Improve error handling (#204)

* Improve error handling

1) Let GrpcError implement exception

As the user is (usually) expected to catch these they should implement
Exception.
There is a bigger clean-up in splitting off the GrpcError.internal as
that seems to be for invariant violations

2) Convert some thrown strings into exceptions.

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.0.3
+
+* GrpcError now implements Exception to indicate it can be reasonably handled.
+
 ## 2.0.2
 
 * Fix computation of the audience given to metadata providers to include the scheme.

lib/src/client/http2_connection.dart

@@ -73,8 +73,7 @@ class Http2ClientConnection implements connection.ClientConnection {
     var socket = await Socket.connect(host, port);
     if (_state == ConnectionState.shutdown) {
       socket.destroy();
-      // TODO(sigurdm): Throw something nicer...
-      throw 'Shutting down';
+      throw _ShutdownException();
     }
     final securityContext = credentials.securityContext;
     if (securityContext != null) {
@@ -84,8 +83,7 @@ class Http2ClientConnection implements connection.ClientConnection {
           onBadCertificate: _validateBadCertificate);
       if (_state == ConnectionState.shutdown) {
         socket.destroy();
-        // TODO(sigurdm): Throw something nicer...
-        throw 'Shutting down';
+        throw _ShutdownException();
       }
     }
     socket.done.then((_) => _handleSocketClosed());
@@ -275,3 +273,5 @@ class Http2ClientConnection implements connection.ClientConnection {
     return validator(certificate, authority);
   }
 }
+
+class _ShutdownException implements Exception {}

lib/src/shared/message.dart

@@ -41,7 +41,7 @@ class GrpcMessageSink extends Sink<GrpcMessage> {
   @override
   void add(GrpcMessage data) {
     if (message != null) {
-      throw 'Too many messages received!';
+      throw StateError('Too many messages received!');
     }
     message = data;
   }
@@ -49,7 +49,7 @@ class GrpcMessageSink extends Sink<GrpcMessage> {
   @override
   void close() {
     if (message == null) {
-      throw 'No messages received!';
+      throw StateError('No messages received!');
     }
   }
 }

lib/src/shared/status.dart

@@ -117,7 +117,7 @@ class StatusCode {
   static const unauthenticated = 16;
 }
 
-class GrpcError {
+class GrpcError implements Exception {
   final int code;
   final String message;
 
@@ -214,6 +214,7 @@ class GrpcError {
 
   /// Internal errors. Means some invariants expected by underlying system has
   /// been broken. If you see one of these errors, something is very broken.
+  // TODO(sigurdm): This should probably not be an [Exception].
   GrpcError.internal([this.message]) : code = StatusCode.internal;
 
   /// The service is currently unavailable.  This is a most likely a transient

pubspec.yaml

@@ -1,7 +1,7 @@
 name: grpc
 description: Dart implementation of gRPC, a high performance, open-source universal RPC framework.
 
-version: 2.0.2
+version: 2.0.3
 
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/grpc-dart