branch: fix branch_checked_out() leaks

On 6/8/2022 4:08 PM, Derrick Stolee via GitGitGadget wrote:
> This is a replacement for some patches from v2 of my 'git rebase
> --update-refs' topic [1]. After some feedback from Philip, I've decided to
> pull that topic while I rework how I track the refs to rewrite [2]. This
> series moves forward with the branch_checked_out() helper that was a bit
> more complicated than expected at first glance. This series is a culmination
> of the discussion started by Junio at [3].
>

Junio pointed out that patch 1 introduced a memory leak when a ref
is checked out in multiple places. Here is a patch to fix that
scenario. It applies cleanly on top of patch 4, so I include it as
a new "patch 5". I will include it in any v2 of the full series, if
needed.

Thanks,
-Stolee

---- >8 ----

From c3842b3 Mon Sep 17 00:00:00 2001
From: Derrick Stolee <[email protected]>
Date: Mon, 13 Jun 2022 10:33:20 -0400
Subject: [PATCH 5/5] branch: fix branch_checked_out() leaks

The branch_checked_out() method populates a strmap linking a refname to
a worktree that has that branch checked out. While unlikely, it is
possible that a bug or filesystem manipulation could create a scenario
where the same ref is checked out in multiple places. Further, there are
some states in an interactive rebase where HEAD and REBASE_HEAD point to
the same ref, leading to multiple insertions into the strmap. In either
case, the strmap_put() method returns the old value which is leaked.

Update branch_checked_out() to consume that pointer and free it.

Add a test in t2407 that checks this erroneous case. The test "checks
itself" by first confirming that the filesystem manipulations it makes
trigger the branch_checked_out() logic, and then sets up similar
manipulations to make it look like there are multiple worktrees pointing
to the same ref.

While TEST_PASSES_SANITIZE_LEAK would be helpful to demonstrate the
leakage and prevent it in the future, t2407 uses helpers such as 'git
clone' that cause the test to fail under that mode.

Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

branch.c

@@ -362,25 +362,29 @@ static void prepare_checked_out_branches(void)
 	worktrees = get_worktrees();
 
 	while (worktrees[i]) {
+		char *old;
 		struct wt_status_state state = { 0 };
 		struct worktree *wt = worktrees[i++];
 
 		if (wt->is_bare)
 			continue;
 
-		if (wt->head_ref)
-			strmap_put(&current_checked_out_branches,
-				   wt->head_ref,
-				   xstrdup(wt->path));
+		if (wt->head_ref) {
+			old = strmap_put(&current_checked_out_branches,
+					 wt->head_ref,
+					 xstrdup(wt->path));
+			free(old);
+		}
 
 		if (wt_status_check_rebase(wt, &state) &&
 		    (state.rebase_in_progress || state.rebase_interactive_in_progress) &&
 		    state.branch) {
 			struct strbuf ref = STRBUF_INIT;
 			strbuf_addf(&ref, "refs/heads/%s", state.branch);
-			strmap_put(&current_checked_out_branches,
-				   ref.buf,
-				   xstrdup(wt->path));
+			old = strmap_put(&current_checked_out_branches,
+					 ref.buf,
+					 xstrdup(wt->path));
+			free(old);
 			strbuf_release(&ref);
 		}
 		wt_status_state_free_buffers(&state);
@@ -389,9 +393,10 @@ static void prepare_checked_out_branches(void)
 		    state.branch) {
 			struct strbuf ref = STRBUF_INIT;
 			strbuf_addf(&ref, "refs/heads/%s", state.branch);
-			strmap_put(&current_checked_out_branches,
-				   ref.buf,
-				   xstrdup(wt->path));
+			old = strmap_put(&current_checked_out_branches,
+					 ref.buf,
+					 xstrdup(wt->path));
+			free(old);
 			strbuf_release(&ref);
 		}
 		wt_status_state_free_buffers(&state);

t/t2407-worktree-heads.sh

@@ -78,4 +78,43 @@ test_expect_success 'refuse to overwrite: worktree in rebase' '
 	grep "refusing to fetch into branch '\''refs/heads/wt-4'\''" err
 '
 
+test_expect_success 'refuse to overwrite when in error states' '
+	test_when_finished rm -rf .git/worktrees/wt-*/rebase-merge &&
+	test_when_finished rm -rf .git/worktrees/wt-*/BISECT_* &&
+
+	git branch -f fake1 &&
+	mkdir -p .git/worktrees/wt-3/rebase-merge &&
+	touch .git/worktrees/wt-3/rebase-merge/interactive &&
+	echo refs/heads/fake1 >.git/worktrees/wt-3/rebase-merge/head-name &&
+	echo refs/heads/fake2 >.git/worktrees/wt-3/rebase-merge/onto &&
+
+	git branch -f fake2 &&
+	touch .git/worktrees/wt-4/BISECT_LOG &&
+	echo refs/heads/fake2 >.git/worktrees/wt-4/BISECT_START &&
+
+	# First, ensure we prevent writing when only one reason to fail.
+	for i in 1 2
+	do
+		test_must_fail git branch -f fake$i HEAD 2>err &&
+		grep "cannot force update the branch '\''fake$i'\'' checked out at" err ||
+			return 1
+	done &&
+
+	# Second, set up duplicate values.
+	mkdir -p .git/worktrees/wt-4/rebase-merge &&
+	touch .git/worktrees/wt-4/rebase-merge/interactive &&
+	echo refs/heads/fake2 >.git/worktrees/wt-4/rebase-merge/head-name &&
+	echo refs/heads/fake1 >.git/worktrees/wt-4/rebase-merge/onto &&
+
+	touch .git/worktrees/wt-1/BISECT_LOG &&
+	echo refs/heads/fake1 >.git/worktrees/wt-1/BISECT_START &&
+
+	for i in 1 2
+	do
+		test_must_fail git branch -f fake$i HEAD 2>err &&
+		grep "cannot force update the branch '\''fake$i'\'' checked out at" err ||
+			return 1
+	done
+'
+
 test_done