +    }
 +
 +    /* Add new lines in reverse order, since it will all be reversed at the
 +     * end.
 +     */
 +    if (action == 'a' || action == 'c') {
 +      int added_end = i;
 +
 +      i++; /* Skip the line with the range and command. */
 +      while (i < diff_len) {
 +        if (line_str_eq(smartlist_get(diff, i), ".")) {
 +          break;
 +        }
 +        if (++i == diff_len) {
 +          log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +              "it has lines to be inserted that don't end with a \".\".");
 +          goto error_cleanup;
 +        }
 +      }
 +
 +      int added_i = i-1;
 +
 +      /* It would make no sense to add zero new lines. */
 +      if (added_i == added_end) {
 +        log_warn(LD_CONSDIFF, "Could not apply consensus diff because "
 +            "it has an ed command that tries to insert zero lines.");
 +        goto error_cleanup;
 +      }
 +
 +      while (added_i > added_end) {
 +        cdline_t *added_line = smartlist_get(diff, added_i--);
 +        smartlist_add(cons2, added_line);
 +      }
 +    }
 +  }
 +
 +  /* Add remaining unchanged lines. */
 +  for (; j > 0; --j) {
 +    cdline_t *cons_line = smartlist_get(cons1, j-1);
 +    smartlist_add(cons2, cons_line);
 +  }
 +
 +  /* Reverse the whole thing since we did it from the end. */
 +  smartlist_reverse(cons2);
 +  return cons2;
 +
 + error_cleanup:
 +
 +  smartlist_free(cons2);
 +
 +  return NULL;
 +}
 +
 +/** Generate a consensus diff as a smartlist from two given consensuses, also
 + * as smartlists. Will return NULL if the consensus diff could not be
 + * generated. Neither of the two consensuses are modified in any way, so it's
 + * up to the caller to free their resources.
 + */
 +smartlist_t *
 +consdiff_gen_diff(const smartlist_t *cons1,
 +                  const smartlist_t *cons2,
 +                  const consensus_digest_t *digests1,
 +                  const consensus_digest_t *digests2,
 +                  memarea_t *area)
 +{
 +  smartlist_t *ed_diff = gen_ed_diff(cons1, cons2, area);
 +  /* ed diff could not be generated - reason already logged by gen_ed_diff. */
 +  if (!ed_diff) {
 +    goto error_cleanup;
 +  }
 +
 +  /* See that the script actually produces what we want. */
 +  smartlist_t *ed_cons2 = apply_ed_diff(cons1, ed_diff, 0);
 +  if (!ed_cons2) {
 +    /* LCOV_EXCL_START -- impossible if diff generation is correct */
 +    log_warn(LD_BUG|LD_CONSDIFF, "Refusing to generate consensus diff because "
 +        "the generated ed diff could not be tested to successfully generate "
 +        "the target consensus.");
 +    goto error_cleanup;
 +    /* LCOV_EXCL_STOP */
 +  }
 +
 +  int cons2_eq = 1;
 +  if (smartlist_len(cons2) == smartlist_len(ed_cons2)) {
 +    SMARTLIST_FOREACH_BEGIN(cons2, const cdline_t *, line1) {
 +      const cdline_t *line2 = smartlist_get(ed_cons2, line1_sl_idx);
 +      if (! lines_eq(line1, line2) ) {
 +        cons2_eq = 0;
 +        break;
 +      }
 +    } SMARTLIST_FOREACH_END(line1);
 +  } else {
 +    cons2_eq = 0;
 +  }
 +  smartlist_free(ed_cons2);
 +  if (!cons2_eq) {
 +    /* LCOV_EXCL_START -- impossible if diff generation is correct. */
 +    log_warn(LD_BUG|LD_CONSDIFF, "Refusing to generate consensus diff because "
 +        "the generated ed diff did not generate the target consensus "
 +        "successfully when tested.");
 +    goto error_cleanup;
 +    /* LCOV_EXCL_STOP */
 +  }
 +
 +  char cons1_hash_hex[HEX_DIGEST256_LEN+1];
 +  char cons2_hash_hex[HEX_DIGEST256_LEN+1];
 +  base16_encode(cons1_hash_hex, HEX_DIGEST256_LEN+1,
 +                (const char*)digests1->sha3_256, DIGEST256_LEN);
 +  base16_encode(cons2_hash_hex, HEX_DIGEST256_LEN+1,
 +                (const char*)digests2->sha3_256, DIGEST256_LEN);
 +
 +  /* Create the resulting consensus diff. */
 +  char buf[160];
 +  smartlist_t *result = smartlist_new();
 +  tor_snprintf(buf, sizeof(buf), "%s", ns_diff_version);
 +  smartlist_add_linecpy(result, area, buf);
 +  tor_snprintf(buf, sizeof(buf), "%s %s %s", hash_token,
 +      cons1_hash_hex, cons2_hash_hex);
 +  smartlist_add_linecpy(result, area, buf);
 +  smartlist_add_all(result, ed_diff);
 +  smartlist_free(ed_diff);
 +  return result;
 +
 + error_cleanup:
 +
 +  if (ed_diff) {
 +    /* LCOV_EXCL_START -- ed_diff is NULL except in unreachable cases above */
 +    smartlist_free(ed_diff);
 +    /* LCOV_EXCL_STOP */
 +  }
 +
 +  return NULL;
 +}
 +
 +/** Fetch the digest of the base consensus in the consensus diff, encoded in
 + * base16 as found in the diff itself. digest1_out and digest2_out must be of
 + * length DIGEST256_LEN or larger if not NULL.
 + */
 +int
 +consdiff_get_digests(const smartlist_t *diff,
 +                     char *digest1_out,
 +                     char *digest2_out)
 +{
 +  smartlist_t *hash_words = NULL;
 +  const cdline_t *format;
 +  char cons1_hash[DIGEST256_LEN], cons2_hash[DIGEST256_LEN];
 +  char *cons1_hash_hex, *cons2_hash_hex;
 +  if (smartlist_len(diff) < 2) {
 +    log_info(LD_CONSDIFF, "The provided consensus diff is too short.");
 +    goto error_cleanup;
 +  }
 +
 +  /* Check that it's the format and version we know. */
 +  format = smartlist_get(diff, 0);
 +  if (!line_str_eq(format, ns_diff_version)) {
 +    log_warn(LD_CONSDIFF, "The provided consensus diff format is not known.");
 +    goto error_cleanup;
 +  }
 +
 +  /* Grab the base16 digests. */
 +  hash_words = smartlist_new();
 +  {
 +    const cdline_t *line2 = smartlist_get(diff, 1);
 +    char *h = tor_memdup_nulterm(line2->s, line2->len);
 +    smartlist_split_string(hash_words, h, " ", 0, 0);
 +    tor_free(h);
 +  }
 +
 +  /* There have to be three words, the first of which must be hash_token. */
 +  if (smartlist_len(hash_words) != 3 ||
 +      strcmp(smartlist_get(hash_words, 0), hash_token)) {
 +    log_info(LD_CONSDIFF, "The provided consensus diff does not include "
 +        "the necessary digests.");
 +    goto error_cleanup;
 +  }
 +
 +  /* Expected hashes as found in the consensus diff header. They must be of
 +   * length HEX_DIGEST256_LEN, normally 64 hexadecimal characters.
 +   * If any of the decodings fail, error to make sure that the hashes are
 +   * proper base16-encoded digests.
 +   */
 +  cons1_hash_hex = smartlist_get(hash_words, 1);
 +  cons2_hash_hex = smartlist_get(hash_words, 2);
 +  if (strlen(cons1_hash_hex) != HEX_DIGEST256_LEN ||
 +      strlen(cons2_hash_hex) != HEX_DIGEST256_LEN) {
 +    log_info(LD_CONSDIFF, "The provided consensus diff includes "
 +        "base16-encoded digests of incorrect size.");
 +    goto error_cleanup;
 +  }
 +
 +  if (base16_decode(cons1_hash, DIGEST256_LEN,
 +          cons1_hash_hex, HEX_DIGEST256_LEN) != DIGEST256_LEN ||
 +      base16_decode(cons2_hash, DIGEST256_LEN,
 +          cons2_hash_hex, HEX_DIGEST256_LEN) != DIGEST256_LEN) {
 +    log_info(LD_CONSDIFF, "The provided consensus diff includes "
 +        "malformed digests.");
 +    goto error_cleanup;
 +  }
 +
 +  if (digest1_out) {
 +    memcpy(digest1_out, cons1_hash, DIGEST256_LEN);
 +  }
 +  if (digest2_out) {
 +    memcpy(digest2_out, cons2_hash, DIGEST256_LEN);
 +  }
 +
 +  SMARTLIST_FOREACH(hash_words, char *, cp, tor_free(cp));
 +  smartlist_free(hash_words);
 +  return 0;
 +
 + error_cleanup:
 +
 +  if (hash_words) {
 +    SMARTLIST_FOREACH(hash_words, char *, cp, tor_free(cp));
 +    smartlist_free(hash_words);
 +  }
 +  return 1;
 +}
 +
 +/** Apply the consensus diff to the given consensus and return a new
 + * consensus, also as a line-based smartlist. Will return NULL if the diff
 + * could not be applied. Neither the consensus nor the diff are modified in
 + * any way, so it's up to the caller to free their resources.
 + */
 +char *
 +consdiff_apply_diff(const smartlist_t *cons1,
 +                    const smartlist_t *diff,
 +                    const consensus_digest_t *digests1)
 +{
 +  smartlist_t *cons2 = NULL;
 +  char *cons2_str = NULL;
 +  char e_cons1_hash[DIGEST256_LEN];
 +  char e_cons2_hash[DIGEST256_LEN];
 +
 +  if (consdiff_get_digests(diff, e_cons1_hash, e_cons2_hash) != 0) {
 +    goto error_cleanup;
 +  }
 +
 +  /* See that the consensus that was given to us matches its hash. */
 +  if (!consensus_digest_eq(digests1->sha3_256,
 +                           (const uint8_t*)e_cons1_hash)) {
 +    char hex_digest1[HEX_DIGEST256_LEN+1];
 +    char e_hex_digest1[HEX_DIGEST256_LEN+1];
 +    log_warn(LD_CONSDIFF, "Refusing to apply consensus diff because "
 +        "the base consensus doesn't match the digest as found in "
 +        "the consensus diff header.");
 +    base16_encode(hex_digest1, HEX_DIGEST256_LEN+1,
 +                  (const char *)digests1->sha3_256, DIGEST256_LEN);
 +    base16_encode(e_hex_digest1, HEX_DIGEST256_LEN+1,
 +                  e_cons1_hash, DIGEST256_LEN);
 +    log_warn(LD_CONSDIFF, "Expected: %s; found: %s",
 +             hex_digest1, e_hex_digest1);
 +    goto error_cleanup;
 +  }
 +
 +  /* Grab the ed diff and calculate the resulting consensus. */
 +  /* Skip the first two lines. */
 +  cons2 = apply_ed_diff(cons1, diff, 2);
 +
 +  /* ed diff could not be applied - reason already logged by apply_ed_diff. */
 +  if (!cons2) {
 +    goto error_cleanup;
 +  }
 +
 +  cons2_str = consensus_join_lines(cons2);
 +
 +  consensus_digest_t cons2_digests;
 +  if (consensus_compute_digest(cons2_str, &cons2_digests) < 0) {
 +    /* LCOV_EXCL_START -- digest can't fail */
 +    log_warn(LD_CONSDIFF, "Could not compute digests of the consensus "
 +        "resulting from applying a consensus diff.");
 +    goto error_cleanup;
 +    /* LCOV_EXCL_STOP */
 +  }
 +
 +  /* See that the resulting consensus matches its hash. */
 +  if (!consensus_digest_eq(cons2_digests.sha3_256,
 +                           (const uint8_t*)e_cons2_hash)) {
 +    log_warn(LD_CONSDIFF, "Refusing to apply consensus diff because "
 +        "the resulting consensus doesn't match the digest as found in "
 +        "the consensus diff header.");
 +    char hex_digest2[HEX_DIGEST256_LEN+1];
 +    char e_hex_digest2[HEX_DIGEST256_LEN+1];
 +    base16_encode(hex_digest2, HEX_DIGEST256_LEN+1,
 +        (const char *)cons2_digests.sha3_256, DIGEST256_LEN);
 +    base16_encode(e_hex_digest2, HEX_DIGEST256_LEN+1,
 +        e_cons2_hash, DIGEST256_LEN);
 +    log_warn(LD_CONSDIFF, "Expected: %s; found: %s",
 +             hex_digest2, e_hex_digest2);
 +    goto error_cleanup;
 +  }
 +
 +  goto done;
 +
 + error_cleanup:
 +  tor_free(cons2_str); /* Sets it to NULL */
 +
 + done:
 +  if (cons2) {
 +    smartlist_free(cons2);
 +  }
 +
 +  return cons2_str;
 +}
 +
 +/** Any consensus line longer than this means that the input is invalid. */
 +#define CONSENSUS_LINE_MAX_LEN (1<<20)
 +
 +/**
 + * Helper: For every NL-terminated line in <b>s</b>, add a cdline referring to
 + * that line (without trailing newline) to <b>out</b>.  Return -1 if there are
 + * any non-NL terminated lines; 0 otherwise.
 + *
 + * Unlike tor_split_lines, this function avoids ambiguity on its
 + * handling of a final line that isn't NL-terminated.
 + *
 + * All cdline_t objects are allocated in the provided memarea.  Strings
 + * are not copied: if <b>s</b> changes or becomes invalid, then all
 + * generated cdlines will become invalid.
 + */
 +STATIC int
 +consensus_split_lines(smartlist_t *out, const char *s, memarea_t *area)
 +{
 +  const char *end_of_str = s + strlen(s);
 +  tor_assert(*end_of_str == '\0');
 +
 +  while (*s) {
 +    const char *eol = memchr(s, '\n', end_of_str - s);
 +    if (!eol) {
 +      /* File doesn't end with newline. */
 +      return -1;
 +    }
 +    if (eol - s > CONSENSUS_LINE_MAX_LEN) {
 +      /* Line is far too long. */
 +      return -1;
 +    }
 +    cdline_t *line = memarea_alloc(area, sizeof(cdline_t));
 +    line->s = s;
 +    line->len = (uint32_t)(eol - s);
 +    smartlist_add(out, line);
 +    s = eol+1;
 +  }
 +  return 0;
 +}
 +
 +/** Given a list of cdline_t, return a newly allocated string containing
 + * all of the lines, terminated with NL, concatenated.
 + *
 + * Unlike smartlist_join_strings(), avoids lossy operations on empty
 + * lists.  */
 +static char *
 +consensus_join_lines(const smartlist_t *inp)
 +{
 +  size_t n = 0;
 +  SMARTLIST_FOREACH(inp, const cdline_t *, cdline, n += cdline->len + 1);
 +  n += 1;
 +  char *result = tor_malloc(n);
 +  char *out = result;
 +  SMARTLIST_FOREACH_BEGIN(inp, const cdline_t *, cdline) {
 +    memcpy(out, cdline->s, cdline->len);
 +    out += cdline->len;
 +    *out++ = '\n';
 +  } SMARTLIST_FOREACH_END(cdline);
 +  *out++ = '\0';
 +  tor_assert(out == result+n);
 +  return result;
 +}
 +
 +/** Given two consensus documents, try to compute a diff between them.  On
 + * success, retun a newly allocated string containing that diff.  On failure,
 + * return NULL. */
 +char *
 +consensus_diff_generate(const char *cons1,
 +                        const char *cons2)
 +{
 +  consensus_digest_t d1, d2;
 +  smartlist_t *lines1 = NULL, *lines2 = NULL, *result_lines = NULL;
 +  int r1, r2;
 +  char *result = NULL;
 +
 +  r1 = consensus_compute_digest_as_signed(cons1, &d1);
 +  r2 = consensus_compute_digest(cons2, &d2);
 +  if (BUG(r1 < 0 || r2 < 0))
 +    return NULL; // LCOV_EXCL_LINE
 +
 +  memarea_t *area = memarea_new();
 +  lines1 = smartlist_new();
 +  lines2 = smartlist_new();
 +  if (consensus_split_lines(lines1, cons1, area) < 0)
 +    goto done;
 +  if (consensus_split_lines(lines2, cons2, area) < 0)
 +    goto done;
 +
 +  result_lines = consdiff_gen_diff(lines1, lines2, &d1, &d2, area);
 +
 + done:
 +  if (result_lines) {
 +    result = consensus_join_lines(result_lines);
 +    smartlist_free(result_lines);
 +  }
 +
 +  memarea_drop_all(area);
 +  smartlist_free(lines1);
 +  smartlist_free(lines2);
 +
 +  return result;
 +}
 +
 +/** Given a consensus document and a diff, try to apply the diff to the
 + * consensus.  On success return a newly allocated string containing the new
 + * consensus.  On failure, return NULL. */
 +char *
 +consensus_diff_apply(const char *consensus,
 +                     const char *diff)
 +{
 +  consensus_digest_t d1;
 +  smartlist_t *lines1 = NULL, *lines2 = NULL;
 +  int r1;
 +  char *result = NULL;
 +  memarea_t *area = memarea_new();
 +
 +  r1 = consensus_compute_digest_as_signed(consensus, &d1);
 +  if (BUG(r1 < 0))
-     return NULL; // LCOV_EXCL_LINE
++    goto done;
 +
 +  lines1 = smartlist_new();
 +  lines2 = smartlist_new();
 +  if (consensus_split_lines(lines1, consensus, area) < 0)
 +    goto done;
 +  if (consensus_split_lines(lines2, diff, area) < 0)
 +    goto done;
 +
 +  result = consdiff_apply_diff(lines1, lines2, &d1);
 +
 + done:
 +  smartlist_free(lines1);
 +  smartlist_free(lines2);
 +  memarea_drop_all(area);
 +
 +  return result;
 +}
 +
 +/** Return true iff, based on its header, <b>document</b> is likely
 + * to be a consensus diff. */
 +int
 +looks_like_a_consensus_diff(const char *document, size_t len)
 +{
 +  return (len >= strlen(ns_diff_version) &&
 +          fast_memeq(document, ns_diff_version, strlen(ns_diff_version)));
 +}



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