React to the debugpyAttach event and try and start a new session; this gets tripped up by the _vimspector.Variables (etc) buffers already existing. Probably need to add the sesssion ID to the buffer name

autoload/vimspector/internal/state.vim

@@ -27,12 +27,10 @@ endif
 function! vimspector#internal#state#Reset() abort
   try
     py3 import vim
-    py3 _vimspector_session_manager = __import__(
+    py3 _vimspector_session =  __import__(
           \ "vimspector",
-          \ fromlist=[ "session_manager" ] ).session_manager.SessionManager()
-
-    py3 _vimspector_session = _vimspector_session_manager =
-          \ _vimspector_session_manager.NewSession( vim.eval( 's:prefix' ) )
+          \ fromlist=[ "session_manager" ] ).session_manager.Get().NewSession(
+          \   vim.eval( 's:prefix' ) )
   catch /.*/
     echohl WarningMsg
     echom 'Exception while loading vimspector:' v:exception

python3/vimspector/custom/python.py

@@ -0,0 +1,46 @@
+# vimspector - A multi-language debugging system for Vim
+# Copyright 2021 Ben Jackson
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from vimspector.debug_session import DebugSession
+from vimspector import session_manager
+
+from typing import Sequence
+
+
+class Debugpy( object ):
+  parent: DebugSession
+  sessions: Sequence[ DebugSession ]
+
+  def __init__( self, debug_session: DebugSession ):
+    self.parent = debug_session
+
+
+  def OnEvent_debugpyAttach( self, message ):
+    # Debugpy sends us the contents of a launch request that we should use. We
+    # probaly just jave to guess the rest
+    launch_argyments = message[ 'body' ]
+    session = session_manager.Get().NewSession( self.parent._api_prefix )
+
+    # Inject the launch config (HACK!). This will actually mean that the
+    # configuration passed below is ignored.
+    session._launch_config = launch_argyments
+
+    # FIXME: We probably do need to add a StartWithLauncArguments and somehow
+    # tell the new session that it shoud not support "Restart" requests ?
+    #
+    # In fact, what even would Reset do... ?
+    session._StartWithConfiguration( self.parent._configuration,
+                                     self.parent._adapter )
+

python3/vimspector/debug_session.py

@@ -1190,9 +1190,10 @@ class DebugSession( object ):
     self._on_init_complete_handlers = []
 
     self._logger.debug( "LAUNCH!" )
-    self._launch_config = {}
-    self._launch_config.update( self._adapter.get( 'configuration', {} ) )
-    self._launch_config.update( self._configuration[ 'configuration' ] )
+    if self._launch_config is None:
+      self._launch_config = {}
+      self._launch_config.update( self._adapter.get( 'configuration', {} ) )
+      self._launch_config.update( self._configuration[ 'configuration' ] )
 
     request = self._configuration.get(
       'remote-request',

python3/vimspector/gadgets.py

@@ -136,7 +136,8 @@ GADGETS = {
           # doesn't support the custom messages)
           # https://github.com/puremourning/vimspector/issues/141
           "subProcess": False,
-        }
+        },
+        'custom_handler': 'vimspector.custom.python.Debugpy'
       }
     },
   },

python3/vimspector/session_manager.py

@@ -15,6 +15,9 @@
 
 from vimspector.debug_session import DebugSession
 
+# Singleton
+_session_manager = None
+
 
 class SessionManager:
   next_session_id = 0
@@ -41,6 +44,13 @@ class SessionManager:
   def GetSession( self, session_id ):
     return self.sessions.get( session_id )
 
-
   def CurrentSession( self ):
     return self.GetSession( self.current_session )
+
+
+def Get():
+  global _session_manager
+  if _session_manager is None:
+    _session_manager = SessionManager()
+
+  return _session_manager

support/test/python/multiprocessing/.vimspector.json

@@ -0,0 +1,24 @@
+{
+  "$schema": "https://puremourning.github.io/vimspector/schema/vimspector.schema.json",
+  "configurations": {
+    "run": {
+      "adapter": "debugpy",
+      "configuration": {
+        "request": "launch",
+        "type": "python",
+        "cwd": "${workspaceRoot}",
+        "program": "${file}",
+        "stopOnEntry": false,
+        "console": "integratedTerminal",
+        "subProcess": true
+      },
+      "breakpoints": {
+        "exception": {
+          "raised": "N",
+          "uncaught": "Y",
+          "userUnhandled": ""
+        }
+      }
+    }
+  }
+}

support/test/python/multiprocessing/multiprocessing_test.py

@@ -0,0 +1,27 @@
+import time
+import multiprocessing as mp
+
+
+def First():
+  for _ in range( 100 ):
+    print( "in first" )
+    time.sleep( 0.1 )
+
+
+def Second():
+  for _ in range( 100 ):
+    print( "in second" )
+    time.sleep( 0.1 )
+
+
+print( "main" )
+p1 = mp.Process( target=First )
+p2 = mp.Process( target=Second )
+
+p1.start()
+p2.start()
+
+p1.join()
+p2.join()
+
+print( "Done" )