bionic: Fix wrong kernel_id in pthread descriptor after fork()

After forking, the kernel_id field in the phtread_internal_t returned by pthread_self()
is incorrect --- it's the tid from the parent, not the new tid of the
child.

The root cause is that: currently the kernel_id is set by
_init_thread(), which is called in 2 cases:
(1) called by __libc_init_common(). That happens when the execv( ) is
called after fork( ). But when the zygote tries to fork the android
application, the child application doesn't call execv( ), instread, it
tries to call the Java main method directly.
(2) called by pthread_create(). That happens when a new thread is
created.

For the lead thread which is the thread created by fork(), it should
call execv() but it doesn't, as described in (1) above. So its kernel_id
will inherit the parent's kernel_id.

Fixed it in this patch.

Change-Id: I63513e82af40ec5fe51fbb69456b1843e4bc0fc7
Signed-off-by: Chenyang Du <chenyang.du@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>

libc/bionic/fork.c

@@ -27,6 +27,7 @@
  */
 #include <unistd.h>
 #include "pthread_internal.h"
+#include "bionic_pthread.h"
 #include "cpuacct.h"
 
 extern int  __fork(void);
@@ -48,6 +49,9 @@ int  fork(void)
         __timer_table_start_stop(0);
         __bionic_atfork_run_parent();
     } else {
+        /* Adjusting the kernel id after a fork */
+        (void)__pthread_settid(pthread_self(), gettid());
+
         /*
          * Newly created process must update cpu accounting.
          * Call cpuacct_add passing in our uid, which will take

libc/bionic/pthread.c

@@ -2011,3 +2011,14 @@ pid_t __pthread_gettid(pthread_t thid)
     pthread_internal_t* thread = (pthread_internal_t*)thid;
     return thread->kernel_id;
 }
+
+int __pthread_settid(pthread_t thid, pid_t tid)
+{
+    if (thid == 0)
+        return EINVAL;
+
+    pthread_internal_t* thread = (pthread_internal_t*)thid;
+    thread->kernel_id = tid;
+
+    return 0;
+}

libc/private/bionic_pthread.h

@@ -35,6 +35,7 @@ __BEGIN_DECLS
 
 /* Internal, not an NDK API */
 extern pid_t __pthread_gettid(pthread_t thid);
+extern int __pthread_settid(pthread_t thid, pid_t tid);
 
 __END_DECLS