zlib: wrap inflateInit2 used to accept only for gzip format

http-backend.c uses inflateInit2() to tell the library that it wants to
accept only gzip format. Wrap it in a helper function so that readers do
not have to wonder what the magic numbers 15 and 16 are for.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -21,6 +21,7 @@
 #endif
 
 void git_inflate_init(z_streamp strm);
+void git_inflate_init_gzip_only(z_streamp strm);
 void git_inflate_end(z_streamp strm);
 int git_inflate(z_streamp strm, int flush);
 

http-backend.c

@@ -275,12 +275,9 @@ static void inflate_request(const char *prog_name, int out)
 	unsigned char in_buf[8192];
 	unsigned char out_buf[8192];
 	unsigned long cnt = 0;
-	int ret;
 
 	memset(&stream, 0, sizeof(stream));
-	ret = inflateInit2(&stream, (15 + 16));
-	if (ret != Z_OK)
-		die("cannot start zlib inflater, zlib err %d", ret);
+	git_inflate_init_gzip_only(&stream);
 
 	while (1) {
 		ssize_t n = xread(0, in_buf, sizeof(in_buf));

zlib.c

@@ -32,6 +32,21 @@ void git_inflate_init(z_streamp strm)
 	    strm->msg ? strm->msg : "no message");
 }
 
+void git_inflate_init_gzip_only(z_streamp strm)
+{
+	/*
+	 * Use default 15 bits, +16 is to accept only gzip and to
+	 * yield Z_DATA_ERROR when fed zlib format.
+	 */
+	const int windowBits = 15 + 16;
+	int status = inflateInit2(strm, windowBits);
+
+	if (status == Z_OK)
+		return;
+	die("inflateInit2: %s (%s)", zerr_to_string(status),
+	    strm->msg ? strm->msg : "no message");
+}
+
 void git_inflate_end(z_streamp strm)
 {
 	int status = inflateEnd(strm);