[PATCH v4 19/44] gdb, gdbserver: allow setting null_ptid as general thread

When debugging a GPU inferior, we may not have any thread selected.

As a common scenario, when adding a new inferior and attaching to GPU
devices when the host process is just starting, we will have an inferior
representing a GPU device but, since no work has been submitted yet, there
will be no thread, so inferior_ptid == null_ptid.
---
 gdb/remote.c        | 15 ++++++++-------
 gdbserver/server.cc | 17 ++++++-----------
 gdbserver/target.cc |  6 ++++++
 3 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/gdb/remote.c b/gdb/remote.c
index 50e90ad8440..753b59a0819 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -3490,10 +3490,11 @@ remote_target::program_signals (gdb::array_view<const unsigned char> signals)
     }
 }
 
-/* If PTID is MAGIC_NULL_PTID, don't set any thread.  If PTID is
-   MINUS_ONE_PTID, set the thread to -1, so the stub returns the
-   thread.  If GEN is set, set the general thread, if not, then set
-   the step/continue thread.  */
+/* If PTID is MAGIC_NULL_PTID, NULL_PTID, or ANY_THREAD_PTID don't set any
+   thread and let the remote stub select the thread.  If PTID is
+   MINUS_ONE_PTID, set the thread to -1, so the stub returns the thread.
+   If GEN is set, set the general thread, if not, then set the
+   step/continue thread.  */
 void
 remote_target::set_thread (ptid_t ptid, int gen)
 {
@@ -3507,9 +3508,9 @@ remote_target::set_thread (ptid_t ptid, int gen)
 
   *buf++ = 'H';
   *buf++ = gen ? 'g' : 'c';
-  if (ptid == magic_null_ptid)
-    xsnprintf (buf, endbuf - buf, "0");
-  else if (ptid == any_thread_ptid)
+  if (ptid == magic_null_ptid
+      || ptid == null_ptid
+      || ptid == any_thread_ptid)
     xsnprintf (buf, endbuf - buf, "0");
   else if (ptid == minus_one_ptid)
     xsnprintf (buf, endbuf - buf, "-1");
diff --git a/gdbserver/server.cc b/gdbserver/server.cc
index a7e976c0f75..03a40c7a19e 100644
--- a/gdbserver/server.cc
+++ b/gdbserver/server.cc
@@ -4858,14 +4858,8 @@ process_serial_event (void)
 	    {
 	      /* The ptid represents a pid.  */
 	      thread_info *thread = find_any_thread_of_pid (thread_id.pid ());
-
-	      if (thread == NULL)
-		{
-		  write_enn (cs.own_buf);
-		  break;
-		}
-
-	      thread_id = thread->id;
+	      if (thread != nullptr)
+		thread_id = thread->id;
 	    }
 	  else
 	    {
@@ -4885,14 +4879,15 @@ process_serial_event (void)
 		     the currently selected thread is still valid. If
 		     it is not, select the first available.  */
 		  thread_info *thread = find_thread_ptid (cs.general_thread);
-		  if (thread == NULL)
+		  if (thread == nullptr)
 		    thread = get_first_thread ();
-		  thread_id = thread->id;
+
+		  if (thread != nullptr)
+		    thread_id = thread->id;
 		}
 
 	      cs.general_thread = thread_id;
 	      set_desired_thread ();
-	      gdb_assert (current_thread != NULL);
 	    }
 	  else if (cs.own_buf[1] == 'c')
 	    cs.cont_thread = thread_id;
diff --git a/gdbserver/target.cc b/gdbserver/target.cc
index d97d2334344..5f0d9000254 100644
--- a/gdbserver/target.cc
+++ b/gdbserver/target.cc
@@ -34,6 +34,12 @@ bool
 set_desired_thread ()
 {
   client_state &cs = get_client_state ();
+  if (cs.general_thread == null_ptid)
+    {
+      switch_to_thread (nullptr);
+      return false;
+    }
+
   if (cs.general_thread.is_pid ())
     {
       process_info *proc = find_process_pid (cs.general_thread.pid ());
-- 
2.43.0

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