zstream: fail when asked to inflate garbage

When we are provided some input buffer (with a length) to inflate,
and it contains more data than simply the deflated data, fail.
zlib will helpfully tell us when it is done reading (via Z_STREAM_END),
so if there is data leftover in the input buffer, fail lest we
continually try to inflate it.

src/zstream.c

@@ -86,6 +86,11 @@ int git_zstream_get_output(void *out, size_t *out_len, git_zstream *zstream)
 	int zflush = Z_FINISH;
 	size_t out_remain = *out_len;
 
+	if (zstream->in_len && zstream->zerr == Z_STREAM_END) {
+		giterr_set(GITERR_ZLIB, "zlib input had trailing garbage");
+		return -1;
+	}
+
 	while (out_remain > 0 && zstream->zerr != Z_STREAM_END) {
 		size_t out_queued, in_queued, out_used, in_used;
 

tests/core/zstream.c

@@ -58,6 +58,25 @@ void test_core_zstream__basic(void)
 	assert_zlib_equal(data, strlen(data) + 1, out, outlen);
 }
 
+void test_core_zstream__fails_on_trailing_garbage(void)
+{
+	git_buf deflated = GIT_BUF_INIT, inflated = GIT_BUF_INIT;
+	size_t i = 0;
+
+	/* compress a simple string */
+	git_zstream_deflatebuf(&deflated, "foobar!!", 8);
+
+	/* append some garbage */
+	for (i = 0; i < 10; i++) {
+		git_buf_putc(&deflated, i);
+	}
+
+	cl_git_fail(git_zstream_inflatebuf(&inflated, deflated.ptr, deflated.size));
+
+	git_buf_free(&deflated);
+	git_buf_free(&inflated);
+}
+
 void test_core_zstream__buffer(void)
 {
 	git_buf out = GIT_BUF_INIT;