fix(config): lock config read-modify-write across processes - #960
fix(config): lock config read-modify-write across processes#960PierrunoYT wants to merge 5 commits into
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Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review. WalkthroughConfiguration mutations and MCP commands now use a shared sibling lock file. The lock covers complete read-modify-write workflows, supports cross-process exclusion, propagates release errors, and is validated by concurrent and integration tests. ChangesConfiguration write serialization
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🔵 Low · up to The change serializes configuration mutations across processes and is supported by focused tests and validation, so it is generally mergeable. Owner follow-up is still needed for two setup-script compatibility concerns: failed Go replacement could remove an existing toolchain, and unsupported Node.js versions may be accepted. Sequence Diagram(s)sequenceDiagram
participant Writer
participant LockFile
participant ConfigFile
participant MCPCommand
Writer->>LockFile: acquire configuration lock
MCPCommand->>LockFile: wait for configuration lock
Writer->>ConfigFile: read, modify, and publish configuration
Writer->>LockFile: release configuration lock
MCPCommand->>ConfigFile: read, modify, and publish configuration
MCPCommand->>LockFile: release configuration lock
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✅ Passed checks (4 passed)
Full details: Linked Issues checkExplanation The changes satisfy issue
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In @.agents/setup:
- Around line 29-36: Update the toolchain replacement flow in the setup script
to stage and validate the extracted Go tree under the destination parent, rather
than deleting the existing go_root first. Publish the staged tree atomically
while preserving the prior toolchain, and restore it if publication fails; avoid
relying on a cross-filesystem mv from temp_dir. Ensure rollback removes only
resources created by this run.
- Around line 64-67: Update the Node.js prerequisite checks in .agents/setup
lines 64-67 and .agents/resume lines 9-13 to validate the installed Node.js
major version is at least 18, not merely that node and npm are available; reject
unsupported versions before npm ci in setup and before the ready message in
resume, while preserving the existing failure behavior.
In `@internal/config/lock.go`:
- Line 55: Update the lock-release callback in the relevant mutator flow to
return the error from lock.Release instead of discarding it, and change the
unlock contract accordingly. Ensure mutation methods return the release error
when the mutation succeeds, while preserving any existing mutation error
precedence.
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.agents/resume.agents/setup.gitignoreinternal/config/concurrent_writer_test.gointernal/config/credentials.gointernal/config/export_test.gointernal/config/lock.gointernal/config/writer.go
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Every config mutator loads the whole document, edits independent fields, and publishes a complete replacement by rename. The rename is atomic, so a reader never sees partial JSON — but two processes that loaded the same revision each write a full document, and the second rename silently discards the first one's acknowledged update. The result is valid JSON with one update missing and no error anywhere. lockConfigFile takes a cross-process advisory lock through lockutil, using the retry-with-deadline idiom cron, hooks and oauth already share (10s timeout, 20ms retry). Callers acquire BEFORE their first read, so the lock spans load, mutation, validation and publication and the read inside it is authoritative — holding it only around the write would still let both processes start from the same stale revision. The lock file is a sibling (config.json.lock), never the config itself: an advisory lock is held against an inode, and publishing by rename installs a new one, so locking the config directly would leave each process holding a different inode. Covered: all 14 mutators in writer.go, plus ClearProviderKeyStored and MigratePlaintextProviderKeys in credentials.go. The migration matters most — it rewrites the config on every startup, so it is the likeliest writer to collide with an interactive mutation in another Zero. The SetProviderDescription test seam locks too, so it cannot stand in as the one unsynchronized writer. Two shapes needed care: - The lock is not reentrant. EnsureCatalogProvider scans for an existing profile and then upserts, so UpsertProvider is split into a locking wrapper and upsertProviderLocked; one lock now spans the scan and the upsert, which also closes the window where two processes could both create the same catalog profile. - SetPet edits raw bytes to preserve unknown members and formatting rather than round-tripping the struct. It takes the same lock, so it neither clobbers nor is clobbered by the struct writers. Regression tests, each verified to FAIL with the lock disabled: - TestConcurrentMutationsDoNotLoseUpdates — theme, pet, recaps, favorites and a provider mutated at once; all five are independent fields, so a lost update shows up as a zero value in exactly one of them. - TestConcurrentProviderUpsertsAllSurvive — 16 distinct providers added concurrently, all must be present. - TestConcurrentSameFieldMutationsSerialize — 24 writers contending on one field; every call succeeds and the document stays readable. - TestCrossProcessMutationExcludesAndPreserves — the coordinated two-process case. Goroutines share this process's descriptors, so only a second OS process shows the lock is kernel-held. The child announces itself, then the parent asserts its write cannot land while the lock is held elsewhere, does its own mutation, releases, and requires both updates to survive. Fixes Gitlawb#832 Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01U389qmQUhoZB3YtXkFmiTq
The previous commit locked internal/config's mutators, but that is not every writer of the user config document. internal/cli's MCP editor reads the SAME file, edits it, and republishes it with the identical temp-file+rename shape, at three sites (add/update, remove, disable/enable). Locking only one package left `zero mcp add` free to clobber a concurrent provider or preference write — and to be clobbered by one — with the file still valid JSON afterwards, which is the same silent lost update issue Gitlawb#832 reports. config.LockFile exports the existing helper so the lock is one authority across packages rather than a private detail of internal/config. A second, adjacent implementation would drift from the first exactly the way this writer already drifted from the mutators. TestRunMCPAddParticipatesInConfigLock is the regression. Racing the two writers and waiting to observe a lost update is NOT reliable — the losing interleaving is narrow, and a straightforward concurrent version of this test passed five consecutive runs against the unlocked code, so it would have shipped as reassurance that proved nothing. It instead asserts the deterministic property: while the config lock is held elsewhere, the MCP writer's update cannot land, and it completes once the lock is released. That fails immediately against the unlocked code and passes with it. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01U389qmQUhoZB3YtXkFmiTq
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Inline comments:
In `@internal/cli/mcp_config_lock_test.go`:
- Around line 47-50: Extend the lock regression tests around runWithDeps to
cover mcp add, mcp remove, mcp enable, and mcp disable, including both lock
contention and config.LockFile acquisition failures. Verify each command returns
the expected error and does not proceed with configuration changes.
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internal/cli/mcp_config.gointernal/cli/mcp_config_lock_test.gointernal/config/lock.go
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AGENTS.md requires that a mutator never report success when unlock failed, and
the release error was being dropped on the floor. A failed Release can leave the
lock held for the rest of the process, so returning (cfg, nil) after one claims
a state the next mutation cannot reproduce — it will block for the full lock
timeout and then fail.
lockConfigFile now returns lock.Release directly, and every caller joins it into
its own result with the idiom credstore already uses:
defer func() { err = errors.Join(err, unlock()) }()
Joined, not chosen between: a release failure annotates the result rather than
masking the mutation error that explains what actually went wrong. That covers
all 16 mutators in writer.go, both in credentials.go, and the test seam. The
three MCP sites return an exit code rather than an error, so they convert a
release failure into a crash exit and a stderr message, without overwriting a
failure the command had already reported.
A release failure cannot be provoked through the public API — lockutil.Release
is idempotent and reports nil once released — so the mutators call through a
lockConfigFileFn seam that a test can substitute. My first attempt at this test
could only ever t.Skip, which asserts nothing; the seam is what turns it into a
real assertion.
- TestMutationReportsUnlockFailure: SetTheme surfaces the release error AND the
mutation is still published, since a release failure annotates the result
rather than undoing the write.
- TestMutationErrorSurvivesUnlockFailure: an unknown-provider error and the
release failure are both present in the joined error.
Both verified to fail when the defer discards the release error.
Note for review: cron, hooks, oauth and swarm all still discard their
lockutil.Release error, so this makes internal/config stricter than its
siblings. Worth deciding whether the guideline should be applied to them too —
out of scope here.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U389qmQUhoZB3YtXkFmiTq
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Addressed in Findings 1 and 2 (
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Inline comments:
In `@internal/cli/mcp_config.go`:
- Around line 77-84: Update the command success paths around the deferred unlock
in the MCP configuration handlers to call unlock before writing JSON or text
success output, so unlock failures can change the result before success is
emitted. Retain deferred unlock cleanup for earlier failure paths and apply the
same ordering to all corresponding success branches, including those near the
other reported locations.
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internal/cli/mcp_config.gointernal/config/concurrent_writer_test.gointernal/config/credentials.gointernal/config/export_test.gointernal/config/lock.gointernal/config/writer.go
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Amp-Thread-ID: https://ampcode.com/threads/T-01a043da-1703-70c5-9d8d-904cd8fd964b Co-authored-by: Pierre Bruno <pierrebruno@hotmail.ch>
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Addressed the current MCP lock review findings in
Regression proof: with the old output ordering restored, The two Validation passed:
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.
Inline comments:
In `@internal/cli/mcp_config_lock_test.go`:
- Around line 112-134: Synchronize the lock assertion in the test by wrapping
lockMCPConfigFile with a channel signal and waiting for that signal after
starting runWithDeps. Begin the config-integrity and command-blocking polling
only once lockMCPConfigFile has confirmed participation, while preserving the
existing release and completion checks.
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internal/cli/mcp_config.gointernal/cli/mcp_config_lock_test.go
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Amp-Thread-ID: https://ampcode.com/threads/T-01a0448f-5860-721c-8a47-5119fc57f685 Co-authored-by: Amp <amp@ampcode.com>
Vasanthdev2004
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The core of this is right and I could not break it.
Coverage is complete. I enumerated every primitive that can publish the user config document, writeConfigFile/writeConfigData and writeMCPWritableConfig, and traced every caller. All 15 exported mutators plus ClearProviderKeyStored and MigratePlaintextProviderKeys acquire before their first read, and all three MCP editors go through config.LockFile. No writer bypasses it, and a grep for "config.json" and os.Rename across the repo turns up no fourth one.
No deadlock and no reentrancy. The only in-package nesting is EnsureCatalogProvider into upsertProviderLocked, which does not re-lock. migrateStoredProviderKey touches only the credential store, which has its own lock path, and the ordering is one directional (config to credstore only, since SecureProviderProfile runs as an argument expression before UpsertProvider takes the lock), so there is no inversion to worry about.
Windows behaviour I probed instead of reasoning about. With a live holder in a second process, 3s of continuous acquisition attempts produced zero steals, which makes sense given there is no stale-reclaim protocol to race. Killing the holder released it in about 1ms. A plain read while the lock was held returned immediately, so a wedged writer cannot hang ordinary readers. Against a wedged holder SetTheme returned after 10.02s with config: timed out acquiring config lock for <path> and left the file byte-identical, so it fails closed rather than proceeding unlocked.
I also disabled lockConfigFile to check the tests actually pin the fix. All four new config tests and all four MCP subtests failed, and restoring made them pass. The lock-attempt hook you added in 472bcd1 makes that stronger, since it now asserts the acquisition was attempted as well as that the file stayed untouched.
Three small things, none of which undermine the fix.
The TUI throws away the new failure mode. switchProviderModel does _, _ = on both SetActiveProvider and SetProviderModel. Before this PR those effectively could not fail; now a contended lock is a routine outcome and each can burn the full 10s. Both run on the Bubble Tea update goroutine, so against a wedged holder the UI stops repainting for around 20s and then prints "Switched to ..." for a switch that did not happen, with the old provider and model still there on next launch. Worth surfacing the error, and ideally moving those off the update goroutine.
go doc ./internal/config LockFile prints the wrong function's docs. There is no blank // line between the 32-line rationale written for lockConfigFile and the LockFile paragraph, so the whole block became LockFile's comment and it opens with "lockConfigFile serializes a config read-modify-write across processes". One line fixes it.
TestConcurrentSameFieldMutationsSerialize has no headroom of its own. 24 in-process writers share the production 10s configLockTimeout, which is an absolute deadline over the whole wait rather than per attempt, and any error is fatal. I saw it fail four times in one -count=5 run at roughly 10.3s each while the box was saturated, though I could not reproduce it in about 20 subsequent runs, so treat it as thin rather than broken. Either fewer writers or a test-local timeout would take the timing coupling out.
Two housekeeping notes. The red Windows Smoke is unrelated: it is TestExecCommandForegroundServerReturnsSessionAndServesHTTP in internal/tools, a package this PR does not touch, failing with the known startup race ("server output did not include listening address"). I have kicked a re-run. And this collides with #892 to #895 on internal/config/writer.go and internal/config/credentials.go, so whichever goes in second will need a rebase. Given this one is seven files and self-contained, it is the easier one to land first.
Approving.
3 participants
Summary
Config mutations lose updates when two processes write concurrently. Every mutator loads the whole document, edits its fields, and publishes a complete replacement by rename. The rename is atomic — a reader never sees partial JSON — but two processes that loaded the same revision each write a full document, and the second rename silently discards the first one's acknowledged update. The result is valid JSON with one update missing and no error anywhere.
Fixes #832
The fix
lockConfigFile(internal/config/lock.go) takes a cross-process advisory lock throughlockutil, using the retry-with-deadline idiomcron,hooksandoauthalready share (10s timeout, 20ms retry).Callers acquire before their first read, so the lock spans load → mutation → validation → publication and the read inside it is authoritative. Holding it only around the write would still let both processes start from the same stale revision — which is the whole bug.
The lock file is a sibling (
config.json.lock), never the config itself. An advisory lock is held against an inode, and publishing by rename installs a new one, so locking the config directly would leave each process holding a different inode.lockutilkeeps the sibling's path stable and never removes it, for the same reason.This also serializes goroutines inside one process: each acquisition opens its own file description, so a second in-process attempt contends exactly as another process would.
Coverage
All 14 mutators in
writer.go, plusClearProviderKeyStoredandMigratePlaintextProviderKeysincredentials.go.MigratePlaintextProviderKeysmatters most: it rewrites the config on every startup, so it is the likeliest writer to collide with an interactive mutation in another Zero. TheSetProviderDescriptiontest seam locks too, so it cannot stand in as the one unsynchronized writer in a future concurrency test.Two shapes that needed care
The lock is not reentrant.
EnsureCatalogProviderscans for an existing profile and then upserts. Going through the publicUpsertProviderwould have spun in the retry loop until the 10s deadline and then failed.UpsertProvideris split into a locking wrapper plusupsertProviderLocked, and one lock now spans the scan and the upsert — which additionally closes the window where two processes could both create the same catalog profile.SetPetedits raw bytes rather than round-tripping the struct, to preserve unknown members and existing formatting. It takes the same lock, so it neither clobbers nor is clobbered by the struct writers. It is deliberately included in the mixed-mutation regression below for that reason.Regression tests
Each was verified to fail with the lock disabled and pass with it.
TestConcurrentMutationsDoNotLoseUpdatesTestConcurrentProviderUpsertsAllSurviveTestConcurrentSameFieldMutationsSerializeTestCrossProcessMutationExcludesAndPreservesOn the cross-process test. Goroutines share this process's descriptors, so only a second OS process shows the lock is held by the kernel rather than by in-process state. My first version of this test was worthless — it passed with and without the lock, because child-process startup latency meant the child usually read after the parent had already written. It is now written so the child announces itself before contending, and the parent asserts the child's write provably cannot land while the lock is held elsewhere, then mutates, releases, and requires both updates to survive. Deterministic in the passing direction rather than a scheduling coin flip.
Validation
go build ./...,go vet ./...,gofmtcleango test ./internal/config/ -count=1— greengo test ./...— onlyinternal/clifails, with 15 pre-existing ambient-config failures (no active provider configured: active provider "chatgpt" not found). I diffed the failing test names against agit stashbaseline of this same tree: identical set, so none of them come from this change.Note on scope
This implements the issue's primary suggestion (hold a cross-process lock across the whole transaction, re-read inside it). It does not add the optional generation/revision field — with the lock spanning load-through-publish there is no stale-writer window left for a revision check to catch, and adding one would change the on-disk schema. Happy to add it if you would rather have defence in depth against a future caller that mutates without the lock.
Summary by CodeRabbit
Bug Fixes
Tests