Implement DAP "source" request

This implements the DAP "source" request.  I renamed the
"loadedSources" function from "sources" to "loaded_sources" to avoid
any confusion.  I also moved the loadedSources test to the new
sources.exp.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30691

gdb/python/lib/gdb/dap/sources.py

@@ -82,5 +82,27 @@ def _sources():
 
 @request("loadedSources")
 @capability("supportsLoadedSourcesRequest")
-def sources(**extra):
+def loaded_sources(**extra):
     return send_gdb_with_response(_sources)
+
+
+# This helper is needed because we must only access the globals here
+# from the gdb thread.
+@in_gdb_thread
+def _get_source(source):
+    filename = decode_source(source)
+    with open(filename) as f:
+        content = f.read()
+    return {
+        "content": content,
+    }
+
+
+@request("source")
+def source(*, source=None, sourceReference: int, **extra):
+    # The 'sourceReference' parameter is required by the spec, but is
+    # for backward compatibility, which I take to mean that the
+    # 'source' is preferred.
+    if source is None:
+        source = {"sourceReference": sourceReference}
+    return send_gdb_with_response(lambda: _get_source(source))

gdb/testsuite/gdb.dap/basic-dap.exp

@@ -207,7 +207,4 @@ set obj [dap_check_request_and_response "command repl" \
 set response [lindex $obj 0]
 gdb_assert {[dict get $response body result] == 23}
 
-set obj [dap_check_request_and_response sources loadedSources]
-gdb_assert {[string first basic-dap.c $obj] != -1}
-
 dap_shutdown

gdb/testsuite/gdb.dap/sources.c

@@ -0,0 +1,22 @@
+/* Copyright 2023 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/>.  */
+
+int
+main ()
+{
+  /* Distinguishing comment.  */
+}

gdb/testsuite/gdb.dap/sources.exp

@@ -0,0 +1,52 @@
+# Copyright 2023 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 'sources' and 'loadedSources'.
+
+require allow_dap_tests
+
+load_lib dap-support.exp
+
+standard_testfile
+
+if {[build_executable ${testfile}.exp $testfile] == -1} {
+    return
+}
+
+if {[dap_launch $testfile stop_at_main 1] == ""} {
+    return
+}
+
+set obj [dap_check_request_and_response loadedSources loadedSources]
+set ref 0
+foreach src [dict get [lindex $obj 0] body sources] {
+    if {[file tail [dict get $src name]] == "sources.c"} {
+	set ref [dict get $src sourceReference]
+    }	
+}
+
+if {$ref == 0} {
+    fail "sources.c in loadedSources"
+} else {
+    pass "sources.c in loadedSources"
+
+    set obj [dap_check_request_and_response "get source" source \
+		 [format {o sourceReference [i %d]} $ref]]
+    set text [dict get [lindex $obj 0] body content]
+    gdb_assert {[string first "Distinguishing comment" $text] != -1}
+}
+
+
+dap_shutdown