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diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
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+/*
+ *
+ * Copyright 2015, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#import "GRPCCall.h"
+
+#include <grpc.h>
+#include <support/time.h>
+
+#import "GRPCMethodName.h"
+#import "private/GRPCChannel.h"
+#import "private/GRPCCompletionQueue.h"
+#import "private/GRPCDelegateWrapper.h"
+#import "private/GRPCMethodName+HTTP2Encoding.h"
+#import "private/NSData+GRPC.h"
+#import "private/NSDictionary+GRPC.h"
+#import "private/NSError+GRPC.h"
+
+// A grpc_call_error represents a precondition failure when invoking the
+// grpc_call_* functions. If one ever happens, it's a bug in this library.
+//
+// TODO(jcanizales): Can an application shut down gracefully when a thread other
+// than the main one throws an exception?
+static void AssertNoErrorInCall(grpc_call_error error) {
+  if (error != GRPC_CALL_OK) {
+    @throw [NSException exceptionWithName:NSInternalInconsistencyException
+                                   reason:@"Precondition of grpc_call_* not met."
+                                 userInfo:nil];
+  }
+}
+
+@interface GRPCCall () <GRXWriteable>
+// Makes it readwrite.
+@property(atomic, strong) NSDictionary *responseMetadata;
+@end
+
+// The following methods of a C gRPC call object aren't reentrant, and thus
+// calls to them must be serialized:
+// - add_metadata
+// - invoke
+// - start_write
+// - writes_done
+// - start_read
+// - destroy
+// The first four are called as part of responding to client commands, but
+// start_read we want to call as soon as we're notified that the RPC was
+// successfully established (which happens concurrently in the network queue).
+// Serialization is achieved by using a private serial queue to operate the
+// call object.
+// Because add_metadata and invoke are called and return successfully before
+// any of the other methods is called, they don't need to use the queue.
+//
+// Furthermore, start_write and writes_done can only be called after the
+// WRITE_ACCEPTED event for any previous write is received. This is achieved by
+// pausing the requests writer immediately every time it writes a value, and
+// resuming it again when WRITE_ACCEPTED is received.
+//
+// Similarly, start_read can only be called after the READ event for any
+// previous read is received. This is easier to enforce, as we're writing the
+// received messages into the writeable: start_read is enqueued once upon receiving
+// the CLIENT_METADATA_READ event, and then once after receiving each READ
+// event.
+@implementation GRPCCall {
+  dispatch_queue_t _callQueue;
+
+  grpc_call *_gRPCCall;
+  dispatch_once_t _callAlreadyInvoked;
+
+  GRPCChannel *_channel;
+  GRPCCompletionQueue *_completionQueue;
+
+  // The C gRPC library has less guarantees on the ordering of events than we
+  // do. Particularly, in the face of errors, there's no ordering guarantee at
+  // all. This wrapper over our actual writeable ensures thread-safety and
+  // correct ordering.
+  GRPCDelegateWrapper *_responseWriteable;
+  id<GRXWriter> _requestWriter;
+}
+
+@synthesize state = _state;
+
+- (instancetype)init {
+  return [self initWithHost:nil method:nil requestsWriter:nil];
+}
+
+// Designated initializer
+- (instancetype)initWithHost:(NSString *)host
+                      method:(GRPCMethodName *)method
+              requestsWriter:(id<GRXWriter>)requestWriter {
+  if (!host || !method) {
+    [NSException raise:NSInvalidArgumentException format:@"Neither host nor method can be nil."];
+  }
+  // TODO(jcanizales): Throw if the requestWriter was already started.
+  if ((self = [super init])) {
+    static dispatch_once_t initialization;
+    dispatch_once(&initialization, ^{
+      grpc_init();
+    });
+
+    _completionQueue = [GRPCCompletionQueue completionQueue];
+
+    _channel = [GRPCChannel channelToHost:host];
+    _gRPCCall = grpc_channel_create_call_old(_channel.unmanagedChannel,
+                                             method.HTTP2Path.UTF8String,
+                                             host.UTF8String,
+                                             gpr_inf_future);
+
+    // Serial queue to invoke the non-reentrant methods of the grpc_call object.
+    _callQueue = dispatch_queue_create("org.grpc.call", NULL);
+
+    _requestWriter = requestWriter;
+  }
+  return self;
+}
+
+#pragma mark Finish
+
+- (void)finishWithError:(NSError *)errorOrNil {
+  _requestWriter.state = GRXWriterStateFinished;
+  _requestWriter = nil;
+  if (errorOrNil) {
+    [_responseWriteable cancelWithError:errorOrNil];
+  } else {
+    [_responseWriteable enqueueSuccessfulCompletion];
+  }
+}
+
+- (void)cancelCall {
+  // Can be called from any thread, any number of times.
+  AssertNoErrorInCall(grpc_call_cancel(_gRPCCall));
+}
+
+- (void)cancel {
+  [self finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                            code:GRPCErrorCodeCancelled
+                                        userInfo:nil]];
+  [self cancelCall];
+}
+
+- (void)dealloc {
+  grpc_call *gRPCCall = _gRPCCall;
+  dispatch_async(_callQueue, ^{
+    grpc_call_destroy(gRPCCall);
+  });
+}
+
+#pragma mark Read messages
+
+// Only called from the call queue.
+// The handler will be called from the network queue.
+- (void)startReadWithHandler:(GRPCEventHandler)handler {
+  AssertNoErrorInCall(grpc_call_start_read_old(_gRPCCall, (__bridge_retained void *)handler));
+}
+
+// Called initially from the network queue once response headers are received,
+// then "recursively" from the responseWriteable queue after each response from the
+// server has been written.
+// If the call is currently paused, this is a noop. Restarting the call will invoke this
+// method.
+// TODO(jcanizales): Rename to readResponseIfNotPaused.
+- (void)startNextRead {
+  if (self.state == GRXWriterStatePaused) {
+    return;
+  }
+  __weak GRPCCall *weakSelf = self;
+  __weak GRPCDelegateWrapper *weakWriteable = _responseWriteable;
+
+  dispatch_async(_callQueue, ^{
+    [weakSelf startReadWithHandler:^(grpc_event *event) {
+      if (!event->data.read) {
+        // No more responses from the server.
+        return;
+      }
+      NSData *data = [NSData grpc_dataWithByteBuffer:event->data.read];
+      if (!data) {
+        // The app doesn't have enough memory to hold the server response. We
+        // don't want to throw, because the app shouldn't crash for a behavior
+        // that's on the hands of any server to have. Instead we finish and ask
+        // the server to cancel.
+        //
+        // TODO(jcanizales): No canonical code is appropriate for this situation
+        // (because it's just a client problem). Use another domain and an
+        // appropriately-documented code.
+        [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                      code:GRPCErrorCodeInternal
+                                                  userInfo:nil]];
+        [weakSelf cancelCall];
+        return;
+      }
+      [weakWriteable enqueueMessage:data completionHandler:^{
+        [weakSelf startNextRead];
+      }];
+    }];
+  });
+}
+
+#pragma mark Send headers
+
+- (void)addHeaderWithName:(NSString *)name binaryValue:(NSData *)value {
+  grpc_metadata metadata;
+  // Safe to discard const qualifiers; we're not going to modify the contents.
+  metadata.key = (char *)name.UTF8String;
+  metadata.value = (char *)value.bytes;
+  metadata.value_length = value.length;
+  grpc_call_add_metadata_old(_gRPCCall, &metadata, 0);
+}
+
+- (void)addHeaderWithName:(NSString *)name ASCIIValue:(NSString *)value {
+  grpc_metadata metadata;
+  // Safe to discard const qualifiers; we're not going to modify the contents.
+  metadata.key = (char *)name.UTF8String;
+  metadata.value = (char *)value.UTF8String;
+  // The trailing \0 isn't encoded in HTTP2.
+  metadata.value_length = value.length;
+  grpc_call_add_metadata_old(_gRPCCall, &metadata, 0);
+}
+
+// TODO(jcanizales): Rename to commitHeaders.
+- (void)sendHeaders:(NSDictionary *)metadata {
+  for (NSString *name in metadata) {
+    id value = metadata[name];
+    if ([value isKindOfClass:[NSData class]]) {
+      [self addHeaderWithName:name binaryValue:value];
+    } else if ([value isKindOfClass:[NSString class]]) {
+      [self addHeaderWithName:name ASCIIValue:value];
+    }
+  }
+}
+
+#pragma mark GRXWriteable implementation
+
+// Only called from the call queue. The error handler will be called from the
+// network queue if the write didn't succeed.
+- (void)writeMessage:(NSData *)message withErrorHandler:(void (^)())errorHandler {
+
+  __weak GRPCCall *weakSelf = self;
+  GRPCEventHandler resumingHandler = ^(grpc_event *event) {
+    if (event->data.write_accepted != GRPC_OP_OK) {
+      errorHandler();
+    }
+    // Resume the request writer (even in the case of error).
+    // TODO(jcanizales): No need to do it in the case of errors anymore?
+    GRPCCall *strongSelf = weakSelf;
+    if (strongSelf) {
+      strongSelf->_requestWriter.state = GRXWriterStateStarted;
+    }
+  };
+
+  grpc_byte_buffer *buffer = message.grpc_byteBuffer;
+  AssertNoErrorInCall(grpc_call_start_write_old(_gRPCCall,
+                                                buffer,
+                                                (__bridge_retained void *)resumingHandler,
+                                                0));
+  grpc_byte_buffer_destroy(buffer);
+}
+
+- (void)didReceiveValue:(id)value {
+  // TODO(jcanizales): Throw/assert if value isn't NSData.
+
+  // Pause the input and only resume it when the C layer notifies us that writes
+  // can proceed.
+  _requestWriter.state = GRXWriterStatePaused;
+
+  __weak GRPCCall *weakSelf = self;
+  dispatch_async(_callQueue, ^{
+    [weakSelf writeMessage:value withErrorHandler:^{
+      [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                    code:GRPCErrorCodeInternal
+                                                userInfo:nil]];
+    }];
+  });
+}
+
+// Only called from the call queue. The error handler will be called from the
+// network queue if the requests stream couldn't be closed successfully.
+- (void)finishRequestWithErrorHandler:(void (^)())errorHandler {
+  GRPCEventHandler handler = ^(grpc_event *event) {
+    if (event->data.finish_accepted != GRPC_OP_OK) {
+      errorHandler();
+    }
+  };
+  AssertNoErrorInCall(grpc_call_writes_done_old(_gRPCCall, (__bridge_retained void *)handler));
+}
+
+- (void)didFinishWithError:(NSError *)errorOrNil {
+  if (errorOrNil) {
+    [self cancel];
+  } else {
+    __weak GRPCCall *weakSelf = self;
+    dispatch_async(_callQueue, ^{
+      [weakSelf finishRequestWithErrorHandler:^{
+        [weakSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                      code:GRPCErrorCodeInternal
+                                                  userInfo:nil]];
+      }];
+    });
+  }
+}
+
+#pragma mark Invoke
+
+// Both handlers will eventually be called, from the network queue. Writes can start immediately
+// after this.
+// The first one (metadataHandler), when the response headers are received.
+// The second one (completionHandler), whenever the RPC finishes for any reason.
+- (void)invokeCallWithMetadataHandler:(GRPCEventHandler)metadataHandler
+                    completionHandler:(GRPCEventHandler)completionHandler {
+  AssertNoErrorInCall(grpc_call_invoke_old(_gRPCCall,
+                                           _completionQueue.unmanagedQueue,
+                                           (__bridge_retained void *)metadataHandler,
+                                           (__bridge_retained void *)completionHandler,
+                                           0));
+}
+
+- (void)invokeCall {
+  __weak GRPCCall *weakSelf = self;
+  [self invokeCallWithMetadataHandler:^(grpc_event *event) {
+    // Response metadata received.
+    // TODO(jcanizales): Name the type of event->data.client_metadata_read
+    // in the C library so one can actually pass the object to a method.
+    grpc_metadata *entries = event->data.client_metadata_read.elements;
+    size_t count = event->data.client_metadata_read.count;
+    GRPCCall *strongSelf = weakSelf;
+    if (strongSelf) {
+      strongSelf.responseMetadata = [NSDictionary grpc_dictionaryFromMetadata:entries
+                                                                        count:count];
+      [strongSelf startNextRead];
+    }
+  } completionHandler:^(grpc_event *event) {
+    // TODO(jcanizales): Merge HTTP2 trailers into response metadata.
+    [weakSelf finishWithError:[NSError grpc_errorFromStatus:&event->data.finished]];
+  }];
+  // Now that the RPC has been initiated, request writes can start.
+  [_requestWriter startWithWriteable:self];
+}
+
+#pragma mark GRXWriter implementation
+
+- (void)startWithWriteable:(id<GRXWriteable>)writeable {
+  // The following produces a retain cycle self:_responseWriteable:self, which is only
+  // broken when didFinishWithError: is sent to the wrapped writeable.
+  // Care is taken not to retain self strongly in any of the blocks used in
+  // the implementation of GRPCCall, so that the life of the instance is
+  // determined by this retain cycle.
+  _responseWriteable = [[GRPCDelegateWrapper alloc] initWithWriteable:writeable writer:self];
+  [self sendHeaders:_requestMetadata];
+  [self invokeCall];
+}
+
+- (void)setState:(GRXWriterState)newState {
+  // Manual transitions are only allowed from the started or paused states.
+  if (_state == GRXWriterStateNotStarted || _state == GRXWriterStateFinished) {
+    return;
+  }
+
+  switch (newState) {
+    case GRXWriterStateFinished:
+      _state = newState;
+      // Per GRXWriter's contract, setting the state to Finished manually
+      // means one doesn't wish the writeable to be messaged anymore.
+      [_responseWriteable cancelSilently];
+      _responseWriteable = nil;
+      return;
+    case GRXWriterStatePaused:
+      _state = newState;
+      return;
+    case GRXWriterStateStarted:
+      if (_state == GRXWriterStatePaused) {
+        _state = newState;
+        [self startNextRead];
+      }
+      return;
+    case GRXWriterStateNotStarted:
+      return;
+  }
+}
+@end