Properly fix SIGBUS crash issue (#358)

* Fix SIGBUS crash.

* Workaround annoying and buggy style checker.

* Fix revealed SIGSEGV error.

* Add comments.

chronos/streams/tlsstream.nim

@@ -35,7 +35,7 @@ type
     RSA, EC
 
   TLSResult {.pure.} = enum
-    Success, Error, EOF
+    Success, Error, Stopped, WriteEof, ReadEof
 
   TLSPrivateKey* = ref object
     case kind: TLSKeyType
@@ -126,6 +126,9 @@ template newTLSStreamProtocolImpl[T](message: T): ref TLSStreamProtocolError =
   err.errCode = code
   err
 
+template newTLSUnexpectedProtocolError(): ref TLSStreamProtocolError =
+  newException(TLSStreamProtocolError, "Unexpected internal error")
+
 proc newTLSStreamProtocolError[T](message: T): ref TLSStreamProtocolError =
   newTLSStreamProtocolImpl(message)
 
@@ -142,12 +145,14 @@ proc tlsWriteRec(engine: ptr SslEngineContext,
     sslEngineSendrecAck(engine[], length)
     return TLSResult.Success
   except AsyncStreamError as exc:
-    if writer.state == AsyncStreamState.Running:
-      writer.state = AsyncStreamState.Error
-      writer.error = exc
+    writer.state = AsyncStreamState.Error
+    writer.error = exc
+    return TLSResult.Error
   except CancelledError:
     if writer.state == AsyncStreamState.Running:
       writer.state = AsyncStreamState.Stopped
+    return TLSResult.Stopped
+
   return TLSResult.Error
 
 proc tlsWriteApp(engine: ptr SslEngineContext,
@@ -157,6 +162,13 @@ proc tlsWriteApp(engine: ptr SslEngineContext,
     if item.size > 0:
       var length = 0'u
       var buf = sslEngineSendappBuf(engine[], length)
+      if isNil(buf) or (length == 0):
+        # This situation could happen when connection is closing, no
+        # application data can be sent, but some can still be received
+        # (and discarded).
+        writer.state = AsyncStreamState.Finished
+        return TLSResult.WriteEof
+
       let toWrite = min(int(length), item.size)
       copyOut(buf, item, toWrite)
       if int(length) >= item.size:
@@ -180,6 +192,8 @@ proc tlsWriteApp(engine: ptr SslEngineContext,
   except CancelledError:
     if writer.state == AsyncStreamState.Running:
       writer.state = AsyncStreamState.Stopped
+    return TLSResult.Stopped
+
   return TLSResult.Error
 
 proc tlsReadRec(engine: ptr SslEngineContext,
@@ -191,17 +205,18 @@ proc tlsReadRec(engine: ptr SslEngineContext,
     sslEngineRecvrecAck(engine[], uint(res))
     if res == 0:
       sslEngineClose(engine[])
-
-      return TLSResult.EOF
+      return TLSResult.ReadEof
     else:
       return TLSResult.Success
+  except AsyncStreamError as exc:
+    reader.state = AsyncStreamState.Error
+    reader.error = exc
+    return TLSResult.Error
   except CancelledError:
     if reader.state == AsyncStreamState.Running:
       reader.state = AsyncStreamState.Stopped
-  except AsyncStreamError as exc:
-    if reader.state == AsyncStreamState.Running:
-      reader.state = AsyncStreamState.Error
-      reader.error = exc
+    return TLSResult.Stopped
+
   return TLSResult.Error
 
 proc tlsReadApp(engine: ptr SslEngineContext,
@@ -215,13 +230,15 @@ proc tlsReadApp(engine: ptr SslEngineContext,
   except CancelledError:
     if reader.state == AsyncStreamState.Running:
       reader.state = AsyncStreamState.Stopped
+    return TLSResult.Stopped
+
   return TLSResult.Error
 
 template readAndReset(fut: untyped) =
   if fut.finished():
     let res = fut.read()
     case res
-    of TLSResult.Success:
+    of TLSResult.Success, TLSResult.WriteEof, TLSResult.Stopped:
       fut = nil
       continue
     of TLSResult.Error:
@@ -229,7 +246,7 @@ template readAndReset(fut: untyped) =
       if loopState == AsyncStreamState.Running:
         loopState = AsyncStreamState.Error
       break
-    of TLSResult.EOF:
+    of TLSResult.ReadEof:
       fut = nil
       if loopState == AsyncStreamState.Running:
         loopState = AsyncStreamState.Finished
@@ -301,14 +318,14 @@ proc tlsLoop*(stream: TLSAsyncStream) {.async.} =
 
     if isNil(sendAppFut):
       if (state and SSL_SENDAPP) == SSL_SENDAPP:
-        # Application data can be sent over stream.
-        if not(stream.writer.handshaked):
-          stream.reader.handshaked = true
-          stream.writer.handshaked = true
-          if not(isNil(stream.writer.handshakeFut)):
-            stream.writer.handshakeFut.complete()
-
-        sendAppFut = tlsWriteApp(engine, stream.writer)
+        if stream.writer.state == AsyncStreamState.Running:
+          # Application data can be sent over stream.
+          if not(stream.writer.handshaked):
+            stream.reader.handshaked = true
+            stream.writer.handshaked = true
+            if not(isNil(stream.writer.handshakeFut)):
+              stream.writer.handshakeFut.complete()
+          sendAppFut = tlsWriteApp(engine, stream.writer)
     else:
       sendAppFut.readAndReset()
 
@@ -353,8 +370,10 @@ proc tlsLoop*(stream: TLSAsyncStream) {.async.} =
     of AsyncStreamState.Error:
       if not(isNil(stream.writer.error)):
         stream.writer.error
-      else:
+      elif not(isNil(stream.reader.error)):
         newTLSStreamWriteError(stream.reader.error)
+      else:
+        newTLSUnexpectedProtocolError()
     of AsyncStreamState.Finished:
       let err = engine[].sslEngineLastError()
       if err != 0:
@@ -389,7 +408,8 @@ proc tlsLoop*(stream: TLSAsyncStream) {.async.} =
       if not(isNil(stream.writer.handshakeFut)):
         if not(stream.writer.handshakeFut.finished()):
           stream.writer.handshakeFut.fail(
-            newTLSStreamProtocolError("Connection with remote peer lost")
+            newTLSStreamProtocolError(
+              "Connection to the remote peer has been lost")
           )
 
   # Completing readers