Unable to loop through golang dynamic channels

Question

I want to loop through the menu's options. However, it stops at the first option, since the select without "default:" is blocking and it does not know more options will appear dynamically.

Bellow is the broken code:

package main

import (
    "bytes"
    "fmt"
    "io/ioutil"
    "log"
    "os/exec"
    "strings"
    "time"

    "github.com/getlantern/systray"
    "gopkg.in/yaml.v3"
)

var menuItensPtr []*systray.MenuItem
var config map[string]string
var commands []string

func main() {
    config = readconfig()
    systray.Run(onReady, onExit)
}

func onReady() {
    systray.SetIcon(getIcon("assets/menu.ico"))
    menuItensPtr = make([]*systray.MenuItem,0)
    commands = make([]string,0)
    for k, v := range config {
        menuItemPtr := systray.AddMenuItem(k, k)
        menuItensPtr = append(menuItensPtr, menuItemPtr)
        commands = append(commands, v)
   }
   systray.AddSeparator()
    // mQuit := systray.AddMenuItem("Quit", "Quits this app")
    go func() {
        for {
            systray.SetTitle("My tray menu")
            systray.SetTooltip("https://github.com/evandrojr/my-tray-menu")
            time.Sleep(1 * time.Second)
        }
    }()

    go func() {
        for{
            for i, menuItenPtr := range menuItensPtr {
                

          select { 
/// EXECUTION GETS STUCK HERE!!!!!!!
                case<-menuItenPtr.ClickedCh:
                    execute(commands[i])
                }
            }   
            // select {
            // case <-mQuit.ClickedCh:
            //  systray.Quit()
            //  return
            // // default:
            // }
        }
    }()
}

func onExit() {
    // Cleaning stuff will go here. 
}

func getIcon(s string) []byte {
    b, err := ioutil.ReadFile(s)
    if err != nil {
        fmt.Print(err)
    }
    return b
}

func execute(commands string){
    command_array:= strings.Split(commands, " ")
    command:="" 
    command, command_array = command_array[0], command_array[1:]
    cmd := exec.Command(command, command_array ...)
    var out bytes.Buffer
    cmd.Stdout = &out
    err := cmd.Run()
    if err != nil {
        log.Fatal(err)
    }
    // fmt.Printf("Output %s\n", out.String())
}

func readconfig()  map[string]string{
    yfile, err := ioutil.ReadFile("my-tray-menu.yaml")
    if err != nil {
         log.Fatal(err)
    }
    data := make(map[string]string)
    err2 := yaml.Unmarshal(yfile, &data)
    if err2 != nil {
         log.Fatal(err2)
    }
    for k, v := range data {
         fmt.Printf("%s -> %s\n", k, v)
    }
    return data
}

Bellow is the ugly workaround that works:

package main

import (
    "bytes"
    "fmt"
    "io/ioutil"
    "log"
    "os"
    "os/exec"
    "path/filepath"
    "strings"
    "time"

    "github.com/getlantern/systray"
    "gopkg.in/yaml.v3"
)

var menuItensPtr []*systray.MenuItem
var config map[string]string
var commands []string
var labels []string
var programPath string

func main() {
    setProgramPath()
    config = readconfig()
    time.Sleep(1 * time.Second)
    systray.Run(onReady, onExit)
}

func onReady() {
    systray.SetIcon(getIcon(filepath.Join(programPath,"assets/menu.ico")))
    menuItensPtr = make([]*systray.MenuItem, 0)
    i := 0
    op0 := systray.AddMenuItem(labels[i], commands[i])
    i++
    op1 := systray.AddMenuItem(labels[i], commands[i])
    i++
    op2 := systray.AddMenuItem(labels[i], commands[i])
    i++
    op3 := systray.AddMenuItem(labels[i], commands[i])
    i++

    systray.AddSeparator()
    mQuit := systray.AddMenuItem("Quit", "Quits this app")
    go func() {
        for {
            systray.SetTitle("My tray menu")
            systray.SetTooltip("https://github.com/evandrojr/my-tray-menu")
            time.Sleep(1 * time.Second)
        }
    }()

    go func() {
        for {
            select {
// HERE DOES NOT GET STUCK!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
            case <-op0.ClickedCh:
                execute(commands[0])
            case <-op1.ClickedCh:
                execute(commands[1])
            case <-op2.ClickedCh:
                execute(commands[2])
            case <-op3.ClickedCh:
                execute(commands[3])
            case <-mQuit.ClickedCh:
                systray.Quit()
                return
            }
        }
    }()
}

func onExit() {
    // Cleaning stuff will go here.
}

func getIcon(s string) []byte {
    b, err := ioutil.ReadFile(s)
    if err != nil {
        fmt.Print(err)
    }
    return b
}

func setProgramPath(){
    ex, err := os.Executable()
    if err != nil {
        panic(err)
    }
    programPath = filepath.Dir(ex)
    if err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
}

func execute(commands string) {
    command_array := strings.Split(commands, " ")
    command := ""
    command, command_array = command_array[0], command_array[1:]
    cmd := exec.Command(command, command_array...)
    var out bytes.Buffer
    cmd.Stdout = &out
    err := cmd.Run()
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Output %s\n", out.String())
}

func readconfig() map[string]string {
    yfile, err := ioutil.ReadFile(filepath.Join(programPath,"my-tray-menu.yaml"))
    if err != nil {
        log.Fatal(err)
    }

    data := make(map[string]string)
    err2 := yaml.Unmarshal(yfile, &data)
    if err2 != nil {
        log.Fatal(err2)
    }

    labels = make([]string, 0)
    commands = make([]string, 0)

    for k, v := range data {
        labels = append(labels, k)
        commands = append(commands, v)
        fmt.Printf("%s -> %s\n", k, v)
    }
    fmt.Print(len(labels))
    return data
}

Full source code here: https://github.com/evandrojr/my-tray-menu

Answer 1

select "chooses which of a set of possible send or receive operations will proceed". The spec sets out how this choice is made:

If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the "select" statement blocks until at least one of the communications can proceed.

Your working example:

select {
    case <-op0.ClickedCh:
        execute(commands[0])
    case <-op1.ClickedCh:
        execute(commands[1])
    // ...
}

uses select successfully to choose between one of the offered options. However if you pass a single option e.g.

select { 
    case<-menuItenPtr.ClickedCh:
        execute(commands[i])
    }
}  

The select will block until <-menuItenPtr.ClickedCh is ready to proceed (e.g. something is received). This is effectively the same as not using a select:

<-menuItenPtr.ClickedCh:
execute(commands[i])

The result you were expecting can be achieved by providing a default option:

select { 
    case<-menuItenPtr.ClickedCh:
        execute(commands[i])
    }
    default:
}  

As per the quote from the spec above the default option will be chosen if none of the other options can proceed. While this may work it's not a very good solution because you effectively end up with:

for {
   // Check if event happened (not blocking)           
}

This will tie up CPU time unnecessarily as it continually loops checking for events. A better solution would be to start a goroutine to monitor each channel:

for i, menuItenPtr := range menuItensPtr {
    go func(c chan struct{}, cmd string) {
        for range c { execute(cmd) }
    }(menuItenPtr.ClickedCh, commands[i])
}
// Start another goroutine to handle quit

The above will probably work but does lead to the possibility that execute will be called concurrently (which might cause issues if your code is not threadsafe). One way around this is to use the "fan in" pattern (as suggested by @kostix and in the Rob Pike video suggested by @John); something like:

cmdChan := make(chan int)

for i, menuItenPtr := range menuItensPtr {
    go func(c chan struct{}, cmd string) {
        for range c { cmdChan <- cmd }
    }(menuItenPtr.ClickedCh, commands[i])
}

go func() {
    for {
        select { 
            case cmd := <- cmdChan:
                execute(cmd) // Handle command        
           case <-mQuit.ClickedCh:
                systray.Quit()
                return
        }
    }
}()

note: all code above entered directly into the question so please treat as pseudo code!