fix: data races, resource leaks, and validation gaps

[alexec]

Summary

Fixes a batch of correctness bugs found in a review of the task runner, grouped into three areas. All findings verified; tests pass with go test -race ./....

Concurrency (data races)

• TaskNode Phase/Message/Metrics/cancel were written by task/timer/metrics goroutines while read by the main loop and the HTTP server. Added a status sync.RWMutex with accessors and a locking MarshalJSON; routed every access through them. (-race was failing before this.)
• logWriter.buffer mutated without a lock — the same writer is passed as both stdout and stderr, which os/exec copies on separate goroutines. Added a mutex.
• k8s.pods appended in the informer goroutine while ranged in the metrics goroutine. Guarded with a mutex; GetMetrics snapshots under lock.

Resource leaks

• Probe HTTP response body never closed on the success path.
• SSE /events and /logs handlers never returned on client disconnect (goroutine + memory leak); now select on r.Context().Done(). Broadcast made non-blocking so one slow client can't stall others.
• Docker client closed via defer when Run returned, but the metrics goroutine kept using it → use-after-close. Now closed on ctx-done.
• Host process leaked if Getpgid failed after Start; now killed.
• Envfiles closed per-iteration instead of via deferred close accumulating across the loop.

Correctness

• Env precedence inverted: spec env now overrides the inherited environment (was the other way around).
• EnvVar.Unstring used Split(s,"="), rejecting values containing = (URLs, base64). Now SplitN.
• kit -v could nil-panic when build info is absent; check the ok bool.
• Dependencies on unknown tasks and dependency cycles were not detected (cycle → silent deadlock). Added validation + DFS cycle detection at load time, with a test.
• Probe.URL() nil-derefed when neither TCPSocket nor HTTPGet was set.
• Shutdown could deadlock when goroutines blocked on a full event channel; the main loop now drains events while waiting.

Test plan

• go build ./...
• go vet ./...
• go test -race ./... (passes; added TestDAG_findCycle)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR hardens the task runner by addressing concurrency races, resource leaks, and validation/correctness gaps that could cause hangs, panics, or inconsistent UI/task state reporting.

Changes:

• Adds synchronization around mutable task status/logging and improves shutdown behavior to avoid deadlocks.
• Fixes multiple resource-lifecycle issues (HTTP bodies, SSE/log streaming disconnects, docker client lifetime, envfile handling).
• Adds workflow dependency validation (unknown deps + cycle detection) and tightens parsing/edge-case handling.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
main.go	Avoids nil panic when build info is unavailable (kit -v prints unknown).
internal/types/probe.go	Makes Probe.URL() safe when no action is configured.
internal/types/envfile.go	Prevents deferred-close accumulation by reading envfiles per-iteration via helper.
internal/types/env_var.go	Allows EnvVar values containing = by using SplitN.
internal/terminal.go	Routes task phase reads through synchronized accessor.
internal/task_node.go	Introduces status locking + accessors and a locking MarshalJSON to eliminate task status races.
internal/server.go	Makes SSE broadcast non-blocking and exits /events + /logs on client disconnect.
internal/run.go	Validates unknown deps + cycles; routes task status/cancel/metrics through accessors; drains event channels during shutdown wait.
internal/proc/kubernetes.go	Guards concurrent access to the pod list and snapshots for metrics.
internal/proc/host.go	Fixes env precedence and kills started process when pgid capture fails.
internal/proc/container.go	Defers docker client close until ctx cancellation to avoid use-after-close by metrics goroutine.
internal/probe.go	Closes probe HTTP response bodies on the success path.
internal/log_writer.go	Adds a mutex to avoid data races when the same writer is used for stdout+stderr.
internal/dag.go	Adds DFS-based cycle detection helper.
internal/dag_test.go	Adds a test covering cycle detection.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.