fix!: guard against two runner processes sharing one runner file (#1099)

Starting two runner daemons with the same `.runner` file makes both present an
identical UUID+token, so Gitea treats them as one runner and they cancel each
other's jobs. This adds a non-blocking advisory lock on a sibling
`<runner-file>.lock`: the daemon (and `register`) acquire it at startup, and a
second process on the same host fails fast with a clear error instead of silently
interfering. The OS releases the lock when the process exits — including a hard
kill — so no stale lock is left behind. Legitimate multi-runner setups are
unaffected since each already uses its own `runner.file`.

Note: this covers the common single-host case; two hosts sharing a copied
`.runner` (e.g. over NFS) would still need server-side detection in Gitea.

---------

Co-authored-by: Zettat123 <zettat123@gmail.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1099
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

internal/pkg/lock/lock_test.go

@@ -0,0 +1,54 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !plan9
+
+package lock
+
+import (
+	"errors"
+	"path/filepath"
+	"testing"
+)
+
+func TestTryLock(t *testing.T) {
+	runnerFile := filepath.Join(t.TempDir(), ".runner")
+
+	release, err := TryLock(runnerFile)
+	if err != nil {
+		t.Fatalf("first TryLock failed: %v", err)
+	}
+
+	// A second lock on the same file must be refused while the first is held.
+	if _, err := TryLock(runnerFile); !errors.Is(err, ErrLocked) {
+		t.Fatalf("second TryLock: want ErrLocked, got %v", err)
+	}
+
+	if err := release(); err != nil {
+		t.Fatalf("release failed: %v", err)
+	}
+
+	// After release the lock is available again.
+	release2, err := TryLock(runnerFile)
+	if err != nil {
+		t.Fatalf("TryLock after release failed: %v", err)
+	}
+	if err := release2(); err != nil {
+		t.Fatalf("second release failed: %v", err)
+	}
+}
+
+// TestTryLockUncreatable ensures a lock file that cannot be created reports a
+// non-ErrLocked error, so callers can tell "already locked by another process"
+// apart from "couldn't lock" and degrade gracefully (e.g. a read-only mount).
+func TestTryLockUncreatable(t *testing.T) {
+	runnerFile := filepath.Join(t.TempDir(), "missing-dir", ".runner")
+
+	_, err := TryLock(runnerFile)
+	if err == nil {
+		t.Fatal("TryLock on an uncreatable lock file: want error, got nil")
+	}
+	if errors.Is(err, ErrLocked) {
+		t.Fatal("TryLock on an uncreatable lock file: want non-ErrLocked error, got ErrLocked")
+	}
+}