Avoid uneeded fragmented TLS work around for PHP 7.3.3+

The work around was introduced in order to bring TLS 1.3 support to all
supported versions, but at the same time it may cause very large data
chunks for high throughput scenarios. The underlying bug has been fixed
in PHP 7.3.3 and PHP 7.2.15, so we can avoid this now uneeded work
around on said version.

README.md

@@ -1372,6 +1372,15 @@ This library does not take responsibility over these context options, so it's
 up to consumers of this library to take care of setting appropriate context
 options as described above.
 
+PHP < 7.3.3 (and PHP < 7.2.15) suffers from a bug where feof() might
+block with 100% CPU usage on fragmented TLS records.
+We try to work around this by always consuming the complete receive
+buffer at once to avoid stale data in TLS buffers. This is known to
+work around high CPU usage for well-behaving peers, but this may
+cause very large data chunks for high throughput scenarios. The buggy
+behavior can still be triggered due to network I/O buffers or
+malicious peers on affected versions, upgrading is highly recommended.
+
 PHP < 7.1.4 (and PHP < 7.0.18) suffers from a bug when writing big
 chunks of data over TLS streams at once.
 We try to work around this by limiting the write chunk size to 8192

src/Connection.php

@@ -43,6 +43,17 @@ class Connection extends EventEmitter implements ConnectionInterface
 
     public function __construct($resource, LoopInterface $loop)
     {
+        // PHP < 7.3.3 (and PHP < 7.2.15) suffers from a bug where feof() might
+        // block with 100% CPU usage on fragmented TLS records.
+        // We try to work around this by always consuming the complete receive
+        // buffer at once to avoid stale data in TLS buffers. This is known to
+        // work around high CPU usage for well-behaving peers, but this may
+        // cause very large data chunks for high throughput scenarios. The buggy
+        // behavior can still be triggered due to network I/O buffers or
+        // malicious peers on affected versions, upgrading is highly recommended.
+        // @link https://bugs.php.net/bug.php?id=77390
+        $clearCompleteBuffer = \PHP_VERSION_ID < 70303 || (\PHP_VERSION_ID >= 70200 && \PHP_VERSION_ID < 70215);
+
         // PHP < 7.1.4 (and PHP < 7.0.18) suffers from a bug when writing big
         // chunks of data over TLS streams at once.
         // We try to work around this by limiting the write chunk size to 8192
@@ -53,14 +64,10 @@ public function __construct($resource, LoopInterface $loop)
         // See https://github.com/reactphp/socket/issues/105
         $limitWriteChunks = (\PHP_VERSION_ID < 70018 || (\PHP_VERSION_ID >= 70100 && \PHP_VERSION_ID < 70104));
 
-        // Construct underlying stream to always consume complete receive buffer.
-        // This avoids stale data in TLS buffers and also works around possible
-        // buffering issues in legacy PHP versions. The buffer size is limited
-        // due to TCP/IP buffers anyway, so this should not affect usage otherwise.
         $this->input = new DuplexResourceStream(
             $resource,
             $loop,
-            -1,
+            $clearCompleteBuffer ? -1 : null,
             new WritableResourceStream($resource, $loop, null, $limitWriteChunks ? 8192 : null)
         );