Re: [PATCH v3] merge-ort: fix segmentation fault in read-only repositories

From: Elijah Newren <newren@gmail.com>
Date: Sep 23, 2022, 01:38 UTC
Message-ID: <CABPp-BGizu+dq+8Hbqo5EjKFAWU_Ni9RwQ6p+4oUMLp0oamu7w@mail.gmail.com>
In-Reply-To: <pull.1362.v3.git.1663875999939.gitgitgadget@gmail.com>

On Thu, Sep 22, 2022 at 12:46 PM Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com> wrote:

Show 9 quoted lines

>
> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> If the blob/tree objects cannot be written, we really need the merge
> operations to fail, and not to continue (and then try to access the tree
> object which is however still set to `NULL`).
>
> Let's stop ignoring the return value of `write_object_file()` and
> `write_tree()` and set `clean = -1` in the error case.

:-)

Show 35 quoted lines

>  merge-ort.c                      | 64 +++++++++++++++++++-------------
>  t/t4301-merge-tree-write-tree.sh |  9 +++++
>  2 files changed, 48 insertions(+), 25 deletions(-)
>
> diff --git a/merge-ort.c b/merge-ort.c
> index 99dcee2db8a..f654296220e 100644
> --- a/merge-ort.c
> +++ b/merge-ort.c
> @@ -3571,15 +3571,15 @@ static int tree_entry_order(const void *a_, const void *b_)
>                                  b->string, strlen(b->string), bmi->result.mode);
>  }
>
> -static void write_tree(struct object_id *result_oid,
> -                      struct string_list *versions,
> -                      unsigned int offset,
> -                      size_t hash_size)
> +static int write_tree(struct object_id *result_oid,
> +                     struct string_list *versions,
> +                     unsigned int offset,
> +                     size_t hash_size)
>  {
>         size_t maxlen = 0, extra;
>         unsigned int nr;
>         struct strbuf buf = STRBUF_INIT;
> -       int i;
> +       int i, ret;
>
>         assert(offset <= versions->nr);
>         nr = versions->nr - offset;
> @@ -3605,8 +3605,9 @@ static void write_tree(struct object_id *result_oid,
>         }
>
>         /* Write this object file out, and record in result_oid */
> -       write_object_file(buf.buf, buf.len, OBJ_TREE, result_oid);
> +       ret = write_object_file(buf.buf, buf.len, OBJ_TREE, result_oid);

This sent me into a bit of a hunt to try to figure out what the return value of write_object_file() is. I know it's non-zero on failure, but don't know if it's always positive or always negative or possibly a mix and how that maps to the idea of "clean_merge". I gave up after a few indirections, to be honest.

Anyway, regardless of my inability to find the answer to the above question, would this be a bit easier to read if we initialized ret to 0 and then did something like

    if (write_object_file(buf.buf, buf.len, OBJ_TREE, result_oid))
        ret = -1;

Show 50 quoted lines

>         strbuf_release(&buf);
> +       return ret;
>  }
>
>  static void record_entry_for_tree(struct directory_versions *dir_metadata,
> @@ -3625,13 +3626,13 @@ static void record_entry_for_tree(struct directory_versions *dir_metadata,
>                            basename)->util = &mi->result;
>  }
>
> -static void write_completed_directory(struct merge_options *opt,
> -                                     const char *new_directory_name,
> -                                     struct directory_versions *info)
> +static int write_completed_directory(struct merge_options *opt,
> +                                    const char *new_directory_name,
> +                                    struct directory_versions *info)
>  {
>         const char *prev_dir;
>         struct merged_info *dir_info = NULL;
> -       unsigned int offset;
> +       unsigned int offset, ret = 0;
>
>         /*
>          * Some explanation of info->versions and info->offsets...
> @@ -3717,7 +3718,7 @@ static void write_completed_directory(struct merge_options *opt,
>          * strcmp here.)
>          */
>         if (new_directory_name == info->last_directory)
> -               return;
> +               return 0;
>
>         /*
>          * If we are just starting (last_directory is NULL), or last_directory
> @@ -3739,7 +3740,7 @@ static void write_completed_directory(struct merge_options *opt,
>                  */
>                 string_list_append(&info->offsets,
>                                    info->last_directory)->util = (void*)offset;
> -               return;
> +               return 0;
>         }
>
>         /*
> @@ -3769,8 +3770,9 @@ static void write_completed_directory(struct merge_options *opt,
>                  */
>                 dir_info->is_null = 0;
>                 dir_info->result.mode = S_IFDIR;
> -               write_tree(&dir_info->result.oid, &info->versions, offset,
> -                          opt->repo->hash_algo->rawsz);
> +               if (write_tree(&dir_info->result.oid, &info->versions, offset,
> +                              opt->repo->hash_algo->rawsz) < 0)
> +                       ret = -1;

This makes me wonder about write_tree() again.  What if its call to
write_object_file() returns a value greater than 0?  Is that possible?
 If it is, are those error cases or not?  About half the callers in
the code base that check the return value of write_object_file() check
for a non-zero value, the other half check for a value less than 0.
And I can't find any documentation.  And it seemed like too much time
for me to figure out what its range of return values was.

Show 114 quoted lines

>         }
>
>         /*
> @@ -3798,6 +3800,8 @@ static void write_completed_directory(struct merge_options *opt,
>         /* And, of course, we need to update last_directory to match. */
>         info->last_directory = new_directory_name;
>         info->last_directory_len = strlen(info->last_directory);
> +
> +       return ret;
>  }
>
>  /* Per entry merge function */
> @@ -4214,8 +4218,8 @@ static void prefetch_for_content_merges(struct merge_options *opt,
>         oid_array_clear(&to_fetch);
>  }
>
> -static void process_entries(struct merge_options *opt,
> -                           struct object_id *result_oid)
> +static int process_entries(struct merge_options *opt,
> +                          struct object_id *result_oid)
>  {
>         struct hashmap_iter iter;
>         struct strmap_entry *e;
> @@ -4224,11 +4228,12 @@ static void process_entries(struct merge_options *opt,
>         struct directory_versions dir_metadata = { STRING_LIST_INIT_NODUP,
>                                                    STRING_LIST_INIT_NODUP,
>                                                    NULL, 0 };
> +       int ret;
>
>         trace2_region_enter("merge", "process_entries setup", opt->repo);
>         if (strmap_empty(&opt->priv->paths)) {
>                 oidcpy(result_oid, opt->repo->hash_algo->empty_tree);
> -               return;
> +               return 0;
>         }
>
>         /* Hack to pre-allocate plist to the desired size */
> @@ -4270,8 +4275,11 @@ static void process_entries(struct merge_options *opt,
>                  */
>                 struct merged_info *mi = entry->util;
>
> -               write_completed_directory(opt, mi->directory_name,
> -                                         &dir_metadata);
> +               if (write_completed_directory(opt, mi->directory_name,
> +                                             &dir_metadata) < 0) {
> +                       ret = -1;
> +                       goto cleanup;
> +               }
>                 if (mi->clean)
>                         record_entry_for_tree(&dir_metadata, path, mi);
>                 else {
> @@ -4291,12 +4299,15 @@ static void process_entries(struct merge_options *opt,
>                 fflush(stdout);
>                 BUG("dir_metadata accounting completely off; shouldn't happen");
>         }
> -       write_tree(result_oid, &dir_metadata.versions, 0,
> -                  opt->repo->hash_algo->rawsz);
> +       ret = write_tree(result_oid, &dir_metadata.versions, 0,
> +                        opt->repo->hash_algo->rawsz);
> +cleanup:
>         string_list_clear(&plist, 0);
>         string_list_clear(&dir_metadata.versions, 0);
>         string_list_clear(&dir_metadata.offsets, 0);
>         trace2_region_leave("merge", "process_entries cleanup", opt->repo);
> +
> +       return ret;
>  }
>
>  /*** Function Grouping: functions related to merge_switch_to_result() ***/
> @@ -4928,15 +4939,18 @@ redo:
>         }
>
>         trace2_region_enter("merge", "process_entries", opt->repo);
> -       process_entries(opt, &working_tree_oid);
> +       if (process_entries(opt, &working_tree_oid) < 0)
> +               result->clean = -1;
>         trace2_region_leave("merge", "process_entries", opt->repo);
>
>         /* Set return values */
>         result->path_messages = &opt->priv->conflicts;
>
> -       result->tree = parse_tree_indirect(&working_tree_oid);
> -       /* existence of conflicted entries implies unclean */
> -       result->clean &= strmap_empty(&opt->priv->conflicted);
> +       if (result->clean >= 0) {
> +               result->tree = parse_tree_indirect(&working_tree_oid);
> +               /* existence of conflicted entries implies unclean */
> +               result->clean &= strmap_empty(&opt->priv->conflicted);
> +       }
>         if (!opt->priv->call_depth) {
>                 result->priv = opt->priv;
>                 result->_properly_initialized = RESULT_INITIALIZED;
> diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
> index 28ca5c38bb5..013b77144bd 100755
> --- a/t/t4301-merge-tree-write-tree.sh
> +++ b/t/t4301-merge-tree-write-tree.sh
> @@ -810,4 +810,13 @@ test_expect_success 'can override merge of unrelated histories' '
>         test_cmp expect actual
>  '
>
> +test_expect_success SANITY 'merge-ort fails gracefully in a read-only repository' '
> +       git init --bare read-only &&
> +       git push read-only side1 side2 side3 &&
> +       test_when_finished "chmod -R u+w read-only" &&
> +       chmod -R a-w read-only &&
> +       test_must_fail git -C read-only merge-tree side1 side3 &&
> +       test_must_fail git -C read-only merge-tree side1 side2
> +'
> +
>  test_done
>
> base-commit: dda7228a83e2e9ff584bf6adbf55910565b41e14
> --
> gitgitgadget

Nice to see that it didn't take too much work to propagate the -1 value from write_tree() up the hierarchy. Nice work!

I think we've still got some separate problems related to propagating the return value of the other write_object_file() call, from handle_content_merge(). The direct call in that other case is okay, but the higher levels at process_renames() and process_entry() seem to fumble it. Your fix for failed tree writes is still valid without fixing the failed blob writes, and I'm happy to tackle the other half if you'd prefer to hand it off. But, if you'd like to tackle that half too, you might see fewer error messages when it fails to write to the read-only repository, since it'll fail early on the first blob write instead of only failing when it gets around to trying to write a new tree.