[minor] improve error handling in module stream io

modules/examplepersist/examplepersist.go

@@ -43,6 +43,7 @@ func buildResults(e elements, r *modules.Result) (buf []byte, err error) {
 }
 
 var logChan chan string
+var handlerErrChan chan error
 
 func runSomeTasks() {
 	for {
@@ -84,17 +85,23 @@ func requestHandler(p interface{}) (ret string) {
 
 func (r *run) RunPersist(in io.ReadCloser, out io.WriteCloser) {
 	// Create a string channel, used to send log messages up to the agent
-	// from the module tasks
+	// from the module tasks. Functions in the persistent module can
+	// log messages through the agent by writing to this channel.
 	logChan = make(chan string, 64)
-	// Start up an example background task
+	// Create an error channel we will pass to the handlers. Writing an
+	// error to this channel will cause DefaultPersistHandlers() to return
+	// and the module to exit.
+	handlerErrChan = make(chan error, 64)
+	// Start up an example background task we want our module to run
+	// continuously.
 	go runSomeTasks()
 	_ = os.Remove(modules.PersistSockPath("examplepersist"))
 	l, err := net.Listen("unix", modules.PersistSockPath("examplepersist"))
 	if err != nil {
 		panic(err)
 	}
-	go modules.HandlePersistRequest(l, requestHandler)
-	modules.DefaultPersistHandlers(in, out, logChan)
+	go modules.HandlePersistRequest(l, requestHandler, handlerErrChan)
+	modules.DefaultPersistHandlers(in, out, logChan, handlerErrChan)
 }
 
 func (r *run) Run(in io.Reader) (resStr string) {

modules/modules.go

@@ -20,7 +20,9 @@ import (
 	"io"
 	"io/ioutil"
 	"net"
+	"os"
 	"path"
+	"time"
 )
 
 var ModuleRunDir string
@@ -124,16 +126,12 @@ func MakeMessage(class MessageClass, params interface{}, comp bool) (rawMsg []by
 }
 
 func MakeMessageLog(f string, args ...interface{}) (rawMsg []byte, err error) {
-	defer func() {
-		if e := recover(); e != nil {
-			err = fmt.Errorf("Failed to make module log message: %v", e)
-		}
-	}()
 	param := LogParams{Message: fmt.Sprintf(f, args...)}
 	msg := Message{Class: MsgClassLog, Parameters: param}
 	rawMsg, err = json.Marshal(&msg)
 	if err != nil {
-		panic(err)
+		err = fmt.Errorf("Failed to make module log message: %v", err)
+		return
 	}
 	return
 }
@@ -236,7 +234,8 @@ func WatchForStop(r io.Reader, stopChan *chan bool) error {
 	}
 }
 
-func DefaultPersistHandlers(in io.ReadCloser, out io.WriteCloser, logch chan string) {
+func DefaultPersistHandlers(in io.ReadCloser, out io.WriteCloser, logch chan string,
+	errch chan error) {
 	inChan := make(chan Message, 0)
 	go func() {
 		for {
@@ -252,6 +251,21 @@ func DefaultPersistHandlers(in io.ReadCloser, out io.WriteCloser, logch chan str
 		failed := false
 
 		select {
+		case em := <-errch:
+			failed = true
+			// An error occurred somewhere in the persistent module and
+			// we want to exit. Try to write the log message, and also
+			// schedule a hard exit to ensure we do not run into a blocking
+			// scenario during the write.
+			go func() {
+				time.Sleep(time.Second * 5)
+				os.Exit(1)
+			}()
+			logmsg, err := MakeMessageLog("%v", em)
+			if err == nil {
+				WriteOutput(logmsg, out)
+			}
+			os.Exit(1)
 		case s := <-logch:
 			logmsg, err := MakeMessageLog("%v", s)
 			if err != nil {
@@ -289,29 +303,44 @@ func DefaultPersistHandlers(in io.ReadCloser, out io.WriteCloser, logch chan str
 	}
 }
 
-func HandlePersistRequest(l net.Listener, f func(interface{}) string) {
+func HandlePersistRequest(l net.Listener, f func(interface{}) string, errch chan error) {
 	for {
 		conn, err := l.Accept()
 		if err != nil {
-			panic(err)
+			errch <- err
+			return
 		}
 		go func() {
 			var p interface{}
 			err = ReadInputParameters(conn, &p)
 			if err != nil {
-				panic(err)
+				errch <- err
+				return
 			}
 			resp := f(p)
 			WriteOutput([]byte(resp), conn)
 			err = conn.Close()
 			if err != nil {
-				panic(err)
+				errch <- err
+				return
 			}
 		}()
 	}
 }
 
-func SendPersistRequest(p interface{}, modname string) string {
+func SendPersistRequest(p interface{}, modname string) (res string) {
+	defer func() {
+		// If something goes wrong here we will want to format and
+		// return the result an a Result type, with the error
+		// message set.
+		if e := recover(); e != nil {
+			var r Result
+			r.Errors = append(r.Errors, fmt.Sprintf("%v", e))
+			r.Success = false
+			resbuf, _ := json.Marshal(&r)
+			res = string(resbuf)
+		}
+	}()
 	conn, err := net.Dial("unix", PersistSockPath(modname))
 	if err != nil {
 		panic(err)
@@ -328,7 +357,8 @@ func SendPersistRequest(p interface{}, modname string) string {
 	if err != nil {
 		panic(err)
 	}
-	return string(rb)
+	res = string(rb)
+	return
 }
 
 func PersistSockPath(modname string) string {