[RFC PATCH 1/8] gdb, infrun: do not discard a step-completed pending waitstatus

[Markus Metzger <markus.t.metzger@intel.com>]

Consider a scenario with two breakpoints on adjacent instructions:

    bp1 at 0xf00
    bp2 at 0xf01

as well as two threads in all-stop-on-top-of-non-stop mode.

Assume that threads A hits bp1 and we report the breakpoint hit to the
user.  When the user continues, we start a step-over for thread A at 0xf00.

Assume that thread B now hits bp1 and we report the breakpoint hit to the
user.  We stop all threads to report the event.  Meanwhile, the step-over
of thread A completes, so we save the pending waitstatus (stop_pc=0xf01,
currently_stepping=1) of thread A.

When the user continues, clear_proceed_status_thread() discards the
pending step completed waitstatus of thread A, and proceed() starts
another step-over for thread A at 0xf01.

We skip bp2 for thread A.

Remove the code in clear_proceed_status_thread() that discards a step
completed waitstatus and let it get handled normally.  This involves
calling finish_step_over() to clean up displaced stepping state.

This introduces an additional step completed stop event for thread A that
results in two user-visible changes in behavior when switching to thread A
and resuming that thread:

  - stepi/nexti completes without making progress

  - adding a breakpoint at the current location before resuming hits that
    breakpoint

Fixes gdb/34380.

Tested-By: Guinevere Larsen <guinevere@redhat.com>
---
 gdb/infrun.c                              | 25 ++-----
 gdb/testsuite/gdb.threads/adjacent-bp.c   | 48 +++++++++++++
 gdb/testsuite/gdb.threads/adjacent-bp.exp | 86 +++++++++++++++++++++++
 3 files changed, 139 insertions(+), 20 deletions(-)
 create mode 100644 gdb/testsuite/gdb.threads/adjacent-bp.c
 create mode 100644 gdb/testsuite/gdb.threads/adjacent-bp.exp

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 47966cf72db..a70d88cd4a7 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -3100,28 +3100,13 @@ clear_proceed_status_thread (struct thread_info *tp)
   infrun_debug_printf ("%s", tp->ptid.to_string ().c_str ());
   gdb_assert (tp->internal_state () != THREAD_INT_RUNNING);
 
-  /* If we're starting a new sequence, then the previous finished
-     single-step is no longer relevant.  */
   if (tp->has_pending_waitstatus ())
     {
-      if (tp->stop_reason () == TARGET_STOPPED_BY_SINGLE_STEP)
-	{
-	  infrun_debug_printf ("pending event of %s was a finished step. "
-			       "Discarding.",
-			       tp->ptid.to_string ().c_str ());
-
-	  tp->set_internal_state (THREAD_INT_STOPPED);
-	  tp->clear_pending_waitstatus ();
-	  tp->set_stop_reason (TARGET_STOPPED_BY_NO_REASON);
-	}
-      else
-	{
-	  infrun_debug_printf
-	    ("thread %s has pending wait status %s (currently_stepping=%d).",
-	     tp->ptid.to_string ().c_str (),
-	     tp->pending_waitstatus ().to_string ().c_str (),
-	     tp->control.currently_stepping);
-	}
+      infrun_debug_printf
+	("thread %s has pending wait status %s (currently_stepping=%d).",
+	 tp->ptid.to_string ().c_str (),
+	 tp->pending_waitstatus ().to_string ().c_str (),
+	 tp->control.currently_stepping);
     }
 
   /* If this signal should not be seen by program, give it zero.
diff --git a/gdb/testsuite/gdb.threads/adjacent-bp.c b/gdb/testsuite/gdb.threads/adjacent-bp.c
new file mode 100644
index 00000000000..e72e674f52a
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/adjacent-bp.c
@@ -0,0 +1,48 @@
+/* Copyright 2026 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <pthread.h>
+#include <unistd.h>
+
+static  pthread_barrier_t barrier;
+
+static void *
+test (void *arg)
+{
+  pthread_barrier_wait (&barrier);
+  int a = 0;  /* break here.  */
+  int b = 0;
+  int c = 0;
+  return arg;
+}
+
+int
+main ()
+{
+  pthread_t th;
+
+  alarm (500);
+
+  pthread_barrier_init (&barrier, NULL, 2);
+  pthread_create (&th, NULL, test, NULL);
+  test (NULL);
+
+  pthread_join (th, NULL);
+  pthread_barrier_destroy (&barrier);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/adjacent-bp.exp b/gdb/testsuite/gdb.threads/adjacent-bp.exp
new file mode 100644
index 00000000000..9e7782c01c2
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/adjacent-bp.exp
@@ -0,0 +1,86 @@
+# Copyright 2026 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test that GDB does not skip a breakpoint when a step-over completes
+# while another event leads to a stop.
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" ${testfile} ${srcfile} \
+	  {debug pthreads}]} {
+    return
+}
+
+if {![runto_main]} {
+    return
+}
+
+# Find a sequence of adjacent instructions.
+set bp_line [gdb_get_line_number "break here"]
+set pcs {}
+gdb_test_multiple "info line $bp_line" "" {
+    -re -wrap "starts at address ($hex).*" {
+	pass $gdb_test_name
+
+	set line "\\s+($hex) \[^\r\n\]+"
+	gdb_test_multiple "x/3i $expect_out(1,string)" "disassemble" {
+	    -re -wrap "$line\r\n$line\r\n$line.*" {
+		pass $gdb_test_name
+
+		lappend pcs $expect_out(1,string)
+		lappend pcs $expect_out(2,string)
+		lappend pcs $expect_out(3,string)
+	    }
+	    -re -wrap "" {
+		fail $gdb_test_name
+	    }
+	}
+    }
+    -re -wrap "" {
+	fail $gdb_test_name
+    }
+}
+
+# Set breakpoints on adjacent instructions.
+foreach pc $pcs {
+    gdb_breakpoint "\*$pc"
+}
+
+# Continue from breakpoint to breakpoint.
+set hits [dict create]
+set iter 0
+gdb_test_multiple "continue" "" {
+    -re -wrap "hit Breakpoint.*" {
+	dict incr hits [get_hexadecimal_valueof "\$pc" invalid "stop $iter"]
+	incr iter
+	send_gdb "continue\n"
+	exp_continue
+    }
+    -re -wrap "$inferior_exited_re normally.*" {
+	pass "$gdb_test_name"
+    }
+}
+
+# We expect all breakpoints to be hit by both threads.
+foreach pc $pcs {
+    if {[dict exists $hits $pc]} {
+	gdb_assert {[dict get $hits $pc] eq 2} "breakpoint at $pc"
+	dict unset hits $pc
+    } else {
+	fail "breakpoint at $pc"
+    }
+}
+# And no unrelated stops.
+gdb_assert {[dict size $hits] eq 0} "no extra stops"
-- 
2.53.0

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