gzopen in src/lib/libz and empty files

[Olivier Taïbi]

I believe there is a bug in gzopen(3) when opening an empty file.  It
can read both gzipped and uncompressed files, and obviously an empty
file falls in the second category, but in this case the first read gives
a buffer error (Z_BUF_ERROR in zlib.h) instead of EOF (Z_STREAM_END in
zlib.h) because the stream is incorrectly identified as gzipped.  The
following diff fixes this issue for me.

Index: gzio.c
===================================================================
RCS file: /cvs/src/lib/libz/gzio.c,v
retrieving revision 1.14
diff -u -p -u -p -r1.14 gzio.c
--- gzio.c      20 Jul 2005 15:56:41 -0000      1.14
+++ gzio.c      15 Apr 2020 12:54:37 -0000
@@ -307,7 +307,7 @@ local void check_header(s)
         s->stream.avail_in += len;
         s->stream.next_in = s->inbuf;
         if (s->stream.avail_in < 2) {
-            s->transparent = s->stream.avail_in;
+            s->transparent = 1;
             return;
         }
     }