reftable: avoid leaks on realloc error

When realloc(3) fails, it returns NULL and keeps the original allocation
intact.  REFTABLE_ALLOC_GROW overwrites both the original pointer and
the allocation count variable in that case, simultaneously leaking the
original allocation and misrepresenting the number of storable items.

parse_names() and reftable_buf_add() avoid leaking by restoring the
original pointer value on failure, but all other callers seem to be OK
with losing the old allocation.  Add a new variant of the macro,
REFTABLE_ALLOC_GROW_OR_NULL, which plugs the leak and zeros the
allocation counter.  Use it for those callers.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/basics.h

@@ -129,6 +129,16 @@ char *reftable_strdup(const char *str);
 			REFTABLE_REALLOC_ARRAY(x, alloc); \
 		} \
 	} while (0)
+
+#define REFTABLE_ALLOC_GROW_OR_NULL(x, nr, alloc) do { \
+	void *reftable_alloc_grow_or_null_orig_ptr = (x); \
+	REFTABLE_ALLOC_GROW((x), (nr), (alloc)); \
+	if (!(x)) { \
+		reftable_free(reftable_alloc_grow_or_null_orig_ptr); \
+		alloc = 0; \
+	} \
+} while (0)
+
 #define REFTABLE_FREE_AND_NULL(p) do { reftable_free(p); (p) = NULL; } while (0)
 
 #ifndef REFTABLE_ALLOW_BANNED_ALLOCATORS

reftable/block.c

@@ -53,7 +53,8 @@ static int block_writer_register_restart(struct block_writer *w, int n,
 	if (2 + 3 * rlen + n > w->block_size - w->next)
 		return -1;
 	if (is_restart) {
-		REFTABLE_ALLOC_GROW(w->restarts, w->restart_len + 1, w->restart_cap);
+		REFTABLE_ALLOC_GROW_OR_NULL(w->restarts, w->restart_len + 1,
+					    w->restart_cap);
 		if (!w->restarts)
 			return REFTABLE_OUT_OF_MEMORY_ERROR;
 		w->restarts[w->restart_len++] = w->next;
@@ -176,7 +177,8 @@ int block_writer_finish(struct block_writer *w)
 		 * is guaranteed to return `Z_STREAM_END`.
 		 */
 		compressed_len = deflateBound(w->zstream, src_len);
-		REFTABLE_ALLOC_GROW(w->compressed, compressed_len, w->compressed_cap);
+		REFTABLE_ALLOC_GROW_OR_NULL(w->compressed, compressed_len,
+					    w->compressed_cap);
 		if (!w->compressed) {
 			ret = REFTABLE_OUT_OF_MEMORY_ERROR;
 			return ret;
@@ -235,8 +237,8 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
 		uLong src_len = block->len - block_header_skip;
 
 		/* Log blocks specify the *uncompressed* size in their header. */
-		REFTABLE_ALLOC_GROW(br->uncompressed_data, sz,
-				    br->uncompressed_cap);
+		REFTABLE_ALLOC_GROW_OR_NULL(br->uncompressed_data, sz,
+					    br->uncompressed_cap);
 		if (!br->uncompressed_data) {
 			err = REFTABLE_OUT_OF_MEMORY_ERROR;
 			goto done;

reftable/pq.c

@@ -49,7 +49,7 @@ int merged_iter_pqueue_add(struct merged_iter_pqueue *pq, const struct pq_entry
 {
 	size_t i = 0;
 
-	REFTABLE_ALLOC_GROW(pq->heap, pq->len + 1, pq->cap);
+	REFTABLE_ALLOC_GROW_OR_NULL(pq->heap, pq->len + 1, pq->cap);
 	if (!pq->heap)
 		return REFTABLE_OUT_OF_MEMORY_ERROR;
 	pq->heap[pq->len++] = *e;

reftable/record.c

@@ -246,8 +246,8 @@ static int reftable_ref_record_copy_from(void *rec, const void *src_rec,
 	if (src->refname) {
 		size_t refname_len = strlen(src->refname);
 
-		REFTABLE_ALLOC_GROW(ref->refname, refname_len + 1,
-				    ref->refname_cap);
+		REFTABLE_ALLOC_GROW_OR_NULL(ref->refname, refname_len + 1,
+					    ref->refname_cap);
 		if (!ref->refname) {
 			err = REFTABLE_OUT_OF_MEMORY_ERROR;
 			goto out;
@@ -385,7 +385,7 @@ static int reftable_ref_record_decode(void *rec, struct reftable_buf key,
 	SWAP(r->refname, refname);
 	SWAP(r->refname_cap, refname_cap);
 
-	REFTABLE_ALLOC_GROW(r->refname, key.len + 1, r->refname_cap);
+	REFTABLE_ALLOC_GROW_OR_NULL(r->refname, key.len + 1, r->refname_cap);
 	if (!r->refname) {
 		err = REFTABLE_OUT_OF_MEMORY_ERROR;
 		goto done;
@@ -839,7 +839,7 @@ static int reftable_log_record_decode(void *rec, struct reftable_buf key,
 	if (key.len <= 9 || key.buf[key.len - 9] != 0)
 		return REFTABLE_FORMAT_ERROR;
 
-	REFTABLE_ALLOC_GROW(r->refname, key.len - 8, r->refname_cap);
+	REFTABLE_ALLOC_GROW_OR_NULL(r->refname, key.len - 8, r->refname_cap);
 	if (!r->refname) {
 		err = REFTABLE_OUT_OF_MEMORY_ERROR;
 		goto done;
@@ -947,8 +947,8 @@ static int reftable_log_record_decode(void *rec, struct reftable_buf key,
 	}
 	string_view_consume(&in, n);
 
-	REFTABLE_ALLOC_GROW(r->value.update.message, scratch->len + 1,
-			    r->value.update.message_cap);
+	REFTABLE_ALLOC_GROW_OR_NULL(r->value.update.message, scratch->len + 1,
+				    r->value.update.message_cap);
 	if (!r->value.update.message) {
 		err = REFTABLE_OUT_OF_MEMORY_ERROR;
 		goto done;

reftable/stack.c

@@ -317,7 +317,9 @@ static int reftable_stack_reload_once(struct reftable_stack *st,
 				 * thus need to keep them alive here, which we
 				 * do by bumping their refcount.
 				 */
-				REFTABLE_ALLOC_GROW(reused, reused_len + 1, reused_alloc);
+				REFTABLE_ALLOC_GROW_OR_NULL(reused,
+							    reused_len + 1,
+							    reused_alloc);
 				if (!reused) {
 					err = REFTABLE_OUT_OF_MEMORY_ERROR;
 					goto done;
@@ -949,8 +951,8 @@ int reftable_addition_add(struct reftable_addition *add,
 	if (err < 0)
 		goto done;
 
-	REFTABLE_ALLOC_GROW(add->new_tables, add->new_tables_len + 1,
-			    add->new_tables_cap);
+	REFTABLE_ALLOC_GROW_OR_NULL(add->new_tables, add->new_tables_len + 1,
+				    add->new_tables_cap);
 	if (!add->new_tables) {
 		err = REFTABLE_OUT_OF_MEMORY_ERROR;
 		goto done;

reftable/writer.c

@@ -254,7 +254,8 @@ static int writer_index_hash(struct reftable_writer *w, struct reftable_buf *has
 	if (key->offset_len > 0 && key->offsets[key->offset_len - 1] == off)
 		return 0;
 
-	REFTABLE_ALLOC_GROW(key->offsets, key->offset_len + 1, key->offset_cap);
+	REFTABLE_ALLOC_GROW_OR_NULL(key->offsets, key->offset_len + 1,
+				    key->offset_cap);
 	if (!key->offsets)
 		return REFTABLE_OUT_OF_MEMORY_ERROR;
 	key->offsets[key->offset_len++] = off;
@@ -820,7 +821,7 @@ static int writer_flush_nonempty_block(struct reftable_writer *w)
 	 * Note that this also applies when flushing index blocks, in which
 	 * case we will end up with a multi-level index.
 	 */
-	REFTABLE_ALLOC_GROW(w->index, w->index_len + 1, w->index_cap);
+	REFTABLE_ALLOC_GROW_OR_NULL(w->index, w->index_len + 1, w->index_cap);
 	if (!w->index)
 		return REFTABLE_OUT_OF_MEMORY_ERROR;
 

t/unit-tests/t-reftable-basics.c

@@ -20,6 +20,11 @@ static int integer_needle_lesseq(size_t i, void *_args)
 	return args->needle <= args->haystack[i];
 }
 
+static void *realloc_stub(void *p UNUSED, size_t size UNUSED)
+{
+	return NULL;
+}
+
 int cmd_main(int argc UNUSED, const char *argv[] UNUSED)
 {
 	if_test ("binary search with binsearch works") {
@@ -141,5 +146,30 @@ int cmd_main(int argc UNUSED, const char *argv[] UNUSED)
 		check_int(in, ==, out);
 	}
 
+	if_test ("REFTABLE_ALLOC_GROW_OR_NULL works") {
+		int *arr = NULL;
+		size_t alloc = 0, old_alloc;
+
+		REFTABLE_ALLOC_GROW_OR_NULL(arr, 1, alloc);
+		check(arr != NULL);
+		check_uint(alloc, >=, 1);
+		arr[0] = 42;
+
+		old_alloc = alloc;
+		REFTABLE_ALLOC_GROW_OR_NULL(arr, old_alloc + 1, alloc);
+		check(arr != NULL);
+		check_uint(alloc, >, old_alloc);
+		arr[alloc - 1] = 42;
+
+		old_alloc = alloc;
+		reftable_set_alloc(malloc, realloc_stub, free);
+		REFTABLE_ALLOC_GROW_OR_NULL(arr, old_alloc + 1, alloc);
+		check(arr == NULL);
+		check_uint(alloc, ==, 0);
+		reftable_set_alloc(malloc, realloc, free);
+
+		reftable_free(arr);
+	}
+
 	return test_done();
 }