[fix] cancel a load() thread when the run's context fires

[vt128]

Problem

A module executed by load() runs on its own thread. The run's timeout (RunWithTimeout / RunWithContext) cancelled only the main thread; the load thread carried the run context as a thread-local but had no cancel watcher, so a long computation inside a loaded module ran to completion and defeated the timeout — bounded only by the step budget, or unbounded when a host relies on the timeout alone (e.g. starbox RunTimeout). Since a loaded module can come from an untrusted scriptFS, this is a reachable DoS: a loaded loop outlives the deadline.

Fix

• The load cache arms a per-load-thread context-cancel watcher for the duration of each module's execution. watchContextCancel is generalized to watchThreadCancelCtx(ctx, thread); the machine exposes watchLoadThread, which reads the load thread's run-context thread-local. Main and load threads share the one context, so a timeout cancels both; nested loads each arm and disarm their own watcher, so no goroutine leaks.
• A load aborted by that cancellation is transient (run-scoped), not a property of the module: doLoad reports it and get() evicts the entry — otherwise a machine reused without Reset would replay the stale cancellation error on the next load of the same module. This mirrors the existing step-budget panic eviction.

Tests

TestMachine_LoadInheritsMachineContext gains two subtests (test-first, both fail before the fix): a loaded module running a long interpreter-level loop under a short RunWithTimeout returns at the deadline instead of running to completion; and a timeout-cancelled load does not poison the cache for a later reuse.

Also

Documents that MakeModuleLoaderFromString/Reader/File execute their (host-supplied) source on a bare thread and do not inherit the Machine's step budget or cancellation — a robustness caveat tracked separately for a thread-aware loader API; use the file path for a Machine-bounded module.

Verification

gofmt / go vet clean; go test -race -count=2, full ./..., and docker golang:1.19 go test -race all green. Part of the STAR-67 follow-up (residual flagged during that fix's review).

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

🟢 Metrics 21 complexity · 0 duplication

Metric	Results
Complexity	21
Duplication	0

View in Codacy

🟢 Coverage 100.00% diff coverage · +0.02% coverage variation

Metric	Results
Coverage variation	✅ +0.02% coverage variation (-1.00%)
Diff coverage	✅ 100.00% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d97f2a1)	7874	7458	94.72%
Head commit (06b926a)	7905 (+31)	7489 (+31)	94.74% (+0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#175)	35	35	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

_{NEW Get contextual insights on your PRs based on Codacy's metrics, along with PR and Jira context, without leaving GitHub. Enable AI reviewer
TIP This summary will be updated as you push new changes.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.53%. Comparing base (d97f2a1) to head (06b926a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
+ Coverage   93.52%   93.53%   +0.01%     
==========================================
  Files          50       50              
  Lines        6298     6314      +16     
==========================================
+ Hits         5890     5906      +16     
  Misses        259      259              
  Partials      149      149              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.