git fsck : Cannot fix broken link

Question

I cannot succeed fixing a git broken link.

Here is the context :

$ git pull
Auto packing the repository in background for optimum performance.
See "git help gc" for manual housekeeping.
warning: The last gc run reported the following. Please correct the root cause
and remove .git/gc.log.
Automatic cleanup will not be performed until the file is removed.

error: inflate: data stream error (unknown compression method)
error: unable to unpack 9a684a736e3b97ff7c709138259196fd223e661c header
fatal: loose object 9a684a736e3b97ff7c709138259196fd223e661c (stored in .git/objects/9a/684a736e3b97ff7c709138259196fd223e661c) is corrupt
fatal: failed to run repack

Already up to date.
$ git fsck
error: inflate: data stream error (unknown compression method)
error: unable to unpack header of .git/objects/9a/684a736e3b97ff7c709138259196fd223e661c
error: 9a684a736e3b97ff7c709138259196fd223e661c: object corrupt or missing: .git/objects/9a/684a736e3b97ff7c709138259196fd223e661c
Checking object directories: 100% (256/256), done.
Checking objects: 100% (5766/5766), done.
Checking connectivity: 12924, done.
dangling tree bd85f5cb37f9545d81b7f923978f97c4eaf391ea
dangling tree 978c24540576bfa8d0e748a33e2457c6286a0d64
dangling blob 0d91b9e2423b61d3f941a3571f08459e3f7db21f
dangling blob 4c4112f5aaa60d99ef2c8a4bb66e855eb1bac7e6
missing tree 9a684a736e3b97ff7c709138259196fd223e661c
$ git cat-file -t 9a684a736e3b97ff7c709138259196fd223e661c
error: inflate: data stream error (unknown compression method)
error: unable to unpack 9a684a736e3b97ff7c709138259196fd223e661c header
fatal: git cat-file: could not get object info
$ du .git/objects/9a/684a736e3b97ff7c709138259196fd223e661c
0   .git/objects/9a/684a736e3b97ff7c709138259196fd223e661c

So I removed this empty file according to this :

$ rm .git/objects/9a/684a736e3b97ff7c709138259196fd223e661c
rm: remove write-protected regular file '.git/objects/9a/684a736e3b97ff7c709138259196fd223e661c'? y
$ git fsck
Checking object directories: 100% (256/256), done.
Checking objects: 100% (5766/5766), done.
broken link from  commit fca8efc10a91c7ac2cd0acf5d6744c1cb1ea9765
              to    tree 9a684a736e3b97ff7c709138259196fd223e661c
Checking connectivity: 12924, done.
dangling tree bd85f5cb37f9545d81b7f923978f97c4eaf391ea
dangling tree 978c24540576bfa8d0e748a33e2457c6286a0d64
dangling blob 0d91b9e2423b61d3f941a3571f08459e3f7db21f
dangling blob 4c4112f5aaa60d99ef2c8a4bb66e855eb1bac7e6
missing tree 9a684a736e3b97ff7c709138259196fd223e661c

So I tried the solutions described here :

$ git gc --aggressive
fatal: bad tree object 9a684a736e3b97ff7c709138259196fd223e661c
fatal: failed to run repack
$ git stash clear
$ git reflog expire --expire-unreachable=now --all
$ git gc --prune=now
fatal: bad tree object 9a684a736e3b97ff7c709138259196fd223e661c
fatal: failed to run repack
$ git fsck --name-objects
Checking object directories: 100% (256/256), done.
Checking objects: 100% (5766/5766), done.
broken link from  commit fca8efc10a91c7ac2cd0acf5d6744c1cb1ea9765 (~11)
              to    tree 9a684a736e3b97ff7c709138259196fd223e661c (~11:)
Checking connectivity: 12924, done.
dangling tree bd85f5cb37f9545d81b7f923978f97c4eaf391ea
dangling tree 978c24540576bfa8d0e748a33e2457c6286a0d64
dangling blob 0d91b9e2423b61d3f941a3571f08459e3f7db21f
dangling blob 4c4112f5aaa60d99ef2c8a4bb66e855eb1bac7e6
missing tree 9a684a736e3b97ff7c709138259196fd223e661c (~11:)
$

Answer 1

Beside trying to fix a corrupted repository, make sure to use Git 2.37 (Q3 2022)

See commit 7c89855, commit 4627c67, commit 29d8e21, commit 58407e0 (12 May 2022) by Ævar Arnfjörð Bjarmason (avar).
^{(Merged by Junio C Hamano -- gitster -- in commit 1b8138f, 23 May 2022)}

object-file: fix a unpack_loose_header() regression in 3b6a8db3b03

^{Signed-off-by: Ævar Arnfjörð Bjarmason}

Fix a regression in my 3b6a8db (object-file.c: use , 2021-10-01, Git v2.34.0-rc0 -- merge listed in batch #15) (object-file.c: use "enum" return type for unpack_loose_header(), 2021-10-01) revealed both by running the test suite with --valgrind, and with the amended "git fsck"^(man) test.

In practice this regression in v2.34.0 caused us to claim that we couldn't parse the header, as opposed to not being able to unpack it.

Before the change in the C code the test_cmp added here would emit:

-error: unable to unpack header of ./objects/e6/9de29bb2d1d6434b8b29ae775ad8c2e48c5391
+error: unable to parse header of ./objects/e6/9de29bb2d1d6434b8b29ae775ad8c2e48c5391

I.e.
we'd proceed to call parse_loose_header() on the uninitialized "hdr" value, and it would have been very unlikely for that uninitialized memory to be a valid git object.

The other callers of unpack_loose_header() were already checking the enum values exhaustively.
See 3b6a8db and 5848fb1 (object-file.c: return ULHR_TOO_LONG on , 2021-10-01, Git v2.34.0-rc0 -- merge listed in batch #15) (object-file.c: return ULHR_TOO_LONG on "header too long", 2021-10-01).