compat/zlib: provide stubs for deflateSetHeader()

The function `deflateSetHeader()` has been introduced with zlib v1.2.2.1,
so we don't use it when linking against an older version of it. Refactor
the code to instead provide a central stub via "compat/zlib.h" so that
we can adapt it based on whether or not we use zlib-ng in a subsequent
commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

archive-tar.c

@@ -473,9 +473,7 @@ static const char internal_gzip_command[] = "git archive gzip";
 static int write_tar_filter_archive(const struct archiver *ar,
 				    struct archiver_args *args)
 {
-#if ZLIB_VERNUM >= 0x1221
 	struct gz_header_s gzhead = { .os = 3 }; /* Unix, for reproducibility */
-#endif
 	struct strbuf cmd = STRBUF_INIT;
 	struct child_process filter = CHILD_PROCESS_INIT;
 	int r;
@@ -486,10 +484,8 @@ static int write_tar_filter_archive(const struct archiver *ar,
 	if (!strcmp(ar->filter_command, internal_gzip_command)) {
 		write_block = tgz_write_block;
 		git_deflate_init_gzip(&gzstream, args->compression_level);
-#if ZLIB_VERNUM >= 0x1221
 		if (deflateSetHeader(&gzstream.z, &gzhead) != Z_OK)
 			BUG("deflateSetHeader() called too late");
-#endif
 		gzstream.next_out = outbuf;
 		gzstream.avail_out = sizeof(outbuf);
 

compat/zlib-compat.h

@@ -7,4 +7,23 @@
 # define deflateBound(c,s)  ((s) + (((s) + 7) >> 3) + (((s) + 63) >> 6) + 11)
 #endif
 
+/*
+ * zlib only gained support for setting up the gzip header in v1.2.2.1. In
+ * Git we only set the header to make archives reproducible across different
+ * operating systems, so it's fine to simply make this a no-op when using a
+ * zlib version that doesn't support this yet.
+ */
+#if ZLIB_VERNUM < 0x1221
+struct gz_header_s {
+	int os;
+};
+
+static int deflateSetHeader(z_streamp strm, struct gz_header_s *head)
+{
+	(void)(strm);
+	(void)(head);
+	return Z_OK;
+}
+#endif
+
 #endif /* COMPAT_ZLIB_H */