merge-ort: fix segmentation fault in read-only repositories

If the blob/tree objects cannot be written, we really need the merge
operations to fail, and not to continue (and then try to access the tree
object which is however still set to `NULL`).

Let's stop ignoring the return value of `write_object_file()` and
`write_tree()` and set `clean = -1` in the error case.

Reviewed-by: Elijah Newren <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

merge-ort.c

@@ -3571,15 +3571,15 @@ static int tree_entry_order(const void *a_, const void *b_)
 				 b->string, strlen(b->string), bmi->result.mode);
 }
 
-static void write_tree(struct object_id *result_oid,
-		       struct string_list *versions,
-		       unsigned int offset,
-		       size_t hash_size)
+static int write_tree(struct object_id *result_oid,
+		      struct string_list *versions,
+		      unsigned int offset,
+		      size_t hash_size)
 {
 	size_t maxlen = 0, extra;
 	unsigned int nr;
 	struct strbuf buf = STRBUF_INIT;
-	int i;
+	int i, ret = 0;
 
 	assert(offset <= versions->nr);
 	nr = versions->nr - offset;
@@ -3605,8 +3605,10 @@ static void write_tree(struct object_id *result_oid,
 	}
 
 	/* Write this object file out, and record in result_oid */
-	write_object_file(buf.buf, buf.len, OBJ_TREE, result_oid);
+	if (write_object_file(buf.buf, buf.len, OBJ_TREE, result_oid))
+		ret = -1;
 	strbuf_release(&buf);
+	return ret;
 }
 
 static void record_entry_for_tree(struct directory_versions *dir_metadata,
@@ -3625,13 +3627,13 @@ static void record_entry_for_tree(struct directory_versions *dir_metadata,
 			   basename)->util = &mi->result;
 }
 
-static void write_completed_directory(struct merge_options *opt,
-				      const char *new_directory_name,
-				      struct directory_versions *info)
+static int write_completed_directory(struct merge_options *opt,
+				     const char *new_directory_name,
+				     struct directory_versions *info)
 {
 	const char *prev_dir;
 	struct merged_info *dir_info = NULL;
-	unsigned int offset;
+	unsigned int offset, ret = 0;
 
 	/*
 	 * Some explanation of info->versions and info->offsets...
@@ -3717,7 +3719,7 @@ static void write_completed_directory(struct merge_options *opt,
 	 * strcmp here.)
 	 */
 	if (new_directory_name == info->last_directory)
-		return;
+		return 0;
 
 	/*
 	 * If we are just starting (last_directory is NULL), or last_directory
@@ -3739,7 +3741,7 @@ static void write_completed_directory(struct merge_options *opt,
 		 */
 		string_list_append(&info->offsets,
 				   info->last_directory)->util = (void*)offset;
-		return;
+		return 0;
 	}
 
 	/*
@@ -3769,8 +3771,9 @@ static void write_completed_directory(struct merge_options *opt,
 		 */
 		dir_info->is_null = 0;
 		dir_info->result.mode = S_IFDIR;
-		write_tree(&dir_info->result.oid, &info->versions, offset,
-			   opt->repo->hash_algo->rawsz);
+		if (write_tree(&dir_info->result.oid, &info->versions, offset,
+			       opt->repo->hash_algo->rawsz) < 0)
+			ret = -1;
 	}
 
 	/*
@@ -3798,6 +3801,8 @@ static void write_completed_directory(struct merge_options *opt,
 	/* And, of course, we need to update last_directory to match. */
 	info->last_directory = new_directory_name;
 	info->last_directory_len = strlen(info->last_directory);
+
+	return ret;
 }
 
 /* Per entry merge function */
@@ -4214,8 +4219,8 @@ static void prefetch_for_content_merges(struct merge_options *opt,
 	oid_array_clear(&to_fetch);
 }
 
-static void process_entries(struct merge_options *opt,
-			    struct object_id *result_oid)
+static int process_entries(struct merge_options *opt,
+			   struct object_id *result_oid)
 {
 	struct hashmap_iter iter;
 	struct strmap_entry *e;
@@ -4224,11 +4229,12 @@ static void process_entries(struct merge_options *opt,
 	struct directory_versions dir_metadata = { STRING_LIST_INIT_NODUP,
 						   STRING_LIST_INIT_NODUP,
 						   NULL, 0 };
+	int ret = 0;
 
 	trace2_region_enter("merge", "process_entries setup", opt->repo);
 	if (strmap_empty(&opt->priv->paths)) {
 		oidcpy(result_oid, opt->repo->hash_algo->empty_tree);
-		return;
+		return 0;
 	}
 
 	/* Hack to pre-allocate plist to the desired size */
@@ -4270,8 +4276,11 @@ static void process_entries(struct merge_options *opt,
 		 */
 		struct merged_info *mi = entry->util;
 
-		write_completed_directory(opt, mi->directory_name,
-					  &dir_metadata);
+		if (write_completed_directory(opt, mi->directory_name,
+					      &dir_metadata) < 0) {
+			ret = -1;
+			goto cleanup;
+		}
 		if (mi->clean)
 			record_entry_for_tree(&dir_metadata, path, mi);
 		else {
@@ -4291,12 +4300,16 @@ static void process_entries(struct merge_options *opt,
 		fflush(stdout);
 		BUG("dir_metadata accounting completely off; shouldn't happen");
 	}
-	write_tree(result_oid, &dir_metadata.versions, 0,
-		   opt->repo->hash_algo->rawsz);
+	if (write_tree(result_oid, &dir_metadata.versions, 0,
+		       opt->repo->hash_algo->rawsz) < 0)
+		ret = -1;
+cleanup:
 	string_list_clear(&plist, 0);
 	string_list_clear(&dir_metadata.versions, 0);
 	string_list_clear(&dir_metadata.offsets, 0);
 	trace2_region_leave("merge", "process_entries cleanup", opt->repo);
+
+	return ret;
 }
 
 /*** Function Grouping: functions related to merge_switch_to_result() ***/
@@ -4928,15 +4941,18 @@ static void merge_ort_nonrecursive_internal(struct merge_options *opt,
 	}
 
 	trace2_region_enter("merge", "process_entries", opt->repo);
-	process_entries(opt, &working_tree_oid);
+	if (process_entries(opt, &working_tree_oid) < 0)
+		result->clean = -1;
 	trace2_region_leave("merge", "process_entries", opt->repo);
 
 	/* Set return values */
 	result->path_messages = &opt->priv->conflicts;
 
-	result->tree = parse_tree_indirect(&working_tree_oid);
-	/* existence of conflicted entries implies unclean */
-	result->clean &= strmap_empty(&opt->priv->conflicted);
+	if (result->clean >= 0) {
+		result->tree = parse_tree_indirect(&working_tree_oid);
+		/* existence of conflicted entries implies unclean */
+		result->clean &= strmap_empty(&opt->priv->conflicted);
+	}
 	if (!opt->priv->call_depth) {
 		result->priv = opt->priv;
 		result->_properly_initialized = RESULT_INITIALIZED;

t/t4301-merge-tree-write-tree.sh

@@ -810,4 +810,13 @@ test_expect_success 'can override merge of unrelated histories' '
 	test_cmp expect actual
 '
 
+test_expect_success SANITY 'merge-ort fails gracefully in a read-only repository' '
+	git init --bare read-only &&
+	git push read-only side1 side2 side3 &&
+	test_when_finished "chmod -R u+w read-only" &&
+	chmod -R a-w read-only &&
+	test_must_fail git -C read-only merge-tree side1 side3 &&
+	test_must_fail git -C read-only merge-tree side1 side2
+'
+
 test_done