[PATCH v6] 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() 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 <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-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(-)