[PATCH][PR gdb/28874] gdb/remote: switch to added threads if there isn't one already

[Mikaela Szekely <mikaela.szekely@qyriad.me>]

Bug PR gdb/28874 reports a failed assertion when attaching to remote
targets. This assertion is checked after a call to
gdb::observers::new_thread.notify from set_running.
The call to set_running in remote_target::remote_add_thread, however,
appears to always be called before any call to switch_to_thread
for remote targets that have set non-stop off.

This commit proposes a new internal function has_inferior_thread in
gdb/thread.c, which allows remote_target::remote_add_thread to check if
the current thread has been set or not. And thus I have also adjusted
the logic of ::remote_add_thread to call switch_to_thread upon thread
creation if has_inferior_thread returns false.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28874
---
 gdb/gdbthread.h | 3 +++
 gdb/remote.c    | 6 ++++++
 gdb/thread.c    | 7 +++++++
 3 files changed, 16 insertions(+)

diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
index 1a33eb61221..bf230410a12 100644
--- a/gdb/gdbthread.h
+++ b/gdb/gdbthread.h
@@ -875,6 +875,9 @@ class scoped_restore_current_thread
   enum language m_lang;
 };
 
+/* Returns true if there is a current inferior thread. */
+extern bool has_inferior_thread (void);
+
 /* Returns a pointer into the thread_info corresponding to
    INFERIOR_PTID.  INFERIOR_PTID *must* be in the thread list.  */
 extern struct thread_info* inferior_thread (void);
diff --git a/gdb/remote.c b/gdb/remote.c
index aa6a67a96e0..d35387650b3 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -2572,6 +2572,12 @@ remote_target::remote_add_thread (ptid_t ptid, bool running, bool executing,
   else
     thread = add_thread (this, ptid);
 
+  /* If there isn't already an active thread, then switch to the
+     thread we just added, so bare-metal targets don't assert the
+     current thread as null. */
+  if (!has_inferior_thread())
+    switch_to_thread(thread);
+
   /* We start by assuming threads are resumed.  That state then gets updated
      when we process a matching stop reply.  */
   get_remote_thread_info (thread)->set_resumed ();
diff --git a/gdb/thread.c b/gdb/thread.c
index 9d0693bd0b8..e7b418b1c85 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -79,6 +79,13 @@ is_current_thread (const thread_info *thr)
   return thr == current_thread_;
 }
 
+
+bool
+has_inferior_thread (void)
+{
+  return current_thread_ != nullptr;
+}
+
 struct thread_info*
 inferior_thread (void)
 {
-- 
2.35.1