[PATCH v5] pack-bitmap: remove checks before bitmap_free

From: Lidong Yan <502024330056@smail.nju.edu.cn>

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. 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 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 <502024330056@smail.nju.edu.cn>
---
    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-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1977/brandb97/remove-check-before-bitmap-free-v5
Pull-Request: https://github.com/git/git/pull/1977

Range-diff vs v4:

 1:  fa443065436 ! 1:  4bc90c83a40 pack-bitmap: remove checks before bitmap_free
     @@ Commit message
          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 a pseudo-merge whose size is equal to
     -    or smaller than roots_bitmap (which corresponds to the set of "haves"
     -    commits in prepare_bitmap_walk()). To do this, we can create two
     -    commits: A and B. Add A to the pseudo-merge list 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.
     +    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
     +    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 <502024330056@smail.nju.edu.cn>
      
     @@ t/t5333-pseudo-merge-bitmaps.sh: test_expect_success 'pseudo-merge closure' '
      +		cd pseudo-merge-boundary-traversal &&
      +
      +		git config bitmapPseudoMerge.test.pattern refs/ &&
     -+		git config bitmapPseudoMerge.test.threshold now &&
     -+		git config bitmapPseudoMerge.test.stableThreshold now &&
     ++		git config pack.useBitmapBoundaryTraversal true &&
      +
      +		test_commit A &&
      +		git repack -adb &&
      +		test_commit B &&
      +
     -+		echo '1' >expect &&
     -+		GIT_TEST_PACK_USE_BITMAP_BOUNDARY_TRAVERSAL=1 \
     -+			git rev-list --count --use-bitmap-index HEAD~1..HEAD >actual &&
     -+		test_cmp expect actual
     ++		nr=$(git rev-list --count --use-bitmap-index HEAD~1..HEAD) &&
     ++		test 1 -eq "$nr"
      +	)
      +'
      +

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