[PATCH 2/3] gdb: add some asserts to thread_db_notice_clone

[Andrew Burgess <aburgess@redhat.com>]

This commit adds some asserts to thread_db_notice_clone, and reorders
some code.  This is really just a refactor ahead of the next commit.
The changes made here are:

  1. Add an assert that STOPPED is not NULL.  We pass STOPPED as the
     first argument to thread_from_lwp, which already assumes that the
     pointer is not NULL.  If STOPPED was ever NULL then we would have
     hit a segfault in thread_from_lwp long ago.

  2. Move the thread_from_lwp call relating to the PARENT thread
     before the call relating to the CHILD thread, and assert that the
     value we get back is STOPPED.  The thread_from_lwp might also
     gather thread-db related information about PARENT, so the call is
     important, but even if PARENT cannot be managed via thread-db,
     the result we get back should always still be STOPPED (see
     record_thread for more details).  Moving this call before the
     call relating to the CHILD thread makes the next commit cleaner.

There should be no user visible changes after this commit as I believe
both of the things being asserted have always been true.
---
 gdb/linux-thread-db.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/gdb/linux-thread-db.c b/gdb/linux-thread-db.c
index cd25c55a898..1807352ed76 100644
--- a/gdb/linux-thread-db.c
+++ b/gdb/linux-thread-db.c
@@ -440,12 +440,17 @@ thread_db_notice_clone (ptid_t parent, ptid_t child)
     return false;
 
   thread_info *stopped = linux_target->find_thread (parent);
-
-  thread_from_lwp (stopped, child);
+  gdb_assert (stopped != nullptr);
 
   /* If we do not know about the main thread's pthread info yet, this
-     would be a good time to find it.  */
-  thread_from_lwp (stopped, parent);
+     would be a good time to find it.  This should return the same
+     thread_info as STOPPED, but a side effect of this call is that the
+     pthread/thread-db information might have been filled in if it was not
+     already known.  */
+  thread_info *parent_info = thread_from_lwp (stopped, parent);
+  gdb_assert (parent_info == stopped);
+
+  thread_from_lwp (stopped, child);
   return true;
 }
 
-- 
2.25.4