line-log: protect inner strbuf from free

This fixes a regression introduced in 2.46.0.

The change made in 394affd46d (line-log: always allocate the output prefix, 2024-06-07) made the method more consistent in that it did not return a static empty string that would fail to free(), but it does lead to double-frees when a strbuf buffer is freed multiple times.

In v2, I add Peff's test to the first patch. I also split his diff into two more follow-up patches because the additional clarity seems valuable to me. I have forged his sign-off in all three patches.

Note to the maintainer: feel free to take only the first patch, as Peff replied that he may work on the remaining cleanup independently (but I had already prepared patches 2 & 3).

Thanks, -Stolee

Derrick Stolee (1):
line-log: protect inner strbuf from free

Jeff King (2):
line-log: remove output_prefix()
diff: modify output_prefix function pointer

 diff-lib.c          |  4 ++--
 diff.c              |  8 +++-----
 diff.h              |  2 +-
 graph.c             |  4 ++--
 line-log.c          | 16 ++--------------
 log-tree.c          |  4 ++--
 range-diff.c        |  4 ++--
 t/t4211-line-log.sh | 28 ++++++++++++++++++++++++++++
 8 files changed, 42 insertions(+), 28 deletions(-)

base-commit: e9356ba3ea2a6754281ff7697b3e5a1697b21e24
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1806%2Fderrickstolee%2Fline-log-use-after-free-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1806/derrickstolee/line-log-use-after-free-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1806

Range-diff vs v1:

1: 5d341e42d83 ! 1: 05c21616c35 line-log: protect inner strbuf from free

     @@ Commit message
      
          [1] https://github.com/git-for-windows/git/issues/5185
      
     +    This issue would have been caught by the new test, when Git is compiled
     +    with ASan to catch these double frees.
     +
     +    Co-authored-by: Jeff King <[email protected]>
     +    Signed-off-by: Jeff King <[email protected]>
          Signed-off-by: Derrick Stolee <[email protected]>
      
       ## line-log.c ##
     @@ line-log.c: out:
       
       	while (range) {
       		dump_diff_hacky_one(rev, range);
     +
     + ## t/t4211-line-log.sh ##
     +@@ t/t4211-line-log.sh: test_expect_success 'zero-width regex .* matches any function name' '
     + 	test_cmp expect actual
     + '
     + 
     ++test_expect_success 'show line-log with graph' '
     ++	qz_to_tab_space >expect <<-EOF &&
     ++	* $head_oid Modify func2() in file.c
     ++	|Z
     ++	| diff --git a/file.c b/file.c
     ++	| --- a/file.c
     ++	| +++ b/file.c
     ++	| @@ -6,4 +6,4 @@
     ++	|  int func2()
     ++	|  {
     ++	| -    return F2;
     ++	| +    return F2 + 2;
     ++	|  }
     ++	* $root_oid Add func1() and func2() in file.c
     ++	ZZ
     ++	  diff --git a/file.c b/file.c
     ++	  --- /dev/null
     ++	  +++ b/file.c
     ++	  @@ -0,0 +6,4 @@
     ++	  +int func2()
     ++	  +{
     ++	  +    return F2;
     ++	  +}
     ++	EOF
     ++	git log --graph --oneline -L:func2:file.c >actual &&
     ++	test_cmp expect actual
     ++'
     ++
     + test_done
 -:  ----------- > 2:  94d2c034b4a line-log: remove output_prefix()
 -:  ----------- > 3:  e1d825ad212 diff: modify output_prefix function pointer

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