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

1 files changed, 10 insertions, 0 deletions

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
+}