Fix validation of empty maps/arrays at end of metadata

The size validation added in c1d353a to prevent DoS via exaggerated
map/array sizes used >= when checking offset against data_section_size.
This rejected valid 0-length containers when they were the last field
in metadata (offset_to_next == data_section_size). Change to > since
the existing second condition (size > remaining) already correctly
handles the boundary case for non-empty containers.

Reported in vimt/MaxMind-DB-Writer-python#16.

ENG-4322

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: oschwald

Changes.md

@@ -1,3 +1,10 @@
+## 1.13.3 - 2026-XX-XX
+
+- Fixed validation of empty maps and arrays at the end of the metadata section.
+  `MMDB_open` would incorrectly reject databases where a 0-element map or array
+  was the last field in metadata. Reported in
+  vimt/MaxMind-DB-Writer-python#16.
+
 ## 1.13.2 - 2026-02-25
 
 - Fixed a compilation failure on macOS 26 (Tahoe) where `sys/endian.h` defines

src/maxminddb.c

@@ -1729,7 +1729,7 @@ static int get_entry_data_list(const MMDB_s *const mmdb,
             uint32_t array_size = entry_data_list->entry_data.data_size;
             uint32_t array_offset = entry_data_list->entry_data.offset_to_next;
             /* Each array element needs at least 1 byte. */
-            if (array_offset >= mmdb->data_section_size ||
+            if (array_offset > mmdb->data_section_size ||
                 array_size > mmdb->data_section_size - array_offset) {
                 DEBUG_MSG("array size exceeds remaining data section");
                 return MMDB_INVALID_DATA_ERROR;
@@ -1758,7 +1758,7 @@ static int get_entry_data_list(const MMDB_s *const mmdb,
 
             offset = entry_data_list->entry_data.offset_to_next;
             /* Each map entry needs at least a key and a value (1 byte each). */
-            if (offset >= mmdb->data_section_size ||
+            if (offset > mmdb->data_section_size ||
                 size > (mmdb->data_section_size - offset) / 2) {
                 DEBUG_MSG("map size exceeds remaining data section");
                 return MMDB_INVALID_DATA_ERROR;

t/CMakeLists.txt

@@ -31,6 +31,7 @@ if(UNIX)  # or if (NOT WIN32)
     bad_data_size_t
     bad_epoch_t
     bad_indent_t
+    empty_container_metadata_t
     max_depth_t
     threads_t
   )

t/Makefile.am

@@ -19,7 +19,8 @@ check_PROGRAMS = \
 	bad_pointers_t bad_databases_t bad_data_size_t bad_epoch_t bad_indent_t \
 	bad_search_tree_t \
 	basic_lookup_t data_entry_list_t \
-	data-pool-t data_types_t double_close_t dump_t gai_error_t get_value_t \
+	data-pool-t data_types_t double_close_t dump_t empty_container_metadata_t \
+	gai_error_t get_value_t \
 	get_value_pointer_bug_t \
 	ipv4_start_cache_t ipv6_lookup_in_ipv4_t max_depth_t metadata_t \
 	metadata_pointers_t no_map_get_value_t overflow_bounds_t read_node_t \

t/empty_container_metadata_t.c

@@ -0,0 +1,58 @@
+#include "maxminddb_test_helper.h"
+
+void test_empty_map_last_in_metadata(void) {
+    char *db_file =
+        bad_database_path("libmaxminddb-empty-map-last-in-metadata.mmdb");
+
+    MMDB_s mmdb;
+    int status = MMDB_open(db_file, MMDB_MODE_MMAP, &mmdb);
+    cmp_ok(status,
+           "==",
+           MMDB_SUCCESS,
+           "opened MMDB with empty map at end of metadata");
+
+    if (status != MMDB_SUCCESS) {
+        diag("MMDB_open failed: %s", MMDB_strerror(status));
+        free(db_file);
+        return;
+    }
+
+    int gai_error, mmdb_error;
+    MMDB_lookup_string(&mmdb, "1.2.3.4", &gai_error, &mmdb_error);
+    cmp_ok(mmdb_error, "==", MMDB_SUCCESS, "lookup succeeded");
+
+    MMDB_close(&mmdb);
+    free(db_file);
+}
+
+void test_empty_array_last_in_metadata(void) {
+    char *db_file =
+        bad_database_path("libmaxminddb-empty-array-last-in-metadata.mmdb");
+
+    MMDB_s mmdb;
+    int status = MMDB_open(db_file, MMDB_MODE_MMAP, &mmdb);
+    cmp_ok(status,
+           "==",
+           MMDB_SUCCESS,
+           "opened MMDB with empty array at end of metadata");
+
+    if (status != MMDB_SUCCESS) {
+        diag("MMDB_open failed: %s", MMDB_strerror(status));
+        free(db_file);
+        return;
+    }
+
+    int gai_error, mmdb_error;
+    MMDB_lookup_string(&mmdb, "1.2.3.4", &gai_error, &mmdb_error);
+    cmp_ok(mmdb_error, "==", MMDB_SUCCESS, "lookup succeeded");
+
+    MMDB_close(&mmdb);
+    free(db_file);
+}
+
+int main(void) {
+    plan(NO_PLAN);
+    test_empty_map_last_in_metadata();
+    test_empty_array_last_in_metadata();
+    done_testing();
+}