Always check the return value of `repo_read_object_file()`

There are a couple of places in Git's source code where the return value
is not checked. As a consequence, they are susceptible to segmentation
faults.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -159,6 +159,9 @@ static void show_list(const char *debug, int counted, int nr,
 		const char *subject_start;
 		int subject_len;
 
+		if (!buf)
+			die(_("unable to read %s"), oid_to_hex(&commit->object.oid));
+
 		fprintf(stderr, "%c%c%c ",
 			(commit_flags & TREESAME) ? ' ' : 'T',
 			(commit_flags & UNINTERESTING) ? 'U' : ' ',

builtin/cat-file.c

@@ -222,6 +222,10 @@ static int cat_one_file(int opt, const char *exp_type, const char *obj_name,
 								     &type,
 								     &size);
 				const char *target;
+
+				if (!buffer)
+					die(_("unable to read %s"), oid_to_hex(&oid));
+
 				if (!skip_prefix(buffer, "object ", &target) ||
 				    get_oid_hex(target, &blob_oid))
 					die("%s not a valid tag", oid_to_hex(&oid));
@@ -417,6 +421,8 @@ static void print_object_or_die(struct batch_options *opt, struct expand_data *d
 
 		contents = repo_read_object_file(the_repository, oid, &type,
 						 &size);
+		if (!contents)
+			die("object %s disappeared", oid_to_hex(oid));
 
 		if (use_mailmap) {
 			size_t s = size;
@@ -424,8 +430,6 @@ static void print_object_or_die(struct batch_options *opt, struct expand_data *d
 			size = cast_size_t_to_ulong(s);
 		}
 
-		if (!contents)
-			die("object %s disappeared", oid_to_hex(oid));
 		if (type != data->type)
 			die("object %s changed type!?", oid_to_hex(oid));
 		if (data->info.sizep && size != data->size && !use_mailmap)
@@ -482,6 +486,8 @@ static void batch_object_write(const char *obj_name,
 
 			buf = repo_read_object_file(the_repository, &data->oid, &data->type,
 						    &data->size);
+			if (!buf)
+				die(_("unable to read %s"), oid_to_hex(&data->oid));
 			buf = replace_idents_using_mailmap(buf, &s);
 			data->size = cast_size_t_to_ulong(s);
 

builtin/grep.c

@@ -575,6 +575,8 @@ static int grep_cache(struct grep_opt *opt,
 
 			data = repo_read_object_file(the_repository, &ce->oid,
 						     &type, &size);
+			if (!data)
+				die(_("unable to read tree %s"), oid_to_hex(&ce->oid));
 			init_tree_desc(&tree, data, size);
 
 			hit |= grep_tree(opt, pathspec, &tree, &name, 0, 0);

builtin/notes.c

@@ -718,9 +718,11 @@ static int append_edit(int argc, const char **argv, const char *prefix)
 		struct strbuf buf = STRBUF_INIT;
 		char *prev_buf = repo_read_object_file(the_repository, note, &type, &size);
 
-		if (prev_buf && size)
+		if (!prev_buf)
+			die(_("unable to read %s"), oid_to_hex(note));
+		if (size)
 			strbuf_add(&buf, prev_buf, size);
-		if (d.buf.len && prev_buf && size)
+		if (d.buf.len && size)
 			append_separator(&buf);
 		strbuf_insert(&d.buf, 0, buf.buf, buf.len);
 

combine-diff.c

@@ -338,6 +338,8 @@ static char *grab_blob(struct repository *r,
 		free_filespec(df);
 	} else {
 		blob = repo_read_object_file(r, oid, &type, size);
+		if (!blob)
+			die(_("unable to read %s"), oid_to_hex(oid));
 		if (type != OBJ_BLOB)
 			die("object '%s' is not a blob!", oid_to_hex(oid));
 	}

rerere.c

@@ -975,6 +975,9 @@ static int handle_cache(struct index_state *istate,
 			mmfile[i].ptr = repo_read_object_file(the_repository,
 							      &ce->oid, &type,
 							      &size);
+			if (!mmfile[i].ptr)
+				die(_("unable to read %s"),
+				    oid_to_hex(&ce->oid));
 			mmfile[i].size = size;
 		}
 	}