fix: suppress broker command resends instead of running the work twice - #1361
fix: suppress broker command resends instead of running the work twice#1361ananttheant wants to merge 2 commits into
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ChangesBroker resend suppression
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to This change prevents reconnect resends from executing an in-flight command twice by reusing the original completion, with focused regression coverage. A narrow completion-versus-resend race remains outside this change’s scope, but it does not create an actionable merge blocker; the PR is merge-ready after normal checks. Sequence Diagram(s)sequenceDiagram
participant ClientA
participant ClientB
participant StdioBridgeHost
participant commandQueue
ClientA->>StdioBridgeHost: send command
StdioBridgeHost->>commandQueue: enqueue command with ClientA owner
ClientB->>StdioBridgeHost: resend identical command
StdioBridgeHost->>commandQueue: find matching in-flight command
StdioBridgeHost-->>ClientB: await original completion task
commandQueue-->>StdioBridgeHost: complete original command
StdioBridgeHost-->>ClientA: return command result
StdioBridgeHost-->>ClientB: return shared command result
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Linked Issues checkExplanation The implementation satisfies the primary objective in issue Full details: Out of Scope Changes checkExplanation The code, tests, Unity metadata, queue diagnostic accessor, and logging changes are directly related to broker-resend suppression and regression coverage for issue Full details: Description checkExplanation The description clearly explains the duplicate-command bug, the bridge-side fix, scope limitations, testing, compatibility details, related issue, and documentation status. Required template sections are present and sufficiently complete.
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In `@MCPForUnity/Editor/Services/Transport/Transports/StdioBridgeHost.cs`:
- Line 646: The reconnect resend path around FindBrokerResendTarget and
ExecuteQueuedCommand must preserve completion state using a stable request
identifier rather than payload text. Retain completed responses by identifier
until acknowledged by the broker or removed after a bounded expiry, and have
resend handling return the retained response instead of enqueueing and executing
the command again.
In
`@TestProjects/UnityMCPTests/Assets/Tests/EditMode/Services/StdioBrokerResendTests.cs`:
- Around line 129-131: Update ReadQueueDepth to read commandQueue while holding
the same bridge lock used by HandleClientAsync, preferably by exposing a locked
diagnostic helper on the bridge and calling it from the test; do not access the
underlying Dictionary directly outside that synchronization.
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MCPForUnity/Editor/Services/Transport/Transports/StdioBridgeHost.csTestProjects/UnityMCPTests/Assets/Tests/EditMode/Services/StdioBrokerResendTests.csTestProjects/UnityMCPTests/Assets/Tests/EditMode/Services/StdioBrokerResendTests.cs.meta
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The test sampled Dictionary.Count directly by reflection while listener tasks could be adding entries under lockObj, and Dictionary does not support concurrent reads and writes. Sleeping on the main thread stops ProcessCommands from draining the queue but does nothing to synchronise against the socket threads that fill it. Replaces the reflection with an internal QueuedCommandCount accessor that takes lockObj, documented as diagnostics-only. Addresses CodeRabbit review on CoplayDev#1361.
When a command outlives the broker's patience, the Python side closes the socket, reconnects and resends the same payload. The bridge queued that resend as a brand-new command behind the still-executing original, so the work ran twice — visibly so for commands with side effects such as a long ExecuteMenuItem or a build. The enqueue path now looks for an in-flight command with the same payload that was queued by a different connection, and attaches the new caller to that command's completion source rather than queueing a copy. Comparing the owning connection as well as the payload is what keeps this safe: a single connection handles one command at a time, so two identical payloads on one connection are sequential and genuinely distinct requests. QueuedCommand gains an Owner field, held for identity comparison only and never dereferenced. Closes CoplayDev#1130
The test sampled Dictionary.Count directly by reflection while listener tasks could be adding entries under lockObj, and Dictionary does not support concurrent reads and writes. Sleeping on the main thread stops ProcessCommands from draining the queue but does nothing to synchronise against the socket threads that fill it. Replaces the reflection with an internal QueuedCommandCount accessor that takes lockObj, documented as diagnostics-only. Addresses CodeRabbit review on CoplayDev#1361.
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Description
When a command outlives the broker's patience,
send_command_with_retryon the Python side closes the socket, rediscovers the port, reconnects and resends the same payload. The bridge queued that resend as a brand-newcommandIdbehind the still-executing original, so the work ran twice. For a read that is merely wasteful; forexecute_menu_itemor a build it duplicates real side effects, which is what #1130 reports.The enqueue path now checks for an in-flight command with the same payload that was queued by a different connection, and attaches the new caller to that command's
TaskCompletionSourceinstead of queueing a copy. The resent request still gets a real answer — the original's — it just doesn't cause the work to happen again.Comparing the owning connection as well as the payload is what makes this safe. A single connection handles one command at a time (the read loop awaits each response before reading the next frame), so two identical payloads on one connection are necessarily sequential and genuinely distinct requests, and are never collapsed. Only a payload that reappears on a second connection while the first is still in flight matches — which is precisely the reconnect-and-resend signature, and stdio is single-agent by design (a new connection already closes stale ones).
Type of Change
Changes Made
QueuedCommandgains anOwnerfield, held for identity comparison only and never dereferenced.StdioBridgeHost.IsBrokerResend(...)pure predicate, following the existingShouldAbandonBusyPort/ShouldKeepWaitingForReadypattern, plusFindBrokerResendTargetwhich applies it across the queue underlockObj.Known limitation
This narrows the window rather than closing it completely, and it is worth being explicit about where it stops.
Suppression only applies while the original command is still in
commandQueue. If the original finishes in the gap between the broker's new connection closing the stale socket and the resend arriving,ExecuteQueuedCommandhas already removed the entry, so the resend queues and runs a second time — and the original's response has nowhere to go.Closing that remaining window needs a stable request identifier that survives a reconnect, with the completed response retained under it until the broker collects it or a bounded expiry passes. The broker does not send such an identifier today, so that is a wire-protocol change across both
Server/srcand the bridge, which felt well outside this fix.I deliberately did not approximate it by retaining completed responses keyed on payload text. Payload alone cannot distinguish a late resend from a genuinely new identical request, so that would risk answering a real request with a stale cached response. The in-flight + different-connection test used here cannot make that mistake: a single connection processes commands sequentially, so an overlapping identical payload can only come from a reconnect.
So this covers the reported case — a long-running command still executing when the broker gives up and resends — without introducing a false-positive risk. Happy to follow up with the request-identifier work if you want the protocol change.
Testing/Screenshots/Recordings
Five new tests: four covering the predicate's branches, and one end-to-end regression test that drives real sockets against the live bridge.
I wrote the regression test first and confirmed it fails on
beta:and passes with the fix. Full EditMode suite on 2022.3.62f1 with
UNITY_MCP_ALLOW_BATCH=1: 1126 passed, 0 failed (64 pre-existing ignores).Two notes on how that test is built, since both were dead ends first:
commandQueueby reflection, rather than on a command's side effect. That keeps the assertion about the thing the fix changes, makes it deterministic, and lets the same test compile and run against builds with and without the fix.Compatibility / Package Source
file:(local clone, branchfix/stdio-suppress-broker-resendoffbeta)Packages/packages-lock.json: n/a (file reference)Documentation Updates
Related Issues
Closes #1130
Additional Notes
The regression test only runs where the bridge is up, so like the neighbouring
StdioBridgeReconnectTestsitAssert.Ignores otherwise — which means it is skipped by the standard editmode CI job, where batchmode does not arm the bridge withoutUNITY_MCP_ALLOW_BATCH. The four predicate tests run everywhere. If you would like the socket test to run in CI, settingUNITY_MCP_ALLOW_BATCH=1on the editmode job would also un-skip the existing reconnect tests, which are currently dormant there — happy to do that in a separate PR since it changes what CI covers.The issue also proposes cancelling the command on timeout. I did not go that route:
ExecuteMenuItem's call is synchronous and non-cancellable, so cancellation could not stop work already running, and it would only prevent future duplicates — the same outcome as this change for considerably more surface area. The issue text also refers to a 30s frame timeout, which is now 5 minutes and configurable viaUNITY_MCP_STDIO_COMMAND_TIMEOUT_MS; that raises the bar for reproduction but does not change the underlying defect.Summary by CodeRabbit
Bug Fixes
Tests