Introduce GDB_THREAD_OPTION_EXIT thread option, fix step-over-thread-exit

When stepping over a breakpoint with displaced stepping, GDB needs to
be informed if the stepped thread exits, otherwise the displaced
stepping buffer that was allocated to that thread leaks, and this can
result in deadlock, with other threads waiting for their turn to
displaced step, but their turn never comes.

Similarly, when stepping over a breakpoint in line, GDB also needs to
be informed if the stepped thread exits, so that is can clear the step
over state and re-resume threads.

This commit makes it possible for GDB to ask the target to report
thread exit events for a given thread, using the new "thread options"
mechanism introduced by a previous patch.

This only adds the core bits.  Following patches in the series will
teach the Linux backends (native & gdbserver) to handle the
GDB_THREAD_OPTION_EXIT option, and then a later patch will make use of
these thread exit events to clean up displaced stepping and inline
stepping state properly.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I96b719fdf7fee94709e98bb3a90751d8134f3a38
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27338

2 files changed, 5 insertions, 0 deletions

diff --git a/gdb/target/target.c b/gdb/target/target.c
index 3af7d73..58d0f63 100644
--- a/gdb/target/target.c
+++ b/gdb/target/target.c
@@ -196,6 +196,7 @@ to_string (gdb_thread_options options)
 {
   static constexpr gdb_thread_options::string_mapping mapping[] = {
     MAP_ENUM_FLAG (GDB_THREAD_OPTION_CLONE),
+    MAP_ENUM_FLAG (GDB_THREAD_OPTION_EXIT),
   };
   return options.to_string (mapping);
 }
diff --git a/gdb/target/target.h b/gdb/target/target.h
index 2691f92..bad4daa 100644
--- a/gdb/target/target.h
+++ b/gdb/target/target.h
@@ -34,6 +34,10 @@ enum gdb_thread_option : unsigned
   /* Tell the target to report TARGET_WAITKIND_THREAD_CLONED events
      for the thread.  */
   GDB_THREAD_OPTION_CLONE = 1 << 0,
+
+  /* Tell the target to report TARGET_WAITKIND_THREAD_EXIT events for
+     the thread.  */
+  GDB_THREAD_OPTION_EXIT = 1 << 1,
 };
 
 DEF_ENUM_FLAGS_TYPE (enum gdb_thread_option, gdb_thread_options);