Introduce Group config option

pmux-example.yml

@@ -35,6 +35,14 @@ processes:
     # process to exit before sending it a SIGKILL (aka a kill -9).
     sigKillWait: 10s
 
+    # group can be used to control the order and grouping of processes as they
+    # shut down.
+    #
+    # Processes will not be shut down until all processes with a higher group
+    # number are already shut down. Processes with the same group number will be
+    # shut down simultaneously.
+    group: 1
+
   # This process will not immediately exit when pmux tells it to do so, but pmux
   # will SIGKILL it after sigKillWait has elapsed.
   stubborn-pinger:

pmuxlib/pmuxlib.go

@@ -68,9 +68,25 @@ func (p *Pmux) Restart(name string) {
 func (p *Pmux) Stop() {
 	var wg sync.WaitGroup
 
-	p.sysLogger.Println("killing child processes")
+	var minGroup, maxGroup int
+
+	for _, proc := range p.processes {
+		if maxGroup < proc.cfg.Group {
+			maxGroup = proc.cfg.Group
+		}
+
+		if minGroup > proc.cfg.Group {
+			minGroup = proc.cfg.Group
+		}
+	}
+
+	for group := maxGroup; group >= minGroup; group-- {
+		p.sysLogger.Printf("killing child processes (group %d)", group)
 		for _, proc := range p.processes {
 			proc := proc
+			if proc.cfg.Group != group {
+				continue
+			}
 
 			wg.Add(1)
 			go func() {
@@ -78,7 +94,8 @@ func (p *Pmux) Stop() {
 				proc.Stop()
 			}()
 		}
-
 		wg.Wait()
+	}
+
 	p.sysLogger.Println("exited gracefully, ciao!")
 }

pmuxlib/process.go

@@ -49,6 +49,14 @@ type ProcessConfig struct {
 	// this function returns. If used with RunProcessOnce and an error is
 	// returned, RunProcessOnce will return that error.
 	StartAfterFunc func(context.Context) error
+
+	// Group can be used to control the order and grouping of processes as they
+	// shut down.
+	//
+	// Processes will not be shut down until all processes with a higher group
+	// number are already shut down. Processes with the same group number will
+	// be shut down simultaneously.
+	Group int
 }
 
 func (cfg ProcessConfig) withDefaults() ProcessConfig {