Fix pthread_join.

Let the kernel keep pthread_internal_t::tid updated, including
across forks and for the main thread. This then lets us fix
pthread_join to only return after the thread has really exited.

Also fix the thread attributes of the main thread so we don't
unmap the main thread's stack (which is really owned by the
dynamic linker and contains things like environment variables),
which fixes crashes when joining with an exited main thread
and also fixes problems reported publicly with accessing environment
variables after the main thread exits (for which I've added a new
unit test).

In passing I also fixed a bug where if the clone(2) inside
pthread_create(3) fails, we'd unmap the child's stack and TLS (which
contains the mutex) and then try to unlock the mutex. Boom! It wasn't
until after I'd uploaded the fix for this that I came across a new
public bug reporting this exact failure.

Bug: 8206355
Bug: 11693195
Bug: https://code.google.com/p/android/issues/detail?id=57421
Bug: https://code.google.com/p/android/issues/detail?id=62392
Change-Id: I2af9cf6e8ae510a67256ad93cad891794ed0580b

1 files changed, 3 insertions, 1 deletions

diff --git a/libc/private/bionic_tls.h b/libc/private/bionic_tls.h
index 094c71c..ff13fdb 100644
--- a/libc/private/bionic_tls.h
+++ b/libc/private/bionic_tls.h
@@ -51,7 +51,9 @@ enum {
   TLS_SLOT_THREAD_ID,
   TLS_SLOT_ERRNO,
 
-  /* This slot is used when starting a new thread, before any code that needs errno runs. */
+  /* This slot in the child's TLS is used to synchronize the parent and child
+   * during thread initialization. The child finishes with this mutex before
+   * running any code that can set errno, so we can reuse the errno slot. */
   TLS_SLOT_START_MUTEX = TLS_SLOT_ERRNO,
 
   /* These two aren't used by bionic itself, but allow the graphics code to