Allow creating a NaClAppThread without creating a new host OS thread

src/trusted/service_runtime/linux/thread_suspension.c

Index: src/trusted/service_runtime/linux/thread_suspension.c
diff --git a/src/trusted/service_runtime/linux/thread_suspension.c b/src/trusted/service_runtime/linux/thread_suspension.c
index d00c72bfc67f170eb26486ed467076edf7e3a491..1c2d07d2ba9f7bc348f385998135d9d90022a7ff 100644
--- a/src/trusted/service_runtime/linux/thread_suspension.c
+++ b/src/trusted/service_runtime/linux/thread_suspension.c
@@ -251,7 +251,8 @@ void NaClUntrustedThreadSuspend(struct NaClAppThread *natp,
         NaClLog(LOG_FATAL, "NaClUntrustedThreadSuspend: malloc() failed\n");
       }
     }
-    if (pthread_kill(natp->thread.tid, NACL_THREAD_SUSPEND_SIGNAL) != 0) {
+    CHECK(natp->host_thread_is_defined);

[bsy, 2012/12/17 21:02:05]

if we are trying to make it so that embedders can do other forms of threading,
shouldn't we provide an embedding interface for suspending threads in an
embedding-specific manner?  we should have a proper well defined contract for
what that means, of course.

[Mark Seaborn, 2012/12/17 22:30:14]

For the embedder I'm prototyping at the moment, there's no need
for thread suspension.  The prototype runs on Linux and it just
uses a single thread; untrusted code is now allowed to create new
threads.

But if an embedder does want its thread to be suspendable, it can
set host_thread to the current thread and set
host_thread_is_defined=1.  Before tearing down the NaClAppThread,
it can set host_thread_is_defined=0 to take back
ownership.  (When this functionality is needed, we can add a
function to nacl_threads.h for getting the current thread.)

+    if (pthread_kill(natp->host_thread.tid, NACL_THREAD_SUSPEND_SIGNAL) != 0) {
       NaClLog(LOG_FATAL, "NaClUntrustedThreadSuspend: "
               "pthread_kill() call failed\n");
     }