setup: opt-out of check with safe.directory=*

With the addition of the safe.directory in 8959555ce
(setup_git_directory(): add an owner check for the top-level directory,
2022-03-02) released in v2.35.2, we are receiving feedback from a
variety of users about the feature.

Some users have a very large list of shared repositories and find it
cumbersome to add this config for every one of them.

In a more difficult case, certain workflows involve running Git commands
within containers. The container boundary prevents any global or system
config from communicating `safe.directory` values from the host into the
container. Further, the container almost always runs as a different user
than the owner of the directory in the host.

To simplify the reactions necessary for these users, extend the
definition of the safe.directory config value to include a possible '*'
value. This value implies that all directories are safe, providing a
single setting to opt-out of this protection.

Note that an empty assignment of safe.directory clears all previous
values, and this is already the case with the "if (!value || !*value)"
condition.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

Documentation/config/safe.txt

@@ -25,3 +25,10 @@ Unix' simpler permission model, it can be a bit tricky to figure out why
 a directory is considered unsafe. To help with this, Git will provide
 more detailed information when the environment variable
 `GIT_TEST_DEBUG_UNSAFE_DIRECTORIES` is set to `true`.
++
+To completely opt-out of this security check, set `safe.directory` to the
+string `*`. This will allow all repositories to be treated as if their
+directory was listed in the `safe.directory` list. If `safe.directory=*`
+is set in system config and you want to re-enable this protection, then
+initialize your list with an empty value before listing the repositories
+that you deem safe.

setup.c

@@ -1103,9 +1103,11 @@ static int safe_directory_cb(const char *key, const char *value, void *d)
 	if (strcmp(key, "safe.directory"))
 		return 0;
 
-	if (!value || !*value)
+	if (!value || !*value) {
 		data->is_safe = 0;
-	else {
+	} else if (!strcmp(value, "*")) {
+		data->is_safe = 1;
+	} else {
 		const char *interpolated = NULL;
 
 		if (!git_config_pathname(&interpolated, key, value) &&

t/t0033-safe-directory.sh

@@ -36,4 +36,14 @@ test_expect_success 'safe.directory matches, but is reset' '
 	expect_rejected_dir
 '
 
+test_expect_success 'safe.directory=*' '
+	git config --global --add safe.directory "*" &&
+	git status
+'
+
+test_expect_success 'safe.directory=*, but is reset' '
+	git config --global --add safe.directory "" &&
+	expect_rejected_dir
+'
+
 test_done