[mojo-edk] Simplify multiprocess pipe bootstrap

This introduces a new MergePort message at the Ports layer
for joining two independent port cycles which each have
an unused (i.e. unwritten, unread, unsent) receiving port.

MergePort allows us to create a MessagePipeDispatcher which
is immediately usable but which will eventually be linked to
a MessagePipeDispatcher on another port cycle, potentially in
another process.

The basic idea is to create a fully functional port pair but
only bind one port to an MPD. Do this on each end and
merge the dangling ports asynchronously.

The simplification here allows a lot of code to be deleted
from NodeController, some of which is deleted in this CL.

Future work will convert existing bootstrap sites back to
using synchronous bootstrap, including the token-based APIs.

BUG=584764
TBR=ben@chromium.org for null check in mash shell

Review URL: https://codereview.chromium.org/1675603002

Cr-Commit-Position: refs/heads/master@{#374322}

1 files changed, 6 insertions, 2 deletions

diff --git a/mash/shell/shell_application_delegate.cc b/mash/shell/shell_application_delegate.cc
index 68c01fd..4c2196e 100644
--- a/mash/shell/shell_application_delegate.cc
+++ b/mash/shell/shell_application_delegate.cc
@@ -117,8 +117,12 @@ void ShellApplicationDelegate::StartRestartableService(
   // TODO(beng): This would be the place to insert logic that counted restarts
   //             to avoid infinite crash-restart loops.
   scoped_ptr<mojo::Connection> connection = shell_->Connect(url);
-  connection->SetRemoteServiceProviderConnectionErrorHandler(restart_callback);
-  connections_[url] = std::move(connection);
+  // Note: |connection| may be null if we've lost our connection to the shell.
+  if (connection) {
+    connection->SetRemoteServiceProviderConnectionErrorHandler(
+        restart_callback);
+    connections_[url] = std::move(connection);
+  }
 }
 
 }  // namespace shell