Merge pull request #5720 from jcanizales/reachability-objc-fix

Reachability ObjC fix

9 files changed, 453 insertions, 60 deletions

diff --git a/src/objective-c/GRPCClient/GRPCCall.m b/src/objective-c/GRPCClient/GRPCCall.m
index f79b7d0bc0..2d45818b6e 100644
--- a/src/objective-c/GRPCClient/GRPCCall.m
+++ b/src/objective-c/GRPCClient/GRPCCall.m
@@ -37,6 +37,8 @@
 #include <grpc/support/time.h>
 #import <RxLibrary/GRXConcurrentWriteable.h>
 
+#import "private/GRPCConnectivityMonitor.h"
+#import "private/GRPCHost.h"
 #import "private/GRPCRequestHeaders.h"
 #import "private/GRPCWrappedCall.h"
 #import "private/NSData+GRPC.h"
@@ -71,8 +73,11 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 @implementation GRPCCall {
   dispatch_queue_t _callQueue;
 
+  NSString *_host;
+  NSString *_path;
   GRPCWrappedCall *_wrappedCall;
   dispatch_once_t _callAlreadyInvoked;
+  GRPCConnectivityMonitor *_connectivityMonitor;
 
   // 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
@@ -115,13 +120,11 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
                 format:@"The requests writer can't be already started."];
   }
   if ((self = [super init])) {
-    _wrappedCall = [[GRPCWrappedCall alloc] initWithHost:host path:path];
-    if (!_wrappedCall) {
-      return nil;
-    }
+    _host = [host copy];
+    _path = [path copy];
 
     // Serial queue to invoke the non-reentrant methods of the grpc_call object.
-    _callQueue = dispatch_queue_create("org.grpc.call", NULL);
+    _callQueue = dispatch_queue_create("io.grpc.call", NULL);
 
     _requestWriter = requestWriter;
 
@@ -156,7 +159,7 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
 - (void)cancel {
   [self finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
                                             code:GRPCErrorCodeCancelled
-                                        userInfo:nil]];
+                                        userInfo:@{NSLocalizedDescriptionKey: @"Canceled by app"}]];
   [self cancelCall];
 }
 
@@ -354,8 +357,29 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
   _retainSelf = self;
 
   _responseWriteable = [[GRXConcurrentWriteable alloc] initWithWriteable:writeable];
+
+  _wrappedCall = [[GRPCWrappedCall alloc] initWithHost:_host path:_path];
+  NSAssert(_wrappedCall, @"Error allocating RPC objects. Low memory?");
+
   [self sendHeaders:_requestHeaders];
   [self invokeCall];
+  // TODO(jcanizales): Extract this logic somewhere common.
+  NSString *host = [NSURL URLWithString:[@"https://" stringByAppendingString:_host]].host;
+  if (!host) {
+    // TODO(jcanizales): Check this on init.
+    [NSException raise:NSInvalidArgumentException format:@"host of %@ is nil", _host];
+  }
+  __weak typeof(self) weakSelf = self;
+  _connectivityMonitor = [GRPCConnectivityMonitor monitorWithHost:host];
+  [_connectivityMonitor handleLossWithHandler:^{
+    typeof(self) strongSelf = weakSelf;
+    if (strongSelf) {
+      [strongSelf finishWithError:[NSError errorWithDomain:kGRPCErrorDomain
+                                                      code:GRPCErrorCodeUnavailable
+                                                  userInfo:@{NSLocalizedDescriptionKey: @"Connectivity lost."}]];
+      [[GRPCHost hostWithAddress:strongSelf->_host] disconnect];
+    }
+  }];
 }
 
 - (void)setState:(GRXWriterState)newState {
@@ -385,4 +409,5 @@ NSString * const kGRPCTrailersKey = @"io.grpc.TrailersKey";
       return;
   }
 }
+
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCChannel.h b/src/objective-c/GRPCClient/private/GRPCChannel.h
index 8661ae6f97..e49a6aca29 100644
--- a/src/objective-c/GRPCClient/private/GRPCChannel.h
+++ b/src/objective-c/GRPCClient/private/GRPCChannel.h
@@ -35,6 +35,7 @@
 
 #include <grpc/grpc.h>
 
+@class GRPCCompletionQueue;
 struct grpc_channel_credentials;
 
 
@@ -80,4 +81,6 @@ struct grpc_channel_credentials;
 + (nonnull GRPCChannel *)insecureChannelWithHost:(nonnull NSString *)host
                                      channelArgs:(nullable NSDictionary *)channelArgs;
 
+- (nullable grpc_call *)unmanagedCallWithPath:(nonnull NSString *)path
+                              completionQueue:(nonnull GRPCCompletionQueue *)queue;
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCChannel.m b/src/objective-c/GRPCClient/private/GRPCChannel.m
index 7e55a473d7..d7de025e21 100644
--- a/src/objective-c/GRPCClient/private/GRPCChannel.m
+++ b/src/objective-c/GRPCClient/private/GRPCChannel.m
@@ -38,6 +38,8 @@
 #include <grpc/support/log.h>
 #include <grpc/support/string_util.h>
 
+#import "GRPCCompletionQueue.h"
+
 /**
  * Returns @c grpc_channel_credentials from the specified @c path. If the file at the path could not
  * be read then NULL is returned. If NULL is returned, @c errorPtr may not be NULL if there are
@@ -205,4 +207,16 @@ grpc_channel_args * buildChannelArgs(NSDictionary *dictionary) {
                                channelArgs:channelArgs];
 }
 
+- (grpc_call *)unmanagedCallWithPath:(NSString *)path
+                     completionQueue:(GRPCCompletionQueue *)queue {
+  return grpc_channel_create_call(_unmanagedChannel,
+                                  NULL, GRPC_PROPAGATE_DEFAULTS,
+                                  queue.unmanagedQueue,
+                                  path.UTF8String,
+                                  // Get "host" from "host:port"
+                                  // TODO(jcanizales): Use NSURLs throughout, to clarify these.
+                                  [_host componentsSeparatedByString:@":"][0].UTF8String,
+                                  gpr_inf_future(GPR_CLOCK_REALTIME), NULL);
+}
+
 @end
diff --git a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h
new file mode 100644
index 0000000000..2fae410331
--- /dev/null
+++ b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.h
@@ -0,0 +1,77 @@
+/*
+ *
+ * Copyright 2016, 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 <Foundation/Foundation.h>
+#import <SystemConfiguration/SystemConfiguration.h>
+
+@interface GRPCReachabilityFlags : NSObject
+
++ (nonnull instancetype)flagsWithFlags:(SCNetworkReachabilityFlags)flags;
+
+/**
+ * One accessor method to query each of the different flags. Example:
+
+@property(nonatomic, readonly) BOOL isCell;
+
+ */
+#define GRPC_XMACRO_ITEM(methodName, FlagName) \
+@property(nonatomic, readonly) BOOL methodName;
+
+#include "GRPCReachabilityFlagNames.xmacro.h"
+#undef GRPC_XMACRO_ITEM
+
+@property(nonatomic, readonly) BOOL isHostReachable;
+@end
+
+
+@interface GRPCConnectivityMonitor : NSObject
+
++ (nullable instancetype)monitorWithHost:(nonnull NSString *)hostName;
+
+- (nonnull instancetype)init NS_UNAVAILABLE;
+
+/**
+ * Queue on which callbacks will be dispatched. Default is the main queue. Set it before calling
+ * handleLossWithHandler:.
+ */
+// TODO(jcanizales): Default to a serial background queue instead.
+@property(nonatomic, strong, null_resettable) dispatch_queue_t queue;
+
+/**
+ * Calls handler every time the connectivity to this instance's host is lost. If this instance is
+ * released before that happens, the handler won't be called.
+ * Only one handler is active at a time, so if this method is called again before the previous
+ * handler has been called, it might never be called at all (or yes, if it has already been queued).
+ */
+- (void)handleLossWithHandler:(nonnull void (^)())handler;
+@end
diff --git a/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m
new file mode 100644
index 0000000000..b4061bd5ef
--- /dev/null
+++ b/src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m
@@ -0,0 +1,192 @@
+/*
+ *
+ * Copyright 2016, 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 "GRPCConnectivityMonitor.h"
+
+#pragma mark Flags
+
+@implementation GRPCReachabilityFlags {
+  SCNetworkReachabilityFlags _flags;
+}
+
++ (instancetype)flagsWithFlags:(SCNetworkReachabilityFlags)flags {
+  return [[self alloc] initWithFlags:flags];
+}
+
+- (instancetype)initWithFlags:(SCNetworkReachabilityFlags)flags {
+  if ((self = [super init])) {
+    _flags = flags;
+  }
+  return self;
+}
+
+/*
+ * One accessor method implementation per flag. Example:
+
+- (BOOL)isCell { \
+  return !!(_flags & kSCNetworkReachabilityFlagsIsWWAN); \
+}
+
+ */
+#define GRPC_XMACRO_ITEM(methodName, FlagName) \
+- (BOOL)methodName { \
+  return !!(_flags & kSCNetworkReachabilityFlags ## FlagName); \
+}
+#include "GRPCReachabilityFlagNames.xmacro.h"
+#undef GRPC_XMACRO_ITEM
+
+- (BOOL)isHostReachable {
+  // Note: connectionOnDemand means it'll be reachable only if using the CFSocketStream API or APIs
+  // on top of it.
+  // connectionRequired means we can't tell until a connection is attempted (e.g. for VPN on
+  // demand).
+  return self.reachable && !self.interventionRequired && !self.connectionOnDemand;
+}
+
+- (NSString *)description {
+  NSMutableArray *activeOptions = [NSMutableArray arrayWithCapacity:9];
+
+  /*
+   * For each flag, add its name to the array if it's ON. Example:
+
+  if (self.isCell) {
+    [activeOptions addObject:@"isCell"];
+  }
+
+   */
+#define GRPC_XMACRO_ITEM(methodName, FlagName) \
+  if (self.methodName) { \
+    [activeOptions addObject:@#methodName]; \
+  }
+#include "GRPCReachabilityFlagNames.xmacro.h"
+#undef GRPC_XMACRO_ITEM
+
+  return activeOptions.count == 0 ? @"(none)" : [activeOptions componentsJoinedByString:@", "];
+}
+
+- (BOOL)isEqual:(id)object {
+  return [object isKindOfClass:[GRPCReachabilityFlags class]] &&
+      _flags == ((GRPCReachabilityFlags *)object)->_flags;
+}
+
+- (NSUInteger)hash {
+  return _flags;
+}
+@end
+
+#pragma mark Connectivity Monitor
+
+// Assumes the third argument is a block that accepts a GRPCReachabilityFlags object, and passes the
+// received ones to it.
+static void PassFlagsToContextInfoBlock(SCNetworkReachabilityRef target,
+                                        SCNetworkReachabilityFlags flags,
+                                        void *info) {
+  #pragma unused (target)
+  // This can be called many times with the same info. The info is retained by SCNetworkReachability
+  // while this function is being executed.
+  void (^handler)(GRPCReachabilityFlags *) = (__bridge void (^)(GRPCReachabilityFlags *))info;
+  handler([[GRPCReachabilityFlags alloc] initWithFlags:flags]);
+}
+
+@implementation GRPCConnectivityMonitor {
+  SCNetworkReachabilityRef _reachabilityRef;
+}
+
+- (nullable instancetype)initWithReachability:(nullable SCNetworkReachabilityRef)reachability {
+  if (!reachability) {
+    return nil;
+  }
+  if ((self = [super init])) {
+    _reachabilityRef = CFRetain(reachability);
+    _queue = dispatch_get_main_queue();
+  }
+  return self;
+}
+
++ (nullable instancetype)monitorWithHost:(nonnull NSString *)host {
+  const char *hostName = host.UTF8String;
+  if (!hostName) {
+    [NSException raise:NSInvalidArgumentException
+                format:@"host.UTF8String returns NULL for %@", host];
+  }
+  SCNetworkReachabilityRef reachability =
+      SCNetworkReachabilityCreateWithName(NULL, hostName);
+
+  GRPCConnectivityMonitor *returnValue = [[self alloc] initWithReachability:reachability];
+  if (reachability) {
+    CFRelease(reachability);
+  }
+  return returnValue;
+}
+
+- (void)handleLossWithHandler:(void (^)())handler {
+  [self startListeningWithHandler:^(GRPCReachabilityFlags *flags) {
+    if (!flags.isHostReachable) {
+      handler();
+    }
+  }];
+}
+
+- (void)startListeningWithHandler:(void (^)(GRPCReachabilityFlags *))handler {
+  // Copy to ensure the handler block is in the heap (and so can't be deallocated when this method
+  // returns).
+  void (^copiedHandler)(GRPCReachabilityFlags *) = [handler copy];
+  SCNetworkReachabilityContext context = {
+    .version = 0,
+    .info = (__bridge void *)copiedHandler,
+    .retain = CFRetain,
+    .release = CFRelease,
+  };
+  // The following will retain context.info, and release it when the callback is set to NULL.
+  SCNetworkReachabilitySetCallback(_reachabilityRef, PassFlagsToContextInfoBlock, &context);
+  SCNetworkReachabilitySetDispatchQueue(_reachabilityRef, _queue);
+}
+
+- (void)stopListening {
+  // This releases the block on context.info.
+  SCNetworkReachabilitySetCallback(_reachabilityRef, NULL, NULL);
+  SCNetworkReachabilitySetDispatchQueue(_reachabilityRef, NULL);
+}
+
+- (void)setQueue:(dispatch_queue_t)queue {
+  _queue = queue ?: dispatch_get_main_queue();
+}
+
+- (void)dealloc {
+  if (_reachabilityRef) {
+    [self stopListening];
+    CFRelease(_reachabilityRef);
+  }
+}
+
+@end
diff --git a/src/objective-c/GRPCClient/private/GRPCHost.h b/src/objective-c/GRPCClient/private/GRPCHost.h
index 82c0ad6cf6..987d3e9f59 100644
--- a/src/objective-c/GRPCClient/private/GRPCHost.h
+++ b/src/objective-c/GRPCClient/private/GRPCHost.h
@@ -33,27 +33,39 @@
 
 #import <Foundation/Foundation.h>
 
+NS_ASSUME_NONNULL_BEGIN
+
 @class GRPCCompletionQueue;
 struct grpc_call;
 
 @interface GRPCHost : NSObject
 
 @property(nonatomic, readonly) NSString *address;
-@property(nonatomic, copy) NSString *userAgentPrefix;
+@property(nonatomic, copy, nullable) NSString *userAgentPrefix;
 
 /** The following properties should only be modified for testing: */
 
 @property(nonatomic, getter=isSecure) BOOL secure;
 
-@property(nonatomic, copy) NSString *pathToCertificates;
-@property(nonatomic, copy) NSString *hostNameOverride;
+@property(nonatomic, copy, nullable) NSString *pathToCertificates;
+@property(nonatomic, copy, nullable) NSString *hostNameOverride;
 
+- (nullable instancetype)init NS_UNAVAILABLE;
 /** Host objects initialized with the same address are the same. */
-+ (instancetype)hostWithAddress:(NSString *)address;
-- (instancetype)initWithAddress:(NSString *)address NS_DESIGNATED_INITIALIZER;
++ (nullable instancetype)hostWithAddress:(NSString *)address;
+- (nullable instancetype)initWithAddress:(NSString *)address NS_DESIGNATED_INITIALIZER;
 
 /** Create a grpc_call object to the provided path on this host. */
-- (struct grpc_call *)unmanagedCallWithPath:(NSString *)path
-                            completionQueue:(GRPCCompletionQueue *)queue;
+- (nullable struct grpc_call *)unmanagedCallWithPath:(NSString *)path
+                                     completionQueue:(GRPCCompletionQueue *)queue;
 
+// TODO: There's a race when a new RPC is coming through just as an existing one is getting
+// notified that there's no connectivity. If connectivity comes back at that moment, the new RPC
+// will have its channel destroyed by the other RPC, and will never get notified of a problem, so
+// it'll hang (the C layer logs a timeout, with exponential back off). One solution could be to pass
+// the GRPCChannel to the GRPCCall, renaming this as "disconnectChannel:channel", which would only
+// act on that specific channel.
+- (void)disconnect;
 @end
+
+NS_ASSUME_NONNULL_END
diff --git a/src/objective-c/GRPCClient/private/GRPCHost.m b/src/objective-c/GRPCClient/private/GRPCHost.m
index eb1db899b7..508cb20644 100644
--- a/src/objective-c/GRPCClient/private/GRPCHost.m
+++ b/src/objective-c/GRPCClient/private/GRPCHost.m
@@ -34,33 +34,30 @@
 #import "GRPCHost.h"
 
 #include <grpc/grpc.h>
+#import <GRPCClient/GRPCCall.h>
 #import <GRPCClient/GRPCCall+ChannelArg.h>
 
 #import "GRPCChannel.h"
 #import "GRPCCompletionQueue.h"
 #import "NSDictionary+GRPC.h"
 
+NS_ASSUME_NONNULL_BEGIN
+
 // TODO(jcanizales): Generate the version in a standalone header, from templates. Like
 // templates/src/core/surface/version.c.template .
 #define GRPC_OBJC_VERSION_STRING @"0.13.0"
 
-@interface GRPCHost ()
-// TODO(mlumish): Investigate whether caching channels with strong links is a good idea.
-@property(nonatomic, strong) GRPCChannel *channel;
-@end
-
-@implementation GRPCHost
-
-+ (instancetype)hostWithAddress:(NSString *)address {
-  return [[self alloc] initWithAddress:address];
+@implementation GRPCHost {
+  // TODO(mlumish): Investigate whether caching channels with strong links is a good idea.
+  GRPCChannel *_channel;
 }
 
-- (instancetype)init {
-  return [self initWithAddress:nil];
++ (nullable instancetype)hostWithAddress:(NSString *)address {
+  return [[self alloc] initWithAddress:address];
 }
 
 // Default initializer.
-- (instancetype)initWithAddress:(NSString *)address {
+- (nullable instancetype)initWithAddress:(NSString *)address {
   if (!address) {
     return nil;
   }
@@ -95,46 +92,45 @@
   return self;
 }
 
-- (grpc_call *)unmanagedCallWithPath:(NSString *)path completionQueue:(GRPCCompletionQueue *)queue {
-  if (!queue || !path || !self.channel) {
-    return NULL;
+- (nullable grpc_call *)unmanagedCallWithPath:(NSString *)path
+                              completionQueue:(GRPCCompletionQueue *)queue {
+  GRPCChannel *channel;
+  // This is racing -[GRPCHost disconnect].
+  @synchronized(self) {
+    if (!_channel) {
+      _channel = [self newChannel];
+    }
+    channel = _channel;
   }
-  return grpc_channel_create_call(self.channel.unmanagedChannel,
-                                  NULL, GRPC_PROPAGATE_DEFAULTS,
-                                  queue.unmanagedQueue,
-                                  path.UTF8String,
-                                  self.hostName.UTF8String,
-                                  gpr_inf_future(GPR_CLOCK_REALTIME), NULL);
+  return [channel unmanagedCallWithPath:path completionQueue:queue];
 }
 
-- (GRPCChannel *)channel {
-  // Create it lazily, because we don't want to open a connection just because someone is
-  // configuring a host.
+- (NSDictionary *)channelArgs {
+  NSMutableDictionary *args = [NSMutableDictionary dictionary];
 
-  if (!_channel) {
-    NSMutableDictionary *args = [NSMutableDictionary dictionary];
+  // TODO(jcanizales): Add OS and device information (see
+  // https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md#user-agents ).
+  NSString *userAgent = @"grpc-objc/" GRPC_OBJC_VERSION_STRING;
+  if (_userAgentPrefix) {
+    userAgent = [_userAgentPrefix stringByAppendingFormat:@" %@", userAgent];
+  }
+  args[@GRPC_ARG_PRIMARY_USER_AGENT_STRING] = userAgent;
 
-    // TODO(jcanizales): Add OS and device information (see
-    // https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md#user-agents ).
-    NSString *userAgent = @"grpc-objc/" GRPC_OBJC_VERSION_STRING;
-    if (_userAgentPrefix) {
-      userAgent = [@[_userAgentPrefix, userAgent] componentsJoinedByString:@" "];
-    }
-    args[@GRPC_ARG_PRIMARY_USER_AGENT_STRING] = userAgent;
-
-    if (_secure) {
-      if (_hostNameOverride) {
-        args[@GRPC_SSL_TARGET_NAME_OVERRIDE_ARG] = _hostNameOverride;
-      }
-
-      _channel = [GRPCChannel secureChannelWithHost:_address
-                                 pathToCertificates:_pathToCertificates
-                                        channelArgs:args];
-    } else {
-      _channel = [GRPCChannel insecureChannelWithHost:_address channelArgs:args];
-    }
+  if (_secure && _hostNameOverride) {
+    args[@GRPC_SSL_TARGET_NAME_OVERRIDE_ARG] = _hostNameOverride;
+  }
+  return args;
+}
+
+- (GRPCChannel *)newChannel {
+  NSDictionary *args = [self channelArgs];
+  if (_secure) {
+    return [GRPCChannel secureChannelWithHost:_address
+                           pathToCertificates:_pathToCertificates
+                                  channelArgs:args];
+  } else {
+    return [GRPCChannel insecureChannelWithHost:_address channelArgs:args];
   }
-  return _channel;
 }
 
 - (NSString *)hostName {
@@ -142,7 +138,16 @@
   return _hostNameOverride ?: _address;
 }
 
+- (void)disconnect {
+  // This is racing -[GRPCHost unmanagedCallWithPath:completionQueue:].
+  @synchronized(self) {
+    _channel = nil;
+  }
+}
+
 // TODO(jcanizales): Don't let set |secure| to |NO| if |pathToCertificates| or |hostNameOverride|
 // have been set. Don't let set either of the latter if |secure| has been set to |NO|.
 
 @end
+
+NS_ASSUME_NONNULL_END
diff --git a/src/objective-c/GRPCClient/private/GRPCReachabilityFlagNames.xmacro.h b/src/objective-c/GRPCClient/private/GRPCReachabilityFlagNames.xmacro.h
new file mode 100644
index 0000000000..02871d5d02
--- /dev/null
+++ b/src/objective-c/GRPCClient/private/GRPCReachabilityFlagNames.xmacro.h
@@ -0,0 +1,65 @@
+/*
+ *
+ * Copyright 2016, 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.
+ *
+ */
+
+/**
+ * "X-macro" file that lists the flags names of Apple's Network Reachability API, along with a nice
+ * Objective-C method name used to query each of them.
+ *
+ * Example usage: To generate a dictionary from flag value to name, one can do:
+
+  NSDictionary *flagNames = @{
+#define GRPC_XMACRO_ITEM(methodName, FlagName) \
+    @(kSCNetworkReachabilityFlags ## FlagName): @#methodName,
+#include "GRXReachabilityFlagNames.xmacro.h"
+#undef GRPC_XMACRO_ITEM
+  };
+
+  XCTAssertEqualObjects(flagNames[@(kSCNetworkReachabilityFlagsIsWWAN)], @"isCell");
+
+ */
+
+#ifndef GRPC_XMACRO_ITEM
+#error This file is to be used with the "X-macro" pattern: Please #define \
+       GRPC_XMACRO_ITEM(methodName, FlagName), then #include this file, and then #undef \
+       GRPC_XMACRO_ITEM.
+#endif
+
+GRPC_XMACRO_ITEM(isCell, IsWWAN)
+GRPC_XMACRO_ITEM(reachable, Reachable)
+GRPC_XMACRO_ITEM(transientConnection, TransientConnection)
+GRPC_XMACRO_ITEM(connectionRequired, ConnectionRequired)
+GRPC_XMACRO_ITEM(connectionOnTraffic, ConnectionOnTraffic)
+GRPC_XMACRO_ITEM(interventionRequired, InterventionRequired)
+GRPC_XMACRO_ITEM(connectionOnDemand, ConnectionOnDemand)
+GRPC_XMACRO_ITEM(isLocalAddress, IsLocalAddress)
+GRPC_XMACRO_ITEM(isDirect, IsDirect)
diff --git a/src/objective-c/GRPCClient/private/GRPCWrappedCall.h b/src/objective-c/GRPCClient/private/GRPCWrappedCall.h
index 71e7e0e54e..e37ed1b59f 100644
--- a/src/objective-c/GRPCClient/private/GRPCWrappedCall.h
+++ b/src/objective-c/GRPCClient/private/GRPCWrappedCall.h
@@ -34,7 +34,6 @@
 #import <Foundation/Foundation.h>
 #include <grpc/grpc.h>
 
-#import "GRPCChannel.h"
 #import "GRPCRequestHeaders.h"
 
 @interface GRPCOperation : NSObject
@@ -94,4 +93,5 @@
 - (void)startBatchWithOperations:(NSArray *)ops;
 
 - (void)cancel;
+
 @end