[MeshSync] Unsubscribe exec input on teardown; drop leaked drain goroutine

[leecalcote]

Description

Interactive kubectl exec sessions subscribed to a per-session input.<id> subject (client keystrokes) but could not tear that subscription down, because broker.Handler had no Unsubscribe. streamSession worked around the gap by parking a drain goroutine:

go func() {
    <-done
    for range subCh {}
}()

subCh is never closed, so for range subCh never returns: every exec session leaked a goroutine and a NATS subscription for the process lifetime.

Root cause / fix

• Root cause - the interface gap. MeshKit broker.Handler had no way to release a single subscription short of CloseConnection().
• Fix - MeshKit v1.0.22 (meshery/meshkit#1056) adds Unsubscribe(subject string) error. terminate() (already the single, sync.Once-guarded teardown path for stream EOF/error, explicit stop, and global stop) now calls Unsubscribe("input.<id>"), which releases the subscription and the broker's delivery goroutine. The drain goroutine is removed.
• subCh is given a 1-slot buffer so a delivery already in flight at teardown lands in the buffer instead of blocking the broker's delivery goroutine in the narrow window before Unsubscribe takes effect.

What changed

• go.mod: meshery/meshkit v1.0.20 -> v1.0.22 (schemas bumped transitively).
• meshsync/exec.go: Unsubscribe on teardown via a new unsubscribeSessionInput helper (the testable seam - streamSession itself needs a live SPDY kube client); drain goroutine removed; subCh buffered; stale //nolint:cyclop TODO (hardcoded an out-of-date complexity number) replaced with a rationale.
• meshsync/exec_test.go (new): teardown/goroutine-leak regression test and an error-path test, both under -race, using a recordingBroker that wraps a real ChannelBrokerHandler (records the Unsubscribe subject, delegates the actual teardown) plus a compile-time broker.Handler conformance assertion.
• Docs: docs/agent-instructions/architecture.md gains an Interactive Sessions (exec / log stream) section; docs/design/fd4-jetstream-durable-delivery.md gets a status note - the base Unsubscribe it flagged as an open gap has shipped (as 1-arg Unsubscribe(subject string), not the (subject, queue) it speculated) and this exec leak is fixed; durable JetStream delivery remains the forward work there.

Relationship to #585

Fixes part 2 of #585 (the exec input-subscription / drain-goroutine leak). Part 1 (the channelPool concurrent-map data race) landed in #587. With this, #585 is fully addressed.

Testing

• go test --short ./... -race - all packages pass.
• New TestUnsubscribeSessionInputTearsDownSubscription / TestUnsubscribeSessionInputLogsErrorWithoutPanic pass under -race.
• golangci-lint run --new-from-rev=origin/master ./... - 0 new issues.

Signed commits

• Yes, I signed my commits.

Closes #585.

[gemini-code-assist]

Code Review

This pull request updates the meshkit dependency to v1.0.22 to utilize the new Unsubscribe method on broker.Handler, resolving an exec subscription and goroutine leak during session teardown. Documentation and tests have been updated to reflect and verify this fix. Feedback on the changes highlights a potential nil pointer dereference panic in meshsync/exec.go if subCh is closed and read without checking the channel's open status. Additionally, it is recommended to replace the flaky runtime.NumGoroutine() leak detection in the new tests with a robust library like go.uber.org/goleak.

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[Copilot]

Pull request overview

This PR fixes a lifecycle leak in MeshSync interactive kubectl exec sessions by ensuring the per-session stdin subscription (input.<id>) is explicitly torn down on session termination, leveraging the newly added broker.Handler.Unsubscribe(subject string) error in MeshKit v1.0.22.

Changes:

• Bump MeshKit dependency to v1.0.22 and unsubscribe input.<id> on exec-session teardown (removing the previously leaked drain goroutine).
• Add regression tests around teardown/unsubscribe behavior using a recording broker wrapper.
• Update architecture/design docs to reflect the shipped Unsubscribe capability and the exec leak fix.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
meshsync/exec.go	Unsubscribes exec stdin subject on teardown, removes drain goroutine, buffers input channel.
meshsync/exec_test.go	Adds regression/error-path tests for unsubscribe teardown behavior.
go.mod	Bumps github.com/meshery/meshkit to v1.0.22 (and schemas transitively).
go.sum	Updates dependency checksums for MeshKit v1.0.22 and schemas v1.3.25.
docs/design/fd4-jetstream-durable-delivery.md	Adds status note documenting that base Unsubscribe has shipped and the exec leak is fixed.
docs/agent-instructions/architecture.md	Documents interactive session teardown behavior and session-map separation.

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