Add comment for timeout; assert on negative timeout value; fix timeout test

src/objective-c/GRPCClient/GRPCCall.h

@@ -170,7 +170,9 @@ extern id const kGRPCTrailersKey;
 @property (atomic, copy, readwrite) NSString *serverName;
 
 /**
- * The timeout for the RPC call in milliseconds. If set to 0, the call will not timeout.
+ * The timeout for the RPC call in milliseconds. If set to 0, the call will not timeout. If set to
+ * positive, the gRPC call returns with status GRPCErrorCodeDeadlineExceeded. A negative value is
+ * not allowed.
  */
 @property NSTimeInterval timeout;
 

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

@@ -184,6 +184,10 @@ static grpc_channel_args *BuildChannelArgs(NSDictionary *dictionary) {
                           serverName:(NSString *)serverName
                              timeout:(NSTimeInterval)timeout
                      completionQueue:(GRPCCompletionQueue *)queue {
+  GPR_ASSERT(timeout >= 0);
+  if (timeout < 0) {
+    timeout = 0;
+  }
   grpc_slice host_slice;
   if (serverName) {
     host_slice = grpc_slice_from_copied_string(serverName.UTF8String);

src/objective-c/tests/GRPCClientTests.m

@@ -437,7 +437,7 @@ static GRPCProtoMethod *kUnaryCallMethod;
     [completion fulfill];
   }];
 
-  call.timeout = 1;
+  call.timeout = 0.001;
   [call startWithWriteable:responsesWriteable];
 
   [self waitForExpectationsWithTimeout:TEST_TIMEOUT handler:nil];