Add a maybe-temporary way for apps to clear the channel cache

src/objective-c/GRPCClient/GRPCCall+ChannelArg.h

@@ -32,6 +32,8 @@
  */
 #import "GRPCCall.h"
 
+#include <AvailabilityMacros.h>
+
 /**
  * Methods to configure GRPC channel options.
  */
@@ -43,4 +45,7 @@
  */
 + (void)setUserAgentPrefix:(NSString *)userAgentPrefix forHost:(NSString *)host;
 
++ (void)closeOpenConnections DEPRECATED_MSG_ATTRIBUTE("The API for this feature is experimental, "
+                                                      "and might be removed or modified at any "
+                                                      "time.");
 @end

src/objective-c/GRPCClient/GRPCCall+ChannelArg.m

@@ -46,4 +46,8 @@
   hostConfig.userAgentPrefix = userAgentPrefix;
 }
 
++ (void)closeOpenConnections {
+  [GRPCHost flushChannelCache];
+}
+
 @end

src/objective-c/GRPCClient/private/GRPCHost.h

@@ -41,6 +41,8 @@ struct grpc_channel_credentials;
 
 @interface GRPCHost : NSObject
 
++ (void)flushChannelCache;
+
 @property(nonatomic, readonly) NSString *address;
 @property(nonatomic, copy, nullable) NSString *userAgentPrefix;
 @property(nonatomic, nullable) struct grpc_channel_credentials *channelCreds;

src/objective-c/GRPCClient/private/GRPCHost.m

@@ -48,6 +48,8 @@ NS_ASSUME_NONNULL_BEGIN
 // templates/src/core/surface/version.c.template .
 #define GRPC_OBJC_VERSION_STRING @"0.13.0"
 
+static NSMutableDictionary *kHostCache;
+
 @implementation GRPCHost {
   // TODO(mlumish): Investigate whether caching channels with strong links is a good idea.
   GRPCChannel *_channel;
@@ -79,13 +81,12 @@ NS_ASSUME_NONNULL_BEGIN
   }
 
   // Look up the GRPCHost in the cache.
-  static NSMutableDictionary *hostCache;
   static dispatch_once_t cacheInitialization;
   dispatch_once(&cacheInitialization, ^{
-    hostCache = [NSMutableDictionary dictionary];
+    kHostCache = [NSMutableDictionary dictionary];
   });
-  @synchronized(hostCache) {
-    GRPCHost *cachedHost = hostCache[address];
+  @synchronized(kHostCache) {
+    GRPCHost *cachedHost = kHostCache[address];
     if (cachedHost) {
       return cachedHost;
     }
@@ -93,12 +94,22 @@ NS_ASSUME_NONNULL_BEGIN
     if ((self = [super init])) {
       _address = address;
       _secure = YES;
-      hostCache[address] = self;
+      kHostCache[address] = self;
     }
   }
   return self;
 }
 
++ (void)flushChannelCache {
+  @synchronized(kHostCache) {
+    [kHostCache enumerateKeysAndObjectsUsingBlock:^(id  _Nonnull key,
+                                                    GRPCHost * _Nonnull host,
+                                                    BOOL * _Nonnull stop) {
+      [host disconnect];
+    }];
+  }
+}
+
 - (nullable grpc_call *)unmanagedCallWithPath:(NSString *)path
                               completionQueue:(GRPCCompletionQueue *)queue {
   GRPCChannel *channel;