PNaCl: Hold subprocess_mu while doing StartSrpcServices.

StartSrpcServices will require using the service_runtime pointer,
to set up the srpc_client pointer. So we cannot destroy the
service_runtime at the same time.

E.g., if I add a sleep() to induce the race, I get:

../../native_client/src/include/nacl_scoped_ptr.h:96: C *nacl::scoped_ptr<plugin::SelLdrLauncherChrome>::operator->() const [C = plugin::SelLdrLauncherChrome]: Assertion `ptr_ != __null' failed.
Received signal 6
#0 0x7f687f4aef7e base::debug::StackTrace::StackTrace()
#1 0x7f687f4aeabf base::debug::(anonymous namespace)::StackDumpSignalHandler()
#2 0x7f6879dee340 <unknown>
#3 0x7f6876504cc9 gsignal
#4 0x7f68765080d8 abort
#5 0x7f68764fdb86 <unknown>
#6 0x7f68764fdc32 __assert_fail
#7 0x7f688e2887f2 nacl::scoped_ptr<>::operator->()
#8 0x7f688e288487 plugin::ServiceRuntime::SetupAppChannel()
#9 0x7f688e28cb76 plugin::NaClSubprocess::StartSrpcServices()
#10 0x7f688e281810 plugin::PnaclTranslateThread::DoLink()
#11 0x7f688e27fe7d plugin::PnaclTranslateThread::DoLinkThread()
#12 0x7f6879de6182 start_thread
#13 0x7f68765c847d clone

Should have checked this in previous refactoring:
https://codereview.chromium.org/1128943003

BUG=473474

Review URL: https://codereview.chromium.org/1128373005

Cr-Commit-Position: refs/heads/master@{#329718}

components/nacl/renderer/plugin/pnacl_translate_thread.cc

@@ -190,14 +190,12 @@ void WINAPI PnaclTranslateThread::DoCompileThread(void* arg) {
 }
 
 void PnaclTranslateThread::DoCompile() {
-  // If the main thread asked us to exit in between starting the thread
-  // and now, just leave now.
   {
     nacl::MutexLocker ml(&subprocess_mu_);
+    // If the main thread asked us to exit in between starting the thread
+    // and now, just leave now.
     if (!compiler_subprocess_active_)
       return;
-  }
-
     // Now that we are in helper thread, we can do the the blocking
     // StartSrpcServices operation.
     if (!compiler_subprocess_->StartSrpcServices()) {
@@ -206,6 +204,7 @@ void PnaclTranslateThread::DoCompile() {
           "SRPC connection failure for " + compiler_subprocess_->description());
       return;
     }
+  }
 
   SrpcParams params;
   std::vector<nacl::DescWrapper*> compile_out_files;
@@ -338,14 +337,12 @@ void WINAPI PnaclTranslateThread::DoLinkThread(void* arg) {
 }
 
 void PnaclTranslateThread::DoLink() {
-  // If the main thread asked us to exit in between starting the thread
-  // and now, just leave now.
   {
     nacl::MutexLocker ml(&subprocess_mu_);
+    // If the main thread asked us to exit in between starting the thread
+    // and now, just leave now.
     if (!ld_subprocess_active_)
       return;
-  }
-
     // Now that we are in helper thread, we can do the the blocking
     // StartSrpcServices operation.
     if (!ld_subprocess_->StartSrpcServices()) {
@@ -354,6 +351,7 @@ void PnaclTranslateThread::DoLink() {
           "SRPC connection failure for " + ld_subprocess_->description());
       return;
     }
+  }
 
   SrpcParams params;
   std::vector<nacl::DescWrapper*> ld_in_files;

components/nacl/renderer/plugin/pnacl_translate_thread.h

@@ -113,9 +113,9 @@ class PnaclTranslateThread {
   // subprocesses. The *_subprocess_active flags indicate which subprocesses
   // are active to ensure the subprocesses don't get shutdown more than once.
   // The subprocess_mu_ must be held when shutting down the subprocesses
-  // or checking if it's already shut down (via the active flags).
+  // or otherwise accessing the service_runtime component of the subprocess.
   // There are some accesses to the subprocesses without locks held
-  // (invoking SRPC methods client).
+  // (invoking srpc_client methods -- in contrast to using the service_runtime).
   NaClSubprocess* compiler_subprocess_;
   NaClSubprocess* ld_subprocess_;
   bool compiler_subprocess_active_;