zlib package usability issues

[Nathan Heijermans <nheijermans at gmail dot com>]

Hi all,

I recently needed to use the zlib package, but ran into some issues with 
it in its current state. I have attached a patch that resolved the 
issues for me, but there may be some issues that need to be resolved 
that I am not aware of. I do not have an in-depth understanding of the 
inner workings of zlib, but have read through the relevant portions of 
zlib.h to understand the various compile-time configuration macros they 
allow. My goal was to be able to use the routines in gzio.c to read and 
write compressed files.

The first problem arose from the fact that MAX_WBITS is defined as 
`15+16` (instead of `(15+16)`). The gzopen() call in gzio.c called 
deflateInit2() with windowBits=-15+16, which is not a valid value. 
Fixing the definition of MAX_WBITS resulted in deflateInit2() being 
called with a value of -31. This is also invalid. Disabling the CDL 
option controlling the MAX_WBITS value also didn't work, because it 
completely disable deflate() functionality. Eliminating the definition 
of MAX_WBITS altogether, and using the default value of 15, resolved 
this problem.

I then began trying to add support for the various configuration macros 
provided by zlib, and realized that defining MAX_WBITS to be in the 
-8..-15 (enabling raw deflate()/inflate()) or (8..15)+16 (enabling gzip 
deflate()) ranges is also problematic when using the gz* routines in 
gzio.c. gzopen() passes -MAX_WBITS to deflateInit2(), while deflate() 
passes MAX_WBITS. This is why I totally eliminated the option for 
"inflate() to compress gzip compatible output". If somebody really needs 
this, a change will probably need to be made to the upstream zlib code 
to separately define the MAX_WBITS used in gzio.c from the one used in 
deflate.c, or else pass MAX_WBITS in both cases.

That's all; I hope that this made sense and will help somebody...

Sincerely,
Nathan Heijermans

diff --git a/packages/services/compress/zlib/current/cdl/compress_zlib.cdl b/packages/services/compress/zlib/current/cdl/compress_zlib.cdl
index cb0a749..50252e9 100644
--- a/packages/services/compress/zlib/current/cdl/compress_zlib.cdl
+++ b/packages/services/compress/zlib/current/cdl/compress_zlib.cdl
@@ -65,13 +65,35 @@ cdl_package CYGPKG_COMPRESS_ZLIB {
         display "Override memory allocation routines."
     }
 
-    cdl_option  CYGSEM_COMPRESS_ZLIB_DEFLATE_MAKES_GZIP {
-        display        "Should deflate() produce 'gzip' compatible output?"
-        flavor         bool
-        default_value  1
+    cdl_option CYGSEM_COMPRESS_ZLIB_ENABLE_COMPRESSION {
+        display       "Enable Compression"
+        flavor        bool
+        default_value 1
         description "
-          If this option is set then the output of calling deflate()
-          will be wrapped up as a 'gzip' compatible file."
+            If this option is disabled, zlib will be compiled without the
+            ability to compress. This is useful when decompression is needed,
+            but compression is not."
+
+        cdl_option CYGOPT_COMPRESS_ZLIB_MAX_WBITS {
+            display       "#define MAX_WBITS"
+            define        MAX_WBITS
+            flavor        data
+            default_value 15
+            legal_values  8 to 15
+            description   "
+                MAX_WBITS controls the maximum window size  (i.e. the size of
+                the history buffer, which is 2^MAX_WBITS) used by the deflate()
+                call."
+        }
+    }
+
+    cdl_option CYGSEM_COMPRESS_ZLIB_ENABLE_GZIP {
+        display       "Enable gzip encoding and decoding"
+        flavor        bool
+        default_value 1
+        description   "
+            If this option is disabled, zlib will be compiled without the
+            ability to encode and decode gzip streams."
     }
 
     cdl_option CYGSEM_COMPRESS_ZLIB_NEEDS_MALLOC {
diff --git a/packages/services/compress/zlib/current/include/zconf.h b/packages/services/compress/zlib/current/include/zconf.h
index 09608dc..1284fe1 100644
--- a/packages/services/compress/zlib/current/include/zconf.h
+++ b/packages/services/compress/zlib/current/include/zconf.h
@@ -16,13 +16,15 @@
 #if CYGINT_COMPRESS_ZLIB_LOCAL_ALLOC != 0
 #define MY_ZCALLOC
 #endif
-#ifdef CYGSEM_COMPRESS_ZLIB_DEFLATE_MAKES_GZIP
-#undef  MAX_WBITS
-#define MAX_WBITS   15+16 /* 32K LZ77 window */
-#else
-#define NO_GZIP
-#define NO_GZCOMPRESS
+
+#ifndef CYGSEM_COMPRESS_ZLIB_ENABLE_COMPRESSION
+# define NO_GZCOMPRESS
 #endif
+
+#ifndef CYGSEM_COMPRESS_ZLIB_ENABLE_GZIP
+# define NO_GZIP
+#endif
+
 // #endif
 
 /*