Subject: Re: bug#33133: 26.1.50; zlib-decompress-region too rigid

From: Noam Postavsky <[email-masked]>
Date: Tue, 30 Oct 2018 20:25:10 -0400

Eli Zaretskii <[email-masked]> writes:

>> +data.  If @var{allow-partial} is @code{nil}, on failure, the function
>
> We usually say "nil or omitted" for optional arguments.  Also, I'd say
> "then on failure, ...", otherwise this could be misinterpreted as if
> "on failure" qualifies the "is nil" part.
>
> Same comment regarding the doc string of the function.

Makes sense, done.

>> +leaves the region unchanged and returns @code{nil}.  Otherwise, it
>> +returns the number of bytes that were not decompressed and replaces
>> +the region text by whatever data was successfully decompressed.  This
>> +function can be called only in unibyte buffers.
>
> Maybe it would make sense here to say that this emulates what 'gzip'
> does?

Hmm, maybe.  I've added a mention of this, not sure if it actually
helps.

>> +  Lisp_Object ret = Qt;
>>    if (inflate_status != Z_STREAM_END)
>> +    {
>> +      if (!NILP (allow_partial))
>> +        ret = make_int (iend - pos_byte);

> Hmm... should we display a warning message, like gzip does?

Not unconditionally, I'd say.  In the example which prompted this bug
thread, a warning would just be a nuisance.  We could just leave it up
to the caller to print a warning message if they want, e.g.:

   (unless (eq t (zlib-decompress-region START END t))
     (message "Incomplete decompression"))

Or perhaps instead of returning the byte count, return an error
indicator which the caller could use to contruct a warning message (this
could allow for a slightly more specific message)?  Or maybe it's easier
if the ALLOW-PARTIAL parameter could have another possible value to
control display of the warning message?

>From 43a912181a4a30d826b2c016ca05b5c9d8daf3f4 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <[email-masked]>
Date: Sat, 27 Oct 2018 17:45:52 -0400
Subject: [PATCH v2] Allow partial decompression (Bug#33133)

* src/decompress.c (Fzlib_decompress_region): Add optional
ALLOW-PARTIAL parameter.
* lisp/url/url-http.el (url-handle-content-transfer-encoding): Use it.
* doc/lispref/text.texi (Decompression): Document it.
* etc/NEWS: Announce it.
---
 doc/lispref/text.texi | 11 +++++++----
 etc/NEWS              |  6 ++++++
 lisp/url/url-http.el  |  5 +++--
 src/decompress.c      | 22 +++++++++++++++++-----
 4 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index 6c38d8eed0..9f241a6c3f 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -4462,14 +4462,17 @@ Decompression
 available.
 @end defun
 
-@defun zlib-decompress-region start end
+@defun zlib-decompress-region start end &optional allow-partial
 This function decompresses the region between @var{start} and
 @var{end}, using built-in zlib decompression.  The region should
 contain data that were compressed with gzip or zlib.  On success, the
 function replaces the contents of the region with the decompressed
-data.  On failure, the function leaves the region unchanged and
-returns @code{nil}.  This function can be called only in unibyte
-buffers.
+data.  If @var{allow-partial} is @code{nil} or omitted, then on
+failure, the function leaves the region unchanged and returns
+@code{nil}.  Otherwise, it returns the number of bytes that were not
+decompressed and replaces the region text by whatever data was
+successfully decompressed.  This function can be called only in
+unibyte buffers.
 @end defun

diff --git a/etc/NEWS b/etc/NEWS
index 3f86195695..395169253d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1159,6 +1159,12 @@ to mean that it is not known whether DST is in effect.
 'json-insert', 'json-parse-string', and 'json-parse-buffer'.  These
 are implemented in C using the Jansson library.
 
++++
+** 'zlib-decompress-region' can partially decompress corrupted data.
+If the new optional ALLOW-PARTIAL argument is passed, then the data
+that was decompressed successfully before failing will be inserted
+into the buffer.
+
 ** Mailcap
 
 ---
diff --git a/lisp/url/url-http.el b/lisp/url/url-http.el
index 6b5749e1bc..94ac660fcf 100644
--- a/lisp/url/url-http.el
+++ b/lisp/url/url-http.el
@@ -939,7 +939,8 @@ url-http-parse-headers
     (goto-char (point-min))
     success))
 
-(declare-function zlib-decompress-region "decompress.c" (start end))
+(declare-function zlib-decompress-region "decompress.c"
+                  (start end &optional allow-partial))
 
 (defun url-handle-content-transfer-encoding ()
   (let ((encoding (mail-fetch-field "content-encoding")))
@@ -951,7 +952,7 @@ url-handle-content-transfer-encoding
 	(widen)
 	(goto-char (point-min))
 	(when (search-forward "\n\n")
-	  (zlib-decompress-region (point) (point-max)))))))
+	  (zlib-decompress-region (point) (point-max) t))))))
 
 ;; Miscellaneous
 (defun url-http-activate-callback ()
diff --git a/src/decompress.c b/src/decompress.c
index 2836338216..dd55fd68e8 100644
--- a/src/decompress.c
+++ b/src/decompress.c
@@ -120,12 +120,18 @@ DEFUN ("zlib-available-p", Fzlib_available_p, Szlib_available_p, 0, 0, 0,
 
 DEFUN ("zlib-decompress-region", Fzlib_decompress_region,
        Szlib_decompress_region,
-       2, 2, 0,
+       2, 3, 0,
        doc: /* Decompress a gzip- or zlib-compressed region.
 Replace the text in the region by the decompressed data.
-On failure, return nil and leave the data in place.
+
+If optional parameter ALLOW-PARTIAL is nil or omitted, then on
+failure, return nil and leave the data in place.  Otherwise, return
+the number of bytes that were not decompressed and replace the region
+text by whatever data was successfully decompressed (similar to gzip).
+If decompression is completely successful return t.
+
 This function can be called only in unibyte buffers.  */)
-  (Lisp_Object start, Lisp_Object end)
+  (Lisp_Object start, Lisp_Object end, Lisp_Object allow_partial)
 {
   ptrdiff_t istart, iend, pos_byte;
   z_stream stream;
@@ -206,8 +212,14 @@ DEFUN ("zlib-decompress-region", Fzlib_decompress_region,
     }
   while (inflate_status == Z_OK);
 
+  Lisp_Object ret = Qt;
   if (inflate_status != Z_STREAM_END)
-    return unbind_to (count, Qnil);
+    {
+      if (!NILP (allow_partial))
+        ret = make_int (iend - pos_byte);
+      else
+        return unbind_to (count, Qnil);
+    }
 
   unwind_data.start = 0;
 
@@ -218,7 +230,7 @@ DEFUN ("zlib-decompress-region", Fzlib_decompress_region,
   signal_after_change (istart, iend - istart, unwind_data.nbytes);
   update_compositions (istart, istart, CHECK_HEAD);
 
-  return unbind_to (count, Qt);
+  return unbind_to (count, ret);
 }
 
 
-- 
2.11.0