setup: trace bare repository setups

safe.bareRepository=explicit is a safer default mode of operation, since
it guards against the embedded bare repository attack [1]. Most end
users don't use bare repositories directly, so they should be able to
set safe.bareRepository=explicit, with the expectation that they can
reenable bare repositories by specifying GIT_DIR or --git-dir.

However, the user might use a tool that invokes Git on bare repositories
without setting GIT_DIR (e.g. "go mod" will clone bare repositories
[2]), so even if a user wanted to use safe.bareRepository=explicit, it
wouldn't be feasible until their tools learned to set GIT_DIR.

To make this transition easier, add a trace message to note when we
attempt to set up a bare repository without setting GIT_DIR. This allows
users and tool developers to audit which of their tools are problematic
and report/fix the issue.  When they are sufficiently confident, they
would switch over to "safe.bareRepository=explicit".

Note that this uses trace2_data_string(), which isn't supported by the
"normal" GIT_TRACE2 target, only _EVENT or _PERF.

[1] https://lore.kernel.org/git/kl6lsfqpygsj.fsf@chooglen-macbookpro.roam.corp.google.com/
[2] https://go.dev/ref/mod

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -1352,6 +1352,7 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		}
 
 		if (is_git_directory(dir->buf)) {
+			trace2_data_string("setup", NULL, "implicit-bare-repository", dir->buf);
 			if (get_allowed_bare_repo() == ALLOWED_BARE_REPO_EXPLICIT)
 				return GIT_DIR_DISALLOWED_BARE;
 			if (!ensure_valid_ownership(NULL, NULL, dir->buf, report))

t/t0035-safe-bare-repository.sh

@@ -7,13 +7,26 @@ TEST_PASSES_SANITIZE_LEAK=true
 
 pwd="$(pwd)"
 
-expect_accepted () {
-	git "$@" rev-parse --git-dir
+expect_accepted_implicit () {
+	test_when_finished 'rm "$pwd/trace.perf"' &&
+	GIT_TRACE2_PERF="$pwd/trace.perf" git "$@" rev-parse --git-dir &&
+	# Note: we're intentionally only checking that the bare repo has a
+	# directory *prefix* of $pwd
+	grep -F "implicit-bare-repository:$pwd" "$pwd/trace.perf"
+}
+
+expect_accepted_explicit () {
+	test_when_finished 'rm "$pwd/trace.perf"' &&
+	GIT_DIR="$1" GIT_TRACE2_PERF="$pwd/trace.perf" git rev-parse --git-dir &&
+	! grep -F "implicit-bare-repository:$pwd" "$pwd/trace.perf"
 }
 
 expect_rejected () {
-	test_must_fail git "$@" rev-parse --git-dir 2>err &&
-	grep -F "cannot use bare repository" err
+	test_when_finished 'rm "$pwd/trace.perf"' &&
+	test_env GIT_TRACE2_PERF="$pwd/trace.perf" \
+		test_must_fail git "$@" rev-parse --git-dir 2>err &&
+	grep -F "cannot use bare repository" err &&
+	grep -F "implicit-bare-repository:$pwd" "$pwd/trace.perf"
 }
 
 test_expect_success 'setup bare repo in worktree' '
@@ -22,12 +35,13 @@ test_expect_success 'setup bare repo in worktree' '
 '
 
 test_expect_success 'safe.bareRepository unset' '
-	expect_accepted -C outer-repo/bare-repo
+	test_unconfig --global safe.bareRepository &&
+	expect_accepted_implicit -C outer-repo/bare-repo
 '
 
 test_expect_success 'safe.bareRepository=all' '
 	test_config_global safe.bareRepository all &&
-	expect_accepted -C outer-repo/bare-repo
+	expect_accepted_implicit -C outer-repo/bare-repo
 '
 
 test_expect_success 'safe.bareRepository=explicit' '
@@ -47,7 +61,7 @@ test_expect_success 'safe.bareRepository in the repository' '
 
 test_expect_success 'safe.bareRepository on the command line' '
 	test_config_global safe.bareRepository explicit &&
-	expect_accepted -C outer-repo/bare-repo \
+	expect_accepted_implicit -C outer-repo/bare-repo \
 		-c safe.bareRepository=all
 '
 
@@ -60,4 +74,8 @@ test_expect_success 'safe.bareRepository in included file' '
 	expect_rejected -C outer-repo/bare-repo
 '
 
+test_expect_success 'no trace when GIT_DIR is explicitly provided' '
+	expect_accepted_explicit "$pwd/outer-repo/bare-repo"
+'
+
 test_done