I found a situation where 'got rebase' will proceed even though the
work tree is out-of-date. This happens because a seqence of youngest
common-ancestor and same-branch checks will trivially succeed, leading
'got rebase' to assume that this rebase is a fast-forward. However,
it only looks like a fast-forward because we miss the fact that the
work-tree's base commit is out of date, ignoring newer commits on the
work-tree's branch.

 This situation is not covered by the existing rebase_out_of_date test.
To reproduce the problem, run the new test added with this patch, with
the got/got.c part of this patch backed out:

test_rebase_out_of_date2 rebase succeeded unexpectedly
test failed; leaving test data in /tmp/got-test-rebase_out_of_date2-N3qmgXLx
$ cat /tmp/got-test-rebase_out_of_date2-N3qmgXLx/stdout
refs/heads/newbranch is already based on refs/heads/master
Switching work tree from refs/heads/master to refs/heads/newbranch
U  epsilon/zeta
U  gamma/delta
Updated to refs/heads/newbranch: f5fd50fc48b880a976504bb432c60f1372c5dc9c
$

ok?
 
 fix 'got rebase' not detecting an out-of-date work tree in some cases
 
diff 1a99e0b4097b26cac736de07239a3be7589a48f7 cb22360ec8580f5213a60a01295a4121ac013556
commit - 1a99e0b4097b26cac736de07239a3be7589a48f7
commit + cb22360ec8580f5213a60a01295a4121ac013556
blob - 4e1c7e548dd68228986b5949d2af99e7e032a6ba
blob + 9a89d5b62f1635fb4b385534abdd217ee4a06611
--- got/got.c
+++ got/got.c
@@ -10154,6 +10154,8 @@ cmd_rebase(int argc, char *argv[])
 	struct got_object_id *commit_id = NULL, *parent_id = NULL;
 	struct got_object_id *resume_commit_id = NULL;
 	struct got_object_id *branch_head_commit_id = NULL, *yca_id = NULL;
+	struct got_object_id *head_commit_id = NULL;
+	struct got_reference *head_ref = NULL;
 	struct got_commit_object *commit = NULL;
 	int ch, rebase_in_progress = 0, abort_rebase = 0, continue_rebase = 0;
 	int histedit_in_progress = 0, merge_in_progress = 0;
@@ -10356,7 +10358,19 @@ cmd_rebase(int argc, char *argv[])
 	if (!continue_rebase) {
 		struct got_object_id *base_commit_id;
 
+		error = got_ref_open(&head_ref, repo,
+		    got_worktree_get_head_ref_name(worktree), 0);
+		if (error)
+			goto done;
+		error = got_ref_resolve(&head_commit_id, repo, head_ref);
+		if (error)
+			goto done;
 		base_commit_id = got_worktree_get_base_commit_id(worktree);
+		if (got_object_id_cmp(base_commit_id, head_commit_id) != 0) {
+			error = got_error(GOT_ERR_REBASE_OUT_OF_DATE);
+			goto done;
+		}
+
 		error = got_commit_graph_find_youngest_common_ancestor(&yca_id,
 		    base_commit_id, branch_head_commit_id, 1, repo,
 		    check_cancelled, NULL);
@@ -10537,6 +10551,7 @@ done:
 	got_object_id_queue_free(&commits);
 	free(branch_head_commit_id);
 	free(resume_commit_id);
+	free(head_commit_id);
 	free(yca_id);
 	if (commit)
 		got_object_commit_close(commit);
@@ -10546,6 +10561,8 @@ done:
 		got_ref_close(new_base_branch);
 	if (tmp_branch)
 		got_ref_close(tmp_branch);
+	if (head_ref)
+		got_ref_close(head_ref);
 	if (worktree)
 		got_worktree_close(worktree);
 	if (repo) {
blob - f809879812d293f5b44a4ad0e1a299f1384dc027
blob + f87f1d9e51fc63c6ebf7cc957f15ec52ddd15420
--- regress/cmdline/rebase.sh
+++ regress/cmdline/rebase.sh
@@ -1834,6 +1834,57 @@ test_parseargs "$@"
 	test_done "$testroot" 0
 }
 
+test_rebase_out_of_date2() {
+	local testroot=`test_init rebase_out_of_date2`
+	local commit0=`git_show_head $testroot/repo`
+	local commit0_author_time=`git_show_author_time $testroot/repo`
+
+	(cd $testroot/repo && git checkout -q -b newbranch)
+	echo "modified delta on branch" > $testroot/repo/gamma/delta
+	git_commit $testroot/repo -m "committing to delta on newbranch"
+
+	local orig_commit1=`git_show_parent_commit $testroot/repo`
+	local orig_commit2=`git_show_head $testroot/repo`
+	local orig_author_time2=`git_show_author_time $testroot/repo`
+
+	(cd $testroot/repo && git checkout -q master)
+	echo "modified zeta on master" > $testroot/repo/epsilon/zeta
+	git_commit $testroot/repo -m "committing to zeta on master"
+	local master_commit=`git_show_head $testroot/repo`
+
+	got checkout $testroot/repo $testroot/wt > /dev/null
+	ret=$?
+	if [ $ret -ne 0 ]; then
+		test_done "$testroot" "$ret"
+		return 1
+	fi
+
+	# Backdate the file alpha to an earlier version.
+	# This sets the work tree's base commit ID back to $commit0,
+	# which is out-of-date with respect to the master branch.
+	(cd $testroot/wt && got update -c $commit0 alpha > /dev/null)
+
+	# Rebase into an out-of-date work tree should be refused.
+	(cd $testroot/wt && got rebase newbranch > $testroot/stdout \
+		2> $testroot/stderr)
+	ret=$?
+	if [ $ret -eq 0 ]; then
+		echo "rebase succeeded unexpectedly" >&2
+		test_done "$testroot" "1"
+		return 1
+	fi
+	echo -n > $testroot/stdout.expected
+	echo -n "got: work tree must be updated before it can be used to " \
+		> $testroot/stderr.expected
+	echo "rebase a branch" >> $testroot/stderr.expected
+	cmp -s $testroot/stderr.expected $testroot/stderr
+	ret=$?
+	if [ $ret -ne 0 ]; then
+		diff -u $testroot/stderr.expected $testroot/stderr
+	fi
+	test_done "$testroot" "$ret"
+}
+
 test_parseargs "$@"
 run_test test_rebase_basic
 run_test test_rebase_ancestry_check
@@ -1854,3 +1905,4 @@ run_test test_rebase_umask
 run_test test_rebase_no_author_info
 run_test test_rebase_nonbranch
 run_test test_rebase_umask
+run_test test_rebase_out_of_date2