Add return value to DAP scope

A bug report in the DAP specification repository pointed out that it
is typical for DAP implementations to put a function's return value
into the outermost scope.

This patch changes gdb to follow this convention.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31341
Reviewed-By: Kévin Le Gouguec <legouguec@adacore.com>

gdb/python/lib/gdb/dap/events.py

@@ -15,6 +15,7 @@
 
 import gdb
 
+from .scopes import set_finish_value
 from .server import send_event
 from .startup import exec_and_log, in_gdb_thread, log
 from .modules import is_module, make_module
@@ -218,6 +219,8 @@ def _on_stop(event):
     }
     if isinstance(event, gdb.BreakpointEvent):
         obj["hitBreakpointIds"] = [x.number for x in event.breakpoints]
+    if hasattr(event, "details") and "finish-value" in event.details:
+        set_finish_value(event.details["finish-value"])
 
     global _expected_pause
     global _expected_stop_reason

gdb/python/lib/gdb/dap/scopes.py

@@ -25,17 +25,32 @@ from .varref import BaseReference
 frame_to_scope = {}
 
 
+# If the most recent stop was due to a 'finish', and the return value
+# could be computed, then this holds that value.  Otherwise it holds
+# None.
+_last_return_value = None
+
+
 # When the inferior is re-started, we erase all scope references.  See
 # the section "Lifetime of Objects References" in the spec.
 @in_gdb_thread
 def clear_scopes(event):
     global frame_to_scope
     frame_to_scope = {}
+    global _last_return_value
+    _last_return_value = None
 
 
 gdb.events.cont.connect(clear_scopes)
 
 
+@in_gdb_thread
+def set_finish_value(val):
+    """Set the current 'finish' value on a stop."""
+    global _last_return_value
+    _last_return_value = val
+
+
 # A helper function to compute the value of a symbol.  SYM is either a
 # gdb.Symbol, or an object implementing the SymValueWrapper interface.
 # FRAME is a frame wrapper, as produced by a frame filter.  Returns a
@@ -76,7 +91,7 @@ class _ScopeReference(BaseReference):
         result["presentationHint"] = self.hint
         # How would we know?
         result["expensive"] = False
-        result["namedVariables"] = len(self.var_list)
+        result["namedVariables"] = self.child_count()
         if self.line is not None:
             result["line"] = self.line
             # FIXME construct a Source object
@@ -93,6 +108,22 @@ class _ScopeReference(BaseReference):
         return symbol_value(self.var_list[idx], self.frame)
 
 
+# A _ScopeReference that prepends the most recent return value.  Note
+# that this object is only created if such a value actually exists.
+class _FinishScopeReference(_ScopeReference):
+    def __init__(self, *args):
+        super().__init__(*args)
+
+    def child_count(self):
+        return super().child_count() + 1
+
+    def fetch_one_child(self, idx):
+        if idx == 0:
+            global _last_return_value
+            return ("(return)", _last_return_value)
+        return super().fetch_one_child(idx - 1)
+
+
 class _RegisterReference(_ScopeReference):
     def __init__(self, name, frame):
         super().__init__(
@@ -109,6 +140,7 @@ class _RegisterReference(_ScopeReference):
 
 @request("scopes")
 def scopes(*, frameId: int, **extra):
+    global _last_return_value
     global frame_to_scope
     if frameId in frame_to_scope:
         scopes = frame_to_scope[frameId]
@@ -120,10 +152,13 @@ def scopes(*, frameId: int, **extra):
         args = tuple(frame.frame_args() or ())
         if args:
             scopes.append(_ScopeReference("Arguments", "arguments", frame, args))
+        has_return_value = frameId == 0 and _last_return_value is not None
         # Make sure to handle the None case as well as the empty
         # iterator case.
         locs = tuple(frame.frame_locals() or ())
-        if locs:
+        if has_return_value:
+            scopes.append(_FinishScopeReference("Locals", "locals", frame, locs))
+        elif locs:
             scopes.append(_ScopeReference("Locals", "locals", frame, locs))
         scopes.append(_RegisterReference("Registers", frame))
         frame_to_scope[frameId] = scopes

gdb/testsuite/gdb.dap/step-out.c

@@ -0,0 +1,36 @@
+/* Copyright 2024 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/>.  */
+
+struct result
+{
+  int x;
+};
+
+struct result
+function_breakpoint_here (int q)
+{
+  struct result val;
+  val.x = q;
+  return val;
+}
+
+int
+main ()
+{
+  int q = 23;
+  return function_breakpoint_here (q).x - q;
+}

gdb/testsuite/gdb.dap/step-out.exp

@@ -0,0 +1,82 @@
+# Copyright 2024 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 stepOut puts the return value in scope.
+
+require allow_dap_tests
+
+load_lib dap-support.exp
+
+standard_testfile
+
+if {[build_executable ${testfile}.exp $testfile] == -1} {
+    return
+}
+
+if {[dap_initialize] == ""} {
+    return
+}
+
+set obj [dap_check_request_and_response "set breakpoint on function" \
+	     setFunctionBreakpoints \
+	     {o breakpoints [a [o name [s function_breakpoint_here]]]}]
+set fn_bpno [dap_get_breakpoint_number $obj]
+
+dap_check_request_and_response "configurationDone" configurationDone
+
+if {[dap_launch $testfile] == ""} {
+    return
+}
+dap_wait_for_event_and_check "inferior started" thread "body reason" started
+
+dap_wait_for_event_and_check "stopped at function breakpoint" stopped \
+    "body reason" breakpoint \
+    "body hitBreakpointIds" $fn_bpno
+
+dap_check_request_and_response "return from function" stepOut \
+    {o threadId [i 1]}
+dap_wait_for_event_and_check "stopped after return" stopped \
+    "body reason" step
+
+set bt [lindex [dap_check_request_and_response "backtrace" stackTrace \
+		    {o threadId [i 1]}] \
+	    0]
+set frame_id [dict get [lindex [dict get $bt body stackFrames] 0] id]
+
+set scopes [dap_check_request_and_response "get scopes" scopes \
+		[format {o frameId [i %d]} $frame_id]]
+set scopes [dict get [lindex $scopes 0] body scopes]
+
+gdb_assert {[llength $scopes] == 2} "two scopes"
+
+lassign $scopes scope reg_scope
+gdb_assert {[dict get $scope name] == "Locals"} "scope is locals"
+gdb_assert {[dict get $scope presentationHint] == "locals"} \
+    "locals presentation hint"
+gdb_assert {[dict get $scope namedVariables] == 2} "two vars in scope"
+
+set num [dict get $scope variablesReference]
+set refs [lindex [dap_check_request_and_response "fetch arguments" \
+		      "variables" \
+		      [format {o variablesReference [i %d]} $num]] \
+	      0]
+set varlist [lindex [dict get $refs body variables] 0]
+
+gdb_assert {[dict get $varlist variablesReference] > 0} \
+    "variable has children"
+gdb_assert {[dict get $varlist name] == "(return)"} \
+    "variable is return value"
+
+dap_shutdown