[BUG] git diff --no-index segfaults on large files (NULL object database)

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What did you do before the bug happened? (Steps to reproduce your issue)

`git diff --color -- file1 file2` SIGSEGV in case file1 and file2 are "big enough", that is when the file size is bigger than repo_settings_get_big_file_threshold()

What did you expect to happen? (Expected behavior)

Not a crash, it to say "binary files are different"

What happened instead? (Actual behavior)

Program received signal SIGSEGV, Segmentation fault.
index_fd (istate=istate@entry=0x5555559ccb40,
oid=oid@entry=0x5555559d3c80, fd=4, st=st@entry=0x7fffffffc5c0,
type=type@entry=OBJ_BLOB, path=path@entry=0x5555559d3ce0
"4/root_part", flags=0) at /usr/src/debug/git/git/object-file.c:1634
1634                    transaction =
odb_transaction_begin(the_repository->objects);
(gdb) bt
#0  index_fd (istate=istate@entry=0x5555559ccb40,
oid=oid@entry=0x5555559d3c80, fd=4, st=st@entry=0x7fffffffc5c0,
type=type@entry=OBJ_BLOB, path=path@entry=0x5555559d3ce0
"4/root_part", flags=0)
   at /usr/src/debug/git/git/object-file.c:1634
#1  0x000055555579ab75 in index_path (istate=0x5555559ccb40,
oid=0x5555559d3c80, path=0x5555559d3ce0 "4/root_part",
st=0x7fffffffc5c0, flags=0) at
/usr/src/debug/git/git/object-file.c:1658
#2  0x0000555555720f83 in diff_fill_oid_info (one=0x5555559d3c80,
istate=0x5555559ccb40) at /usr/src/debug/git/git/diff.c:4690
#3  diff_fill_oid_info (one=one@entry=0x5555559d3c80,
istate=0x5555559ccb40) at /usr/src/debug/git/git/diff.c:4679
#4  0x0000555555724aba in run_diff (p=0x5555559ce900,
o=0x7fffffffd188) at /usr/src/debug/git/git/diff.c:4738
#5  diff_flush_patch (p=0x5555559ce900, o=0x7fffffffd188) at
/usr/src/debug/git/git/diff.c:6247
#6  0x0000555555729850 in diff_flush_patch_all_file_pairs
(o=0x7fffffffd188) at /usr/src/debug/git/git/diff.c:6802
#7  diff_flush (options=<optimized out>) at /usr/src/debug/git/git/diff.c:6942
#8  0x00005555555b0d34 in diff_no_index (revs=0x7fffffffcbd0,
algop=0x55555598dbd0 <hash_algos+112>, implicit_no_index=<optimized
out>, argc=2, argv=0x5555559ce2a0) at
/usr/src/debug/git/git/diff-no-index.c:427
#9  cmd_diff (argc=<optimized out>, argv=0x5555559ce2a0,
prefix=<optimized out>, repo=<optimized out>) at builtin/diff.c:516
#10 0x000055555555fe05 in run_builtin (p=0x555555994ec8
<commands.lto_priv+840>, argc=<optimized out>, argv=<optimized out>,
repo=0x5555559a3c00 <the_repo.lto_priv>) at
/usr/src/debug/git/git/git.c:506
#11 handle_builtin (args=args@entry=0x7fffffffd980) at
/usr/src/debug/git/git/git.c:779
#12 0x00005555555612cc in run_argv (args=0x7fffffffd980) at
/usr/src/debug/git/git/git.c:862
#13 cmd_main (argc=<optimized out>, argv=<optimized out>) at
/usr/src/debug/git/git/git.c:984
#14 0x000055555555d794 in main (argc=6, argv=0x7fffffffdc58) at
/usr/src/debug/git/git/common-main.c:9
(gdb) p the_repository->objects
$1 = (struct object_database *) 0x0
(gdb) p istate->repo->objects
$2 = (struct object_database *) 0x0
(gdb) p istate->repo
$3 = (struct repository *) 0x5555559a3c00 <the_repo.lto_priv>
(gdb) p the_repository
$4 = (struct repository *) 0x5555559a3c00 <the_repo.lto_priv>

The same issue happens in master, the same path is taken and when we finally get into `odb_transaction_begin` where odb is NULL, and reading ->transaction causes the illegal segment access once again.

Program received signal SIGSEGV, Segmentation fault. 0x00005555557c6283 in odb_transaction_begin (odb=0x0) at odb.c:1075 1075 if (odb->transaction) (gdb) bt #0 0x00005555557c6283 in odb_transaction_begin (odb=0x0) at odb.c:1075 #1 0x00005555557b94f9 in index_fd (istate=0x555555a88b60, oid=0x555555a8fcb0, fd=4, st=0x7fffffffc730, type=OBJ_BLOB, path=0x555555a8fd10 "4/root_part", flags=0) at object-file.c:1665 #2 0x00005555557b9651 in index_path (istate=0x555555a88b60, oid=0x555555a8fcb0, path=0x555555a8fd10 "4/root_part", st=0x7fffffffc730, flags=0) at object-file.c:1691 #3 0x0000555555730937 in diff_fill_oid_info (one=0x555555a8fcb0, istate=0x555555a88b60) at diff.c:4699 #4 0x0000555555730b81 in run_diff (p=0x555555a8a9a0, o=0x7fffffffd1a8) at diff.c:4747 #5 0x00005555557361d5 in diff_flush_patch (p=0x555555a8a9a0, o=0x7fffffffd1a8) at diff.c:6258 #6 0x0000555555737cae in diff_flush_patch_all_file_pairs (o=0x7fffffffd1a8) at diff.c:6813 #7 0x00005555557382b2 in diff_flush (options=0x7fffffffd1a8) at diff.c:6953 #8 0x00005555557228cc in diff_no_index (revs=0x7fffffffcbf0, algop=0x555555a49b50 <hash_algos+112>, implicit_no_index=1, argc=2, argv=0x555555a8a2f0) at diff-no-index.c:427 #9 0x00005555555c04b1 in cmd_diff (argc=5, argv=0x555555a8a2f0, prefix=0x0, repo=0x0) at builtin/diff.c:516 #10 0x0000555555574f47 in run_builtin (p=0x555555a4aaa8 <commands+840>, argc=5, argv=0x555555a8a2f0, repo=0x555555a7e620 <the_repo>) at git.c:506 #11 0x000055555557544e in handle_builtin (args=0x7fffffffdab0) at git.c:780 #12 0x0000555555575778 in run_argv (args=0x7fffffffdab0) at git.c:863 #13 0x0000555555575c0f in cmd_main (argc=5, argv=0x7fffffffdc50) at git.c:984 #14 0x00005555556a7414 in main (argc=6, argv=0x7fffffffdc48) at common-main.c:9

What's different between what you expected and what actually happened?

Anything else you want to add:

Please review the rest of the bug report below. You can delete any lines you don't wish to share.

[System Info]
git version:
git version 2.53.0
cpu: x86_64
built from commit: 67ad42147a7acc2af6074753ebd03d904476118f
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
rust: enabled
gettext: enabled
libcurl: 8.18.0
OpenSSL: OpenSSL 3.6.1 27 Jan 2026
zlib-ng: 2.3.2
SHA-1: SHA1_DC
SHA-256: SHA256_BLK
default-ref-format: files
default-hash: sha1
uname: Linux 6.19.9-arch1-1 #1 SMP PREEMPT_DYNAMIC Thu, 19 Mar 2026
16:33:17 +0000 x86_64
compiler info: gnuc: 15.2
libc info: glibc: 2.43
$SHELL (typically, interactive shell): /usr/bin/fish

[Enabled Hooks] not run from a git repository - no hooks to show