Fix initial thread state of non-threaded remote targets

This change fixes the initial state of the main thread of remote
targets which have no concept of threading. Such targets are
treated as single-threaded by gdb, and this single thread needs
to be initially set to the "resumed" state, in the same manner as
threads in thread-aware remote targets (see remote.c,
remote_target::remote_add_thread).

Without this fix, the following assert was triggered on thread-
unaware remote targets:

    remote_target::select_thread_for_ambiguous_stop_reply(const target_waitstatus*): Assertion `first_resumed_thread != nullptr' failed.

The bug can be reproduced using gdbserver

    * by disabling packets 'T' and 'qThreadInfo', or
    * by disabling all thread-related packets.

The test suite has been updated to include these two scenarios, see
gdb.server/stop-reply-no-thread.exp.

Change-Id: I2c39c9de17e8d6922a8c1b9e259eb316a554a43d

4 files changed, 36 insertions, 4 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 27b0e0c..74e0dee1 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,13 @@
+2021-02-25  Jan Matyas  <jmatyas@codasip.com>
+
+	PR gdb/26819
+	* remote.c (remote_target::start_remote): Ensure the single
+	thread, automatically added for remote targets without the
+	concept of threading, is initially in set to the "resumed"
+	state.
+	* remote.c (remote_target::add_current_inferior_and_thread):
+	Add return value - return the main thread.
+
 2021-02-25  Jan Vrany  <jan.vrany@labware.com>
 
 	* gdb/mi/mi-interp.c (mi_traceframe_changed): Remove trailing \n from output.
diff --git a/gdb/remote.c b/gdb/remote.c
index 2c85bdc..ae15f41 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -741,7 +741,7 @@ public: /* Remote specific methods.  */
   int remote_resume_with_vcont (ptid_t ptid, int step,
 				gdb_signal siggnal);
 
-  void add_current_inferior_and_thread (const char *wait_status);
+  thread_info *add_current_inferior_and_thread (const char *wait_status);
 
   ptid_t wait_ns (ptid_t ptid, struct target_waitstatus *status,
 		  target_wait_flags options);
@@ -4409,9 +4409,11 @@ remote_target::get_current_thread (const char *wait_status)
    whose response is a stop reply from which we can also try
    extracting the thread.  If the target doesn't support the explicit
    qC query, we infer the current thread from that stop reply, passed
-   in in WAIT_STATUS, which may be NULL.  */
+   in in WAIT_STATUS, which may be NULL.
 
-void
+   The function returns pointer to the main thread of the inferior. */
+
+thread_info *
 remote_target::add_current_inferior_and_thread (const char *wait_status)
 {
   struct remote_state *rs = get_remote_state ();
@@ -4445,6 +4447,8 @@ remote_target::add_current_inferior_and_thread (const char *wait_status)
      yet.  */
   thread_info *tp = add_thread_silent (this, curr_ptid);
   switch_to_thread_no_regs (tp);
+
+  return tp;
 }
 
 /* Print info about a thread that was found already stopped on
@@ -4800,7 +4804,8 @@ remote_target::start_remote (int from_tty, int extended_p)
 	  /* Target has no concept of threads at all.  GDB treats
 	     non-threaded target as single-threaded; add a main
 	     thread.  */
-	  add_current_inferior_and_thread (wait_status);
+	  thread_info *tp = add_current_inferior_and_thread (wait_status);
+	  get_remote_thread_info (tp)->set_resumed ();
 	}
       else
 	{
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 46d27a1..f2c142e 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,10 @@
+2021-02-25  Jan Matyas  <jmatyas@codasip.com>
+
+	PR gdb/26819
+	* gdb.server/stop-reply-no-thread.exp: Add two test
+	scenarios that cover remote targets which do not have
+	the concept of threads.
+
 2021-02-25  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.fortran/associated.exp: Add missing '-wrap' argument.
diff --git a/gdb/testsuite/gdb.server/stop-reply-no-thread.exp b/gdb/testsuite/gdb.server/stop-reply-no-thread.exp
index 68bf42a..823bdf8 100644
--- a/gdb/testsuite/gdb.server/stop-reply-no-thread.exp
+++ b/gdb/testsuite/gdb.server/stop-reply-no-thread.exp
@@ -112,3 +112,13 @@ foreach_with_prefix to_disable { "" Tthread T } {
 	run_test $to_disable $t_nonstop
     }
 }
+
+# Regression for PR gdb/26819: Cover the case when GDBserver does not report
+# any threads (i.e. the remote target has no concept of threads).
+#
+# Scenario 1: Disable 'T' and 'qfThreadInfo' packets
+# Scenario 2: Disable all threading packets
+foreach_with_prefix to_disable { "T,qfThreadInfo" "threads" } {
+    # Non-stop mode not applicable - off.
+    run_test $to_disable off
+}