Verify TCP Connections are open before allowing data to be written. (#…

…1341)

Prevent subtle bugs and potential runaway memory usage that can be introduced by being unfamiliar with the internals of the `TCPConnection` class.

As a result of this change when writing to the connection data will be silently discarded if the connection has not yet been established.

Resolves: #1310
See also: https://github.com/ponylang/rfcs/blob/master/text/0016-tcp-must-be-connected.md

CHANGELOG.md

@@ -18,6 +18,7 @@ All notable changes to the Pony compiler and standard library will be documented
 
 ### Changed
 
+- Ensure TCPConnection is established before writing data to it (issue #1310)
 - Deprecate LLVM 3.6 OSX support (PR #1502)
 - Always allow writing to `_` (dontcare) (PR #1499)
 - Methods returning their receiver to allow call chaining have been changed to return either None or some useful value. Generalised method chaining implemented in version 0.9.0 should be used as a replacement. The full list of updated methods follows. No details means that the method now returns None.

packages/net/tcp_connection.pony

@@ -41,6 +41,9 @@ actor TCPConnection
       end
   ```
 
+  Note: when writing to the connection data will be silently discarded if the
+  connection has not yet been established.
+
   ## Backpressure support
 
   ### Write
@@ -247,19 +250,21 @@ actor TCPConnection
 
   be write(data: ByteSeq) =>
     """
-    Write a single sequence of bytes.
+    Write a single sequence of bytes. Data will be silently discarded if the
+    connection has not yet been established though.
     """
-    if not _closed then
+    if _connected and not _closed then
       _in_sent = true
       write_final(_notify.sent(this, data))
       _in_sent = false
     end
 
   be writev(data: ByteSeqIter) =>
     """
-    Write a sequence of sequences of bytes.
+    Write a sequence of sequences of bytes. Data will be silently discarded if
+    the connection has not yet been established though.
     """
-    if not _closed then
+    if _connected and not _closed then
       _in_sent = true
 
       for bytes in _notify.sentv(this, data).values() do
@@ -416,10 +421,11 @@ actor TCPConnection
   fun ref write_final(data: ByteSeq) =>
     """
     Write as much as possible to the socket. Set `_writeable` to `false` if not
-    everything was written. On an error, close the connection. This is for
-    data that has already been transformed by the notifier.
+    everything was written. On an error, close the connection. This is for data
+    that has already been transformed by the notifier. Data will be silently
+    discarded if the connection has not yet been established though.
     """
-    if not _closed then
+    if _connected and not _closed then
       ifdef windows then
         try
           // Add an IOCP write.