[dartdev] Fix race in DartDev::RunDartDev

It was checking DartDev::result_ was set to determine if
dartdev is done, however the intent of the code is to
only continue once we receive DartDev_Result_Exit from
dartdev (based on the fact that we only Notify in
ExitResultCallback).

This code was incorrect if we encounter a spurios wakeup:
we might reach RunExecResultCallback but only execute it
partially i.e. set result_ to DartDev_Result_RunExec but
only partially initialize argv_ and argc_, and not yet
initialize script_name_. However at this point we might
spuriously wakeup from our Wait in RunDartDev and think
that we can continue - which means we will hit a crash
once we try to use partially initialized information for
Process::Exec which would cause us to crash.

The bug was discovered based on stack traces provided by
Elliott Brooks (elliottbrooks) which occur on Dev Tools
CI.

TEST=too hard to reliably test this

Change-Id: Ied9cace6bec9505b51ac0142795708466d166f28
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445180
Reviewed-by: Daco Harkes <[email protected]>
Commit-Queue: Slava Egorov <[email protected]>

Author: mraleph

runtime/bin/dartdev.cc

@@ -712,6 +712,7 @@ class DartDev {
     // the result from execution of dartdev.
     {
       MonitorLocker locker(monitor_);
+      exited_ = true;
       locker.Notify();
     }
   }
@@ -779,7 +780,7 @@ class DartDev {
     // proceed.
     {
       MonitorLocker locker(monitor_);
-      while (result_ == DartDev_Result_Unknown) {
+      while (!exited_) {
         locker.Wait();
       }
     }
@@ -940,6 +941,7 @@ class DartDev {
   }
 
   static Monitor* monitor_;
+  static bool exited_;
   static DartDev_Result result_;
   static char* script_name_;
   static char* package_config_override_;
@@ -949,6 +951,7 @@ class DartDev {
 };
 
 Monitor* DartDev::monitor_ = new Monitor();
+bool DartDev::exited_ = false;
 DartDev::DartDev_Result DartDev::result_ = DartDev::DartDev_Result_Unknown;
 char* DartDev::script_name_ = nullptr;
 char* DartDev::package_config_override_ = nullptr;