[10/25] Implement DAP attach request

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index d1059e0cb7f..1b967485bc6 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -39020,6 +39020,14 @@  If provided, this is a string that specifies the program to use.  This
 corresponds to the @code{file} command.  @xref{Files}.
 @end table
 
+@value{GDBN} defines a parameter that can be passed to the
+@code{attach} request:
+
+@table @code
+@item pid
+The process ID to which @value{GDBN} should attach.  @xref{Attach}.
+@end table
+
 @node JIT Interface
 @chapter JIT Compilation Interface
 @cindex just-in-time compilation
diff --git a/gdb/python/lib/gdb/dap/launch.py b/gdb/python/lib/gdb/dap/launch.py
index 21499a339e1..a46be1dcca2 100644
--- a/gdb/python/lib/gdb/dap/launch.py
+++ b/gdb/python/lib/gdb/dap/launch.py
@@ -16,7 +16,7 @@ 
 import gdb
 from .events import ExecutionInvoker
 from .server import request, capability
-from .startup import send_gdb, in_gdb_thread
+from .startup import send_gdb, send_gdb_with_response, in_gdb_thread
 
 
 _program = None
@@ -45,6 +45,17 @@  def launch(*, program=None, args=[], env=None, **extra):
         send_gdb(lambda: _set_args_env(args, env))
 
 
+@request("attach")
+def attach(*, pid, **args):
+    # Ensure configurationDone does not try to run.
+    global _program
+    _program = None
+    # Use send_gdb_with_response to ensure we get an error if the
+    # attach fails.
+    send_gdb_with_response("attach " + str(pid))
+    return None
+
+
 @capability("supportsConfigurationDoneRequest")
 @request("configurationDone")
 def config_done(**args):
diff --git a/gdb/testsuite/gdb.dap/attach.c b/gdb/testsuite/gdb.dap/attach.c
new file mode 100644
index 00000000000..24c5283d1e6
--- /dev/null
+++ b/gdb/testsuite/gdb.dap/attach.c
@@ -0,0 +1,25 @@ 
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2011-2023 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/>.  */
+
+#include <unistd.h>
+
+int
+main ()
+{
+  sleep (600);
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.dap/attach.exp b/gdb/testsuite/gdb.dap/attach.exp
new file mode 100644
index 00000000000..5b308f94975
--- /dev/null
+++ b/gdb/testsuite/gdb.dap/attach.exp
@@ -0,0 +1,36 @@ 
+# 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 "attach" in DAP.
+
+require can_spawn_for_attach allow_dap_tests
+
+load_lib dap-support.exp
+
+standard_testfile
+
+if {[build_executable ${testfile}.exp $testfile] == -1} {
+    return
+}
+
+set test_spawn_id [spawn_wait_for_attach $binfile]
+set testpid [spawn_id_get_pid $test_spawn_id]
+
+# We just want to test that attaching works at all.
+if {[dap_attach $testpid] != ""} {
+    dap_shutdown true
+}
+
+kill_wait_spawned_process $test_spawn_id
diff --git a/gdb/testsuite/lib/dap-support.exp b/gdb/testsuite/lib/dap-support.exp
index ead295bdbfe..8667164fa11 100644
--- a/gdb/testsuite/lib/dap-support.exp
+++ b/gdb/testsuite/lib/dap-support.exp
@@ -270,9 +270,22 @@  proc dap_launch {file {args {}} {env {}}} {
     return [dap_check_request_and_response "startup - launch" launch $params]
 }
 
-# Cleanly shut down gdb.  NAME is used as the test name.
-proc dap_shutdown {{name shutdown}} {
-    dap_check_request_and_response $name disconnect
+# Start gdb, send a DAP initialize request, and then an attach request
+# specifying PID as the inferior process ID.  Returns the empty string
+# on failure, or the response object from the attach request.
+proc dap_attach {pid} {
+    if {[_dap_initialize "startup - initialize"] == ""} {
+	return ""
+    }
+    return [dap_check_request_and_response "startup - attach" attach \
+		[format {o pid [i %s]} $pid]]
+}
+
+# Cleanly shut down gdb.  TERMINATE is passed as the terminateDebuggee
+# parameter to the request.
+proc dap_shutdown {{terminate false}} {
+    dap_check_request_and_response "shutdown" disconnect \
+	[format {o terminateDebuggee [l %s]} $terminate]
 }
 
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