Automated rollback of commit 61977f4.

*** Reason for rollback ***

Breaks a test of Bazel on Windows: http://ci.bazel.io/view/Dashboard/job/bazel-tests/1078/testReport/src_test_py_bazel_bazel_clean_test/exe/src_test_py_bazel_bazel_clean_test_exe/

Fixes bazelbuild#3882.

*** Original change description ***

Don't release the client lock while the server executes a command.  The
server still doesn't support concurrency, even for commands like 'help',
so there's no benefit from releasing it.

This also improves progress messages.  Today the client may print

WARNING: Running Blaze server needs to be killed, because the startup options are different.

and then wait indefinitely while the server finishes processing a
request.  With this change, an explanation for the delay is given.

RELNOTES: None.
GO...

***

RELNOTES: None.
PiperOrigin-RevId: 171689429

src/main/cpp/blaze.cc

@@ -1670,6 +1670,11 @@ unsigned int GrpcBlazeServer::Communicate() {
   std::unique_ptr<grpc::ClientReader<command_server::RunResponse>> reader(
       client_->Run(&context, request));
 
+  // Release the server lock because the gRPC handles concurrent clients just
+  // fine. Note that this may result in two "waiting for other client" messages
+  // (one during server startup and one emitted by the server)
+  blaze::ReleaseLock(&blaze_lock_);
+
   std::thread cancel_thread(&GrpcBlazeServer::CancelThread, this);
   bool command_id_set = false;
   bool pipe_broken = false;