write-tree: properly detect failure to write tree objects

Tomasz Fortuna reported that "git commit" does not error out properly when
it cannot write tree objects out.  "git write-tree" shares the same issue,
as the failure to notice the error is deep in the logic to write tree
objects out recursively.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -341,8 +341,11 @@ static int update_one(struct cache_tree *it,
 
 	if (dryrun)
 		hash_sha1_file(buffer.buf, buffer.len, tree_type, it->sha1);
-	else
-		write_sha1_file(buffer.buf, buffer.len, tree_type, it->sha1);
+	else if (write_sha1_file(buffer.buf, buffer.len, tree_type, it->sha1)) {
+		strbuf_release(&buffer);
+		return -1;
+	}
+
 	strbuf_release(&buffer);
 	it->entry_count = i;
 #if DEBUG

t/t0004-unwritable.sh

@@ -0,0 +1,67 @@
+#!/bin/sh
+
+test_description='detect unwritable repository and fail correctly'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+
+	>file &&
+	git add file &&
+	git commit -m initial &&
+	echo >file &&
+	git add file
+
+'
+
+test_expect_success 'write-tree should notice unwritable repository' '
+
+	(
+		chmod a-w .git/objects
+		test_must_fail git write-tree
+	)
+	status=$?
+	chmod 775 .git/objects
+	(exit $status)
+
+'
+
+test_expect_success 'commit should notice unwritable repository' '
+
+	(
+		chmod a-w .git/objects
+		test_must_fail git commit -m second
+	)
+	status=$?
+	chmod 775 .git/objects
+	(exit $status)
+
+'
+
+test_expect_success 'update-index should notice unwritable repository' '
+
+	(
+		echo a >file &&
+		chmod a-w .git/objects
+		test_must_fail git update-index file
+	)
+	status=$?
+	chmod 775 .git/objects
+	(exit $status)
+
+'
+
+test_expect_success 'add should notice unwritable repository' '
+
+	(
+		echo b >file &&
+		chmod a-w .git/objects
+		test_must_fail git add file
+	)
+	status=$?
+	chmod 775 .git/objects
+	(exit $status)
+
+'
+
+test_done