[PATCH] git: avoid segfault on "git --shallow-file" without a value

[Christian Couder <christian.couder@gmail.com>]

In "git.c", the other `handle_options()` options that take their value
as a separate argument, like `--git-dir`, `--namespace` or `-C`, check
that such an argument actually exists before using it, and error out
with a message and the usage string otherwise.

The `--shallow-file` option doesn't perform that check. It blindly
advances past the option and then dereferences the next element of
`argv`, which is the NULL terminator when no value was given. So
`git --shallow-file` segfaults:

  $ git --shallow-file
  Segmentation fault (core dumped)

Let's fix that by checking that a value was given, in the same way and
with a message worded like the ones the other options use.

While at it, let's also set the environment variable before advancing
past the option, instead of advancing first and using `(*argv)[0]`, so
that this option looks like the other ones.

Note that all the in-tree callers passing `--shallow-file` to a `git`
subprocess always pass a value after it, so they are not affected. In
`upload-pack.c` that value is an empty string, which is still accepted.

Signed-off-by: Christian Couder <christian.couder@gmail.com>
---

While working on modernizing `git fast-import`, I noticed that
`--shallow-file` was handled differently than the other options that
take an argument in "git.c", and found this segfault.

I have started working on a better way to handle such options not only
in "git.c" but also in other files. For now though, I think a small
localized bugfix like this is the simplest solution.

Not sure if "t0041-usage.sh" is the best place for testing this, but I
couldn't find a dedicated one.

CI tests all pass, see:

https://github.com/chriscool/git/actions/runs/31478034826

 git.c            | 10 +++++++---
 t/t0041-usage.sh |  7 +++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/git.c b/git.c
index e5f1811b6b..96df15b5cd 100644
--- a/git.c
+++ b/git.c
@@ -304,11 +304,15 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 			if (envchanged)
 				*envchanged = 1;
 		} else if (!strcmp(cmd, "--shallow-file")) {
-			(*argv)++;
-			(*argc)--;
-			setenv(GIT_SHALLOW_FILE_ENVIRONMENT, (*argv)[0], 1);
+			if (*argc < 2) {
+				fprintf(stderr, _("no file given for '%s' option\n" ), "--shallow-file");
+				usage(git_usage_string);
+			}
+			setenv(GIT_SHALLOW_FILE_ENVIRONMENT, (*argv)[1], 1);
 			if (envchanged)
 				*envchanged = 1;
+			(*argv)++;
+			(*argc)--;
 		} else if (!strcmp(cmd, "-C")) {
 			if (*argc < 2) {
 				fprintf(stderr, _("no directory given for '%s' option\n" ), "-C");
diff --git a/t/t0041-usage.sh b/t/t0041-usage.sh
index 51af7cc030..2a9c5eafca 100755
--- a/t/t0041-usage.sh
+++ b/t/t0041-usage.sh
@@ -107,4 +107,11 @@ test_expect_success 'for-each-ref usage error' '
 	test_grep "usage" actual.err
 '
 
+test_expect_success 'git --shallow-file without a value' '
+	test_must_fail git --shallow-file >actual 2>actual.err &&
+	test_line_count = 0 actual &&
+	test_grep "no file given for " actual.err &&
+	test_grep "usage" actual.err
+'
+
 test_done
-- 
2.55.0.530.gdb3615d990.dirty