Update Backpressure.apply/release examples (#2708)

The examples given for how to use manually applied backpressure could lead to deadlocking as it would be possible to dispose of the TCPConnection or close it while backpressure had been applied and then not release it. [skip ci]
ponylang · May 23, 2018 · 2d98584 · 2d98584
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diff --git a/examples/under_pressure/main.pony b/examples/under_pressure/main.pony
@@ -95,6 +95,10 @@ class SlowDown is TCPConnectionNotify
     _out.print("Releasing backpressure!")
     Backpressure.release(_auth)
 
+  fun ref closed(connection: TCPConnection ref) =>
+    _out.print("Releasing backpressure if applied!")
+    Backpressure.release(_auth)
+
   fun ref connect_failed(conn: TCPConnection ref) =>
     @printf[I32]("connect_failed\n".cstring())
     None

diff --git a/packages/net/tcp_connection.pony b/packages/net/tcp_connection.pony
@@ -107,6 +107,12 @@ actor TCPConnection
       _out.print("Releasing backpressure!")
       Backpressure.release(_auth)
 
+    fun ref closed(connection: TCPConnection ref) =>
+      // if backpressure has been applied, make sure we release
+      // when shutting down
+      _out.print("Releasing backpressure if applied!")
+      Backpressure.release(_auth)
+
     fun ref connect_failed(conn: TCPConnection ref) =>
       None