Fix #48725: Support for flushing in zlib stream

When `php_zlib_deflate_filter()` is called with `PSFS_FLAG_FLUSH_INC`
but without new buckets being available (e.g. because a user calls
`rewind()` after writing to the stream), we have to make sure that any
pending data are flushed.  This could basically be done like in the
attached patch[1], but that could cause unnessary flushes, which can be
harmful for compression, and adds unnecessary flush markers to the
stream.  Thus, we use the `php_zlib_filter_data.finished` field, which
has not been used for `zlib.deflate` filters, and properly keep track
of the need to flush.

[1] <https://bugs.php.net/patch-display.php?bug_id=48725&patch=zlib-filter-flush-fix.patch&revision=latest>

Closes GH-6019.

Author: cmb69

NEWS

@@ -46,6 +46,9 @@ PHP                                                                        NEWS
 - Tidy:
   . Fixed bug #77594 (ob_tidyhandler is never reset). (cmb)
 
+- Zlib:
+  . Fixed #48725 (Support for flushing in zlib stream). (cmb)
+
 26 Nov 2020, PHP 7.4.13
 
 - Core:

ext/zlib/tests/bug48725.phpt

@@ -0,0 +1,24 @@
+--TEST--
+Bug #48725 (Support for flushing in zlib stream)
+--SKIPIF--
+<?php
+if (!extension_loaded('zlib')) die('skip zip extension not available');
+?>
+--FILE--
+<?php
+$text = str_repeat('0123456789abcdef', 1000);
+
+$temp = fopen('php://temp', 'r+');
+stream_filter_append($temp, 'zlib.deflate', STREAM_FILTER_WRITE);
+fwrite($temp, $text);
+
+rewind($temp);
+
+var_dump(bin2hex(stream_get_contents($temp)));
+var_dump(ftell($temp));
+
+fclose($temp);
+?>
+--EXPECT--
+string(138) "ecc7c901c0100000b09594bac641d97f840e22f9253c31bdb9d4d6c75cdf3ec1ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddaffc0f0000ffff"
+int(69)

ext/zlib/zlib_filter.c

@@ -29,7 +29,7 @@ typedef struct _php_zlib_filter_data {
 	unsigned char *outbuf;
 	size_t outbuf_len;
 	int persistent;
-	zend_bool finished;
+	zend_bool finished; /* for zlib.deflate: signals that no flush is pending */
 } php_zlib_filter_data;
 
 /* }}} */
@@ -196,14 +196,18 @@ static php_stream_filter_status_t php_zlib_deflate_filter(
 		bucket = php_stream_bucket_make_writeable(bucket);
 
 		while (bin < (unsigned int) bucket->buflen) {
+			int flush_mode;
+
 			desired = bucket->buflen - bin;
 			if (desired > data->inbuf_len) {
 				desired = data->inbuf_len;
 			}
 			memcpy(data->strm.next_in, bucket->buf + bin, desired);
 			data->strm.avail_in = desired;
 
-			status = deflate(&(data->strm), flags & PSFS_FLAG_FLUSH_CLOSE ? Z_FULL_FLUSH : (flags & PSFS_FLAG_FLUSH_INC ? Z_SYNC_FLUSH : Z_NO_FLUSH));
+			flush_mode = flags & PSFS_FLAG_FLUSH_CLOSE ? Z_FULL_FLUSH : (flags & PSFS_FLAG_FLUSH_INC ? Z_SYNC_FLUSH : Z_NO_FLUSH);
+			data->finished = flush_mode != Z_NO_FLUSH;
+			status = deflate(&(data->strm), flush_mode);
 			if (status != Z_OK) {
 				/* Something bad happened */
 				php_stream_bucket_delref(bucket);
@@ -230,11 +234,12 @@ static php_stream_filter_status_t php_zlib_deflate_filter(
 		php_stream_bucket_delref(bucket);
 	}
 
-	if (flags & PSFS_FLAG_FLUSH_CLOSE) {
+	if (flags & PSFS_FLAG_FLUSH_CLOSE || ((flags & PSFS_FLAG_FLUSH_INC) && !data->finished)) {
 		/* Spit it out! */
 		status = Z_OK;
 		while (status == Z_OK) {
-			status = deflate(&(data->strm), Z_FINISH);
+			status = deflate(&(data->strm), (flags & PSFS_FLAG_FLUSH_CLOSE ? Z_FINISH : Z_SYNC_FLUSH));
+			data->finished = 1;
 			if (data->strm.avail_out < data->outbuf_len) {
 				size_t bucketlen = data->outbuf_len - data->strm.avail_out;
 
@@ -395,6 +400,7 @@ static php_stream_filter *php_zlib_filter_create(const char *filtername, zval *f
 			}
 		}
 		status = deflateInit2(&(data->strm), level, Z_DEFLATED, windowBits, memLevel, 0);
+		data->finished = 1;
 		fops = &php_zlib_deflate_ops;
 	} else {
 		status = Z_DATA_ERROR;