odb_loose: reject objects that cannot fit in memory

Check the size of objects being read from the loose odb backend and
reject those that would not fit in memory with an error message that
reflects the actual problem, instead of error'ing later with an
unintuitive error message regarding truncation or invalid hashes.

src/odb_loose.c

@@ -156,6 +156,11 @@ static int parse_header(
 		size < 0)
 		goto on_error;
 
+	if ((uint64_t)size > SIZE_MAX) {
+		giterr_set(GITERR_OBJECT, "object is larger than available memory");
+		return -1;
+	}
+
 	out->size = size;
 
 	if (GIT_ADD_SIZET_OVERFLOW(out_len, i, 1))

tests/odb/largefiles.c

@@ -93,3 +93,24 @@ void test_odb_largefiles__read_into_memory(void)
 
 	git_odb_object_free(obj);
 }
+
+void test_odb_largefiles__read_into_memory_rejected_on_32bit(void)
+{
+	git_oid oid;
+	git_odb_object *obj = NULL;
+
+#ifdef GIT_ARCH_64
+	cl_skip();
+#endif
+
+	if (!cl_is_env_set("GITTEST_INVASIVE_FS_SIZE"))
+		cl_skip();
+
+	if (!cl_is_env_set("GITTEST_INVASIVE_MEMORY"))
+		cl_skip();
+
+	writefile(&oid);
+	cl_git_fail(git_odb_read(&obj, odb, &oid));
+
+	git_odb_object_free(obj);
+}