Work around compressing empty stream on PHP 7+

Author: clue

README.md

@@ -232,7 +232,6 @@ As such, we've spotted several inconsistencies (or *bugs*) between different PHP
 These inconsistencies exist in the underlying PHP engines and there's little we can do about this in this library.
 
 * All Zend PHP versions: Decompressing invalid data does not emit any data (and does not raise an error)
-* PHP 7 only: Compressing an empty string does not emit any data (not a valid compression stream)
 * HHVM only: does not currently support the GZIP and ZLIB format at all (and does not raise an error)
 * HHVM only: The [`zlib.deflate` filter function](https://github.com/facebook/hhvm/blob/fee8ae39ce395c7b9b8910dfde6f22a7745aea83/hphp/system/php/stream/default-filters.php#L77) buffers the whole string. This means that compressing a stream of 100 MB actually stores the whole string in memory before invoking the underlying compression algorithm.
 * PHP 5.3 only: Tends to SEGFAULT occasionally on shutdown?

src/Compressor.php

@@ -46,5 +46,7 @@ public function __construct($encoding, $level = -1)
         parent::__construct(
             Filter\fun('zlib.deflate', array('window' => $encoding, 'level' => $level))
         );
+
+        $this->emptyWrite = $encoding;
     }
 }

src/ZlibFilterStream.php

@@ -74,6 +74,13 @@ public static function createZlibDecompressor()
 
     private $filter;
 
+    /**
+     * @var int|null
+     * @see Compressor
+     * @internal
+     */
+    protected $emptyWrite;
+
     public function __construct($filter)
     {
         $this->filter = $filter;
@@ -85,6 +92,7 @@ protected function transformData($chunk)
         $ret = $filter($chunk);
 
         if ($ret !== '') {
+            $this->emptyWrite = null;
             $this->forwardData($ret);
         }
     }
@@ -94,6 +102,13 @@ protected function transformEnd($chunk)
         $filter = $this->filter;
         $ret = $filter($chunk) . $filter();
 
+        // Stream ends successfully and did not emit any data whatsoever?
+        // This happens when compressing an empty stream with PHP 7 only.
+        // Bypass filter and manually compress/encode empty string.
+        if ($this->emptyWrite !== null && $ret === '') {
+            $ret = \zlib_encode('', $this->emptyWrite);
+        }
+
         if ($ret !== '') {
             $this->forwardData($ret);
         }

tests/ZlibFilterDeflateCompressorTest.php

@@ -13,8 +13,6 @@ public function setUp()
 
     public function testDeflateEmpty()
     {
-        if (PHP_VERSION >= 7) $this->markTestSkipped('Not supported on PHP 7 (empty chunk will not be emitted)');
-
         $this->compressor->on('data', $this->expectCallableOnceWith("\x03\x00"));
         $this->compressor->on('end', $this->expectCallableOnce());
 

tests/ZlibFilterGzipCompressorTest.php

@@ -15,8 +15,6 @@ public function setUp()
 
     public function testCompressEmpty()
     {
-        if (PHP_VERSION >= 7) $this->markTestSkipped('Not supported on PHP 7 (empty chunk will not be emitted)');
-
         $os = "\x03"; // UNIX (0x03) or UNKNOWN (0xFF)
         $this->compressor->on('data', $this->expectCallableOnceWith("\x1f\x8b\x08\x00\x00\x00\x00\x00\x00" . $os . "\x03\x00" . "\x00\x00\x00\x00\x00\x00\x00\x00"));
         $this->compressor->on('end', $this->expectCallableOnce());

tests/ZlibFilterZlibCompressorTest.php

@@ -15,8 +15,6 @@ public function setUp()
 
     public function testCompressEmpty()
     {
-        if (PHP_VERSION >= 7) $this->markTestSkipped('Not supported on PHP 7 (empty chunk will not be emitted)');
-
         $this->compressor->on('data', $this->expectCallableOnceWith("\x78\x9c" . "\x03\x00" . "\x00\x00\x00\x01"));
         $this->compressor->on('end', $this->expectCallableOnce());