core: OnExit hooks (#6128)

* core: OnExit callbacks

* core: Process-global OnExit callbacks

caddy.go

@@ -715,6 +715,7 @@ func exitProcess(ctx context.Context, logger *zap.Logger) {
 	logger.Warn("exiting; byeee!! 👋")
 
 	exitCode := ExitCodeSuccess
+	lastContext := ActiveContext()
 
 	// stop all apps
 	if err := Stop(); err != nil {
@@ -736,6 +737,16 @@ func exitProcess(ctx context.Context, logger *zap.Logger) {
 		}
 	}
 
+	// execute any process-exit callbacks
+	for _, exitFunc := range lastContext.exitFuncs {
+		exitFunc(ctx)
+	}
+	exitFuncsMu.Lock()
+	for _, exitFunc := range exitFuncs {
+		exitFunc(ctx)
+	}
+	exitFuncsMu.Unlock()
+
 	// shut down admin endpoint(s) in goroutines so that
 	// if this function was called from an admin handler,
 	// it has a chance to return gracefully
@@ -774,6 +785,23 @@ var exiting = new(int32) // accessed atomically
 // EXPERIMENTAL API: subject to change or removal.
 func Exiting() bool { return atomic.LoadInt32(exiting) == 1 }
 
+// OnExit registers a callback to invoke during process exit.
+// This registration is PROCESS-GLOBAL, meaning that each
+// function should only be registered once forever, NOT once
+// per config load (etc).
+//
+// EXPERIMENTAL API: subject to change or removal.
+func OnExit(f func(context.Context)) {
+	exitFuncsMu.Lock()
+	exitFuncs = append(exitFuncs, f)
+	exitFuncsMu.Unlock()
+}
+
+var (
+	exitFuncs   []func(context.Context)
+	exitFuncsMu sync.Mutex
+)
+
 // Duration can be an integer or a string. An integer is
 // interpreted as nanoseconds. If a string, it is a Go
 // time.Duration value such as `300ms`, `1.5h`, or `2h45m`;