feat: add runner health admission checks (#1090)

Opt-in local task-admission checks under a `health_check` config section (disabled by default):

- pause new task fetching when free disk space on the workspace volume is below the configured minimum
- optional executable health-check script — a non-zero exit, timeout, or start failure marks the runner unavailable
- checks run only while the runner is idle; the last result is reused while a job is active, and polling resumes automatically on recovery
- `/readyz` reports task-admission readiness (reusing the poll loop's last check); `/healthz` stays a process-liveness endpoint

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1090
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/app/run/runner.go

@@ -66,6 +66,13 @@ type Runner struct {
 	runningCount            atomic.Int64
 	lastIdleCleanupUnixNano atomic.Int64
 	now                     func() time.Time
+	healthCheckLast         time.Time
+	healthCheckReady        bool
+	healthCheckReason       string
+	healthStatusSet         bool
+	healthStatusReady       bool
+	healthStatusReason      string
+	runHealthCheck          func(context.Context, string, time.Duration, map[string]string) error
 }
 
 func NewRunner(cfg *config.Config, reg *config.Registration, cli client.Client) *Runner {
@@ -109,13 +116,14 @@ func NewRunner(cfg *config.Config, reg *config.Registration, cli client.Client)
 	envs["GITEA_ACTIONS_RUNNER_VERSION"] = ver.Version()
 
 	runner := &Runner{
-		name:         reg.Name,
-		cfg:          cfg,
-		client:       cli,
-		labels:       ls,
-		envs:         envs,
-		cacheHandler: cacheHandler,
-		now:          time.Now,
+		name:           reg.Name,
+		cfg:            cfg,
+		client:         cli,
+		labels:         ls,
+		envs:           envs,
+		cacheHandler:   cacheHandler,
+		now:            time.Now,
+		runHealthCheck: executeHealthCheck,
 	}
 	return runner
 }
@@ -579,6 +587,67 @@ func (r *Runner) RunningCount() int64 {
 	return r.runningCount.Load()
 }
 
+// CanAcceptTask checks local admission conditions without consuming a task. It is
+// called only from the poll loop, so the cached health fields need no lock.
+func (r *Runner) CanAcceptTask(ctx context.Context) (bool, string) {
+	if !r.cfg.HealthCheck.Enabled {
+		return true, "ok"
+	}
+
+	if r.RunningCount() > 0 {
+		if !r.healthStatusSet {
+			return true, "health checks deferred while jobs are running"
+		}
+		return r.healthStatusReady, r.healthStatusReason
+	}
+
+	if ready, reason := checkFreeDisk(r.cfg); !ready {
+		r.setHealthStatus(ready, reason)
+		return false, reason
+	}
+	ready, reason := r.checkConfiguredHealth(ctx)
+	r.setHealthStatus(ready, reason)
+	return ready, reason
+}
+
+func (r *Runner) setHealthStatus(ready bool, reason string) {
+	r.healthStatusSet = true
+	r.healthStatusReady = ready
+	r.healthStatusReason = reason
+}
+
+// checkFreeDisk evaluates the configured task-admission disk threshold.
+func checkFreeDisk(cfg *config.Config) (bool, string) {
+	root := cfg.Host.WorkdirParent
+	if cfg.Container.BindWorkdir {
+		root = filepath.FromSlash("/" + strings.TrimLeft(cfg.Container.WorkdirParent, "/"))
+	}
+	root = nearestExistingPath(root)
+	available, err := freeDiskBytes(root)
+	if err != nil {
+		return false, fmt.Sprintf("cannot determine free disk space for %s: %v", root, err)
+	}
+	availableMB := available / (1024 * 1024)
+	if availableMB < uint64(cfg.HealthCheck.MinFreeDiskSpaceMB) {
+		return false, fmt.Sprintf("low disk space on %s: %d MiB available, %d MiB required", root, availableMB, cfg.HealthCheck.MinFreeDiskSpaceMB)
+	}
+	return true, "ok"
+}
+
+func nearestExistingPath(path string) string {
+	for path != "" {
+		if _, err := os.Stat(path); err == nil {
+			return path
+		}
+		parent := filepath.Dir(path)
+		if parent == path {
+			return parent
+		}
+		path = parent
+	}
+	return "."
+}
+
 func (r *Runner) Declare(ctx context.Context, labels []string) (*connect.Response[runnerv1.DeclareResponse], error) {
 	return r.client.Declare(ctx, connect.NewRequest(&runnerv1.DeclareRequest{
 		Version:      ver.Version(),