Fix bug #78326

Similar to what fread() does, truncate the stream_get_contents()
result if the original buffer was way too large.

NEWS

@@ -20,6 +20,8 @@ PHP                                                                        NEWS
 - Standard:
   . Fixed bug #69100 (Bus error from stream_copy_to_stream (file -> SSL stream)
     with invalid length). (Nikita)
+  . Fixed bug #78326 (improper memory deallocation on stream_get_contents()
+    with fixed length buffer). (Albert Casademont)
 
 01 Aug 2019, PHP 7.2.21
 

ext/standard/tests/streams/bug78326.phpt

@@ -0,0 +1,18 @@
+--TEST--
+memory allocation on stream_get_contents()
+--INI--
+memory_limit=32M
+--FILE--
+<?php
+$f = tmpfile();
+fwrite($f, '.');
+
+$chunks = array();
+for ($i = 0; $i < 1000; ++$i) {
+    rewind($f);
+    $chunks[] = stream_get_contents($f, 1000000);
+}
+var_dump(count($chunks));
+?>
+--EXPECT--
+int(1000)

ext/standard/tests/streams/bug78326_1.phpt

@@ -0,0 +1,10 @@
+--TEST--
+proper string length on stream_get_contents()
+--FILE--
+<?php
+$f = fopen('php://memory', 'rw');
+fwrite($f, str_repeat('X', 1000));
+fseek($f, 0);
+var_dump(strlen(stream_get_contents($f, 1024)));
+--EXPECT--
+int(1000)

main/streams/streams.c

@@ -1418,8 +1418,13 @@ PHPAPI zend_string *_php_stream_copy_to_mem(php_stream *src, size_t maxlen, int
 			ptr += ret;
 		}
 		if (len) {
-			*ptr = '\0';
 			ZSTR_LEN(result) = len;
+			ZSTR_VAL(result)[len] = '\0';
+
+			/* Only truncate if the savings are large enough */
+			if (len < maxlen / 2) {
+				result = zend_string_truncate(result, len, persistent);
+			}
 		} else {
 			zend_string_free(result);
 			result = NULL;