Subject: [PATCH v6] pack-bitmap: remove checks before bitmap_free

From: "Lidong Yan via GitGitGadget" <[email-masked]>
Date: Thu, 05 Jun 2025 15:53:39 +0000

From: Lidong Yan <[email-masked]>

In pack-bitmap.c:find_boundary_objects(), the roots_bitmap is only freed
if cascade_pseudo_merges_1() fails. Since cascade_pseudo_merges_1() only
use roots_bitmap as a mutable reference but not takes roots_bitmap's
ownership. Once cascade_pseudo_merges_1() succeeds, roots_bitmap leaks.
And this leak currently lacks a dedicated test to detect it.

To fix this leak, remove if cascade_pseudo_merges_1() succeed check and
always calling bitmap_free(roots_bitmap);

To trigger this leak, we need roots_bitmap that contains at least one
pseudo merge. So that we can use pseudo merge bitmap when we compute roots
reachable bitmap. Here we create two commits: first A then B. Add A
to the pseudo-merge and perform a traversal over the range A..B.
In this scenario, the "haves" set will be {A}, and cascade_pseudo_merges_1
will succeed, thereby exposing the leak due to the missing roots_bitmap
cleanup.

Signed-off-by: Lidong Yan <[email-masked]>
---
    pack-bitmap: remove checks before bitmap_free
    
    In pack-bitmap.c:find_boundary_objects, remove cascade success check and
    always free roots_bitmap afterward to make static analysis tool works
    better.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1977%2Fbrandb97%2Fremove-check-before-bitmap-free-v6
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1977/brandb97/remove-check-before-bitmap-free-v6
Pull-Request: https://github.com/git/git/pull/1977

Range-diff vs v5:

 1:  4bc90c83a40 ! 1:  43cdce190dc pack-bitmap: remove checks before bitmap_free
     @@ Commit message
          In pack-bitmap.c:find_boundary_objects(), the roots_bitmap is only freed
          if cascade_pseudo_merges_1() fails. Since cascade_pseudo_merges_1() only
          use roots_bitmap as a mutable reference but not takes roots_bitmap's
     -    ownership. Once cascade_pseudo_merges_1 succeed(), roots_bitmap leaks.
     +    ownership. Once cascade_pseudo_merges_1() succeeds, roots_bitmap leaks.
          And this leak currently lacks a dedicated test to detect it.
      
          To fix this leak, remove if cascade_pseudo_merges_1() succeed check and
          always calling bitmap_free(roots_bitmap);
      
     -    To trigger this leak, we need roots_bitmap contains at least one pseudo
     -    merge. So that we can use pseudo merge bitmap when we compute roots
     +    To trigger this leak, we need roots_bitmap that contains at least one
     +    pseudo merge. So that we can use pseudo merge bitmap when we compute roots
          reachable bitmap. Here we create two commits: first A then B. Add A
          to the pseudo-merge and perform a traversal over the range A..B.
          In this scenario, the "haves" set will be {A}, and cascade_pseudo_merges_1

 pack-bitmap.c                   |  4 ++--
 t/t5333-pseudo-merge-bitmaps.sh | 17 +++++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/pack-bitmap.c b/pack-bitmap.c
index ac6d62b980c..8727f316de9 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -1363,8 +1363,8 @@ static struct bitmap *find_boundary_objects(struct bitmap_index *bitmap_git,
 			bitmap_set(roots_bitmap, pos);
 		}
 
-		if (!cascade_pseudo_merges_1(bitmap_git, cb.base, roots_bitmap))
-			bitmap_free(roots_bitmap);
+		cascade_pseudo_merges_1(bitmap_git, cb.base, roots_bitmap);
+		bitmap_free(roots_bitmap);
 	}
 
 	/*
diff --git a/t/t5333-pseudo-merge-bitmaps.sh b/t/t5333-pseudo-merge-bitmaps.sh
index 56674db562f..ba5ae6a00c9 100755
--- a/t/t5333-pseudo-merge-bitmaps.sh
+++ b/t/t5333-pseudo-merge-bitmaps.sh
@@ -445,4 +445,21 @@ test_expect_success 'pseudo-merge closure' '
 	)
 '
 
+test_expect_success 'use pseudo-merge in boundary traversal' '
+	git init pseudo-merge-boundary-traversal &&
+	(
+		cd pseudo-merge-boundary-traversal &&
+
+		git config bitmapPseudoMerge.test.pattern refs/ &&
+		git config pack.useBitmapBoundaryTraversal true &&
+
+		test_commit A &&
+		git repack -adb &&
+		test_commit B &&
+
+		nr=$(git rev-list --count --use-bitmap-index HEAD~1..HEAD) &&
+		test 1 -eq "$nr"
+	)
+'
+
 test_done

base-commit: 845c48a16a7f7b2c44d8cb137b16a4a1f0140229
-- 
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