This has annoted me forever. Make gunzip report empty files as
what they are than what they are not (not a valid gzip file).

empty files are relitively common when pickey websites refuse to
downloads by hget as they have no respect for the "Plan9"
user agent header (sigh).

Reference: /n/sources/patch/gunzip-empty-file
Date: Wed May 13 11:53:04 CES 2015
Signed-off-by: steve@quintile.net

--- /sys/src/cmd/gzip/gunzip.c	Wed May 13 11:41:58 2015
+++ /sys/src/cmd/gzip/gunzip.c	Wed May 13 11:41:54 2015
@@ -95,7 +95,7 @@
 gunzipf(char *file, int stdout)
 {
 	char ofile[256], *s;
-	int ofd, ifd, ok;
+	int ofd, ifd, ok, n;
 
 	infile = file;
 	ifd = open(file, OREAD);
@@ -105,8 +105,11 @@
 	}
 
 	Binit(&bin, ifd, OREAD);
-	if(Bgetc(&bin) != GZMAGIC1 || Bgetc(&bin) != GZMAGIC2 || Bgetc(&bin) != GZDEFLATE){
-		fprint(2, "gunzip: %s is not a gzip deflate file\n", file);
+	if((n = Bgetc(&bin)) != GZMAGIC1 || Bgetc(&bin) != GZMAGIC2 || Bgetc(&bin) != GZDEFLATE){
+		if(n == -1)
+			fprint(2, "gunzip: %s empty file\n", file);
+		else
+			fprint(2, "gunzip: %s is not a gzip deflate file\n", file);
 		Bterm(&bin);
 		close(ifd);
 		return 0;