compress .tar.gz using gzip as a filter

ui-snapshot.c

@@ -8,13 +8,49 @@
 
 #include "cgit.h"
 
+static int write_tar_gzip_archive(struct archiver_args *args)
+{
+    int rw[2];
+    pid_t gzpid;
+    int stdout2;
+    int status;
+    int rv;
+
+    stdout2 = chk_non_negative(dup(STDIN_FILENO), "Preserving STDOUT before compressing");
+    chk_zero(pipe(rw), "Opening pipe from compressor subprocess");
+    gzpid = chk_non_negative(fork(), "Forking compressor subprocess");
+    if(gzpid==0) {
+	    /* child */
+	    chk_zero(close(rw[1]), "Closing write end of pipe in child");
+	    chk_zero(close(STDIN_FILENO), "Closing STDIN");
+	    chk_non_negative(dup2(rw[0],STDIN_FILENO), "Redirecting compressor input to stdin");
+	    execlp("gzip","gzip",NULL);
+	    _exit(-1);
+    }
+    /* parent */
+    chk_zero(close(rw[0]), "Closing read end of pipe");
+    chk_non_negative(dup2(rw[1],STDOUT_FILENO), "Redirecting output to compressor");
+    
+    rv = write_tar_archive(args);
+
+    chk_zero(close(STDOUT_FILENO), "Closing STDOUT redirected to compressor");
+    chk_non_negative(dup2(stdout2,STDOUT_FILENO), "Restoring uncompressed STDOUT");
+    chk_zero(close(stdout2), "Closing uncompressed STDOUT");
+    chk_zero(close(rw[1]), "Closing write end of pipe in parent");
+    chk_positive(waitpid(gzpid,&status,0), "Waiting on compressor process");
+    if(! ( WIFEXITED(status) && WEXITSTATUS(status)==0 ) )
+	    cgit_print_error("Failed to compress archive");
+
+    return rv;
+}
+
 static const struct snapshot_archive_t {
     	const char *suffix;
 	const char *mimetype;
 	write_archive_fn_t write_func;
 }	snapshot_archives[] = {
 	{ ".zip", "application/x-zip", write_zip_archive },
-	{ ".tar.gz", "application/x-gzip", write_tar_archive }
+	{ ".tar.gz", "application/x-gzip", write_tar_gzip_archive }
 };
 
 void cgit_print_snapshot(struct cacheitem *item, const char *hex,