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

@@ -4513,14 +4513,17 @@ This function returns non-@code{nil} if built-in zlib decompression is
 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
 
 

etc/NEWS

@@ -1672,6 +1672,12 @@ are implemented in C using the Jansson library.
 and 'flatten' it such that the result is a list of all the terminal
 nodes.
 
++++
+** '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
 
 ---

lisp/url/url-http.el

@@ -939,7 +939,8 @@ should be shown to the user."
     (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 @@ should be shown to the user."
 	(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 ()

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 @@ This function can be called only in unibyte buffers.  */)
     }
   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 @@ This function can be called only in unibyte buffers.  */)
   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);
 }