optionally protect against multiple starts with flock (#1873)

* optionally protect against multiple starts with flock

Fixes #1823

* use traditional .lock extension

* move config key to top level

irc/flock/flock.go

@@ -0,0 +1,24 @@
+//go:build !plan9
+
+package flock
+
+import (
+	"errors"
+
+	"github.com/gofrs/flock"
+)
+
+var (
+	CouldntAcquire = errors.New("Couldn't acquire flock (is another Ergo running?)")
+)
+
+func TryAcquireFlock(path string) (fl Flocker, err error) {
+	f := flock.New(path)
+	success, err := f.TryLock()
+	if err != nil {
+		return nil, err
+	} else if !success {
+		return nil, CouldntAcquire
+	}
+	return f, nil
+}

irc/flock/flock_iface.go

@@ -0,0 +1,14 @@
+package flock
+
+// documentation for github.com/gofrs/flock incorrectly claims that
+// Flock implements sync.Locker; it does not because the Unlock method
+// has a return type (err).
+type Flocker interface {
+	Unlock() error
+}
+
+type noopFlocker struct{}
+
+func (n *noopFlocker) Unlock() error {
+	return nil
+}

irc/flock/flock_plan9.go

@@ -0,0 +1,7 @@
+//go:build plan9
+
+package flock
+
+func TryAcquireFlock(path string) (fl Flocker, err error) {
+	return &noopFlocker{}, nil
+}