feat(python): support attaching to a running debugpy server over TCP - #10
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When a Python process is started with `python -m debugpy --listen <port>`,
the process itself is the DAP server. The previous attach flow would spawn
a second debugpy.adapter process and try to proxy through it, which failed.
This PR adds two fixes:
1. TCP attach mode (useTcpAttach flag)
- New `DapClient.connectTcp(host, port)` static method that opens a
direct TCP connection to a running DAP server instead of spawning
a subprocess.
- New `useTcpAttach` flag on `DapRuntimeConfig`. When set, `attach()`
connects directly over TCP and sends the standard DAP sequence
(initialize → attach → configurationDone) without spawning an adapter.
- `debugpyConfig` sets `useTcpAttach: true`.
2. Race condition fix in initializeAdapter()
- Debugpy fires the DAP `initialized` event during the `initialize`
round-trip, before `waitForInitialized()` had a chance to register
its listener — causing a guaranteed timeout.
- Fixed by registering the `initialized` listener inside
`initializeAdapter()` before sending the `initialize` request, and
storing it as `_initializedPromise` for `waitForInitialized()` to await.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
pldesch-chift
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April 17, 2026 15:19
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Thank you for you submission ! But I prefer seperating the concerns between the client / the transporter / and the planner that decides the strategy to either attach or launch. See #11 I close this one |
waltoss
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Apr 24, 2026
…ctor Refactor DAP attach: transport + connector strategies (replaces #10)
pldesch-chift
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Jun 1, 2026
Replaces the `useTcpAttach` boolean and dual-mode DapClient with two orthogonal abstractions: DapTransport — byte-level I/O (Stdio/Tcp) DapConnector — transport provisioning (SpawnAdapter/TcpAttach) Each DapRuntimeConfig now returns a DapConnectPlan (connector + request args) from launch() and optional attach(). DapSession.launch/attach collapse into a single runHandshake() template, and the debugpy-fires-`initialized`-during-initialize race is now fixed structurally (listener armed before the initialize request is sent) instead of via a separate one-shot promise field. Fixes PR theodo-group#10's issue (adapter-on-adapter when attaching to debugpy): debugpyConfig.attach() returns a TcpAttachConnector, so dbg speaks DAP directly over the socket to the running debugpy — no subprocess in between. Adds an integration test (tests/integration/python/python-attach.test.ts) that spawns `python -m debugpy --listen <port> --wait-for-client`, attaches, and verifies a breakpoint hits. RED on pre-refactor main (times out with the adapter-on-adapter bug), GREEN here. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Problem
dbg attach <port> --runtime pythonwas broken. When called, it would:debugpy.adapterprocessattachrequest to connect to the running debugpy serverThis is adapter-on-top-of-adapter — it doesn't work. The new adapter process exited immediately, causing
DAP adapter process terminated.There was also a separate race condition: debugpy fires the
initializedDAP event during theinitializeround-trip, beforewaitForInitialized()had registered its listener — causing a guaranteed timeout.Solution
1. TCP attach mode (
useTcpAttachflag)Added
DapClient.connectTcp(host, port)— a new static constructor that opens a direct TCP connection to a running DAP server instead of spawning a subprocess. WhenuseTcpAttach: trueis set on a runtime config,attach()uses this path and speaks DAP directly over the socket (the same way VS Code does).debugpyConfignow setsuseTcpAttach: true.Usage:
2. Race condition fix in
initializeAdapter()Moved the
initializedevent listener registration to before sending theinitializerequest, storing it as_initializedPromise.waitForInitialized()now awaits this pre-registered promise instead of creating a new listener after the fact.Files changed
src/dap/client.ts— TCP transport support (connectTcp,readLoopTcp, updateddisconnect/writeMessage)src/dap/runtimes/python.ts— adduseTcpAttach: truesrc/dap/runtimes/types.ts— adduseTcpAttach?: booleantoDapRuntimeConfigsrc/dap/session.ts— TCP attach branch +_initializedPromiserace condition fixTesting
Tested end-to-end against a FastAPI/uvicorn app running under debugpy. Breakpoints hit, variables inspectable, step/continue working.
Note: the existing Python integration tests were already failing on
mainbefore these changes (pre-existing issue unrelated to this PR).