Make it capable of detecting process automatically

main.go

@@ -102,27 +102,31 @@ func (c *cli) run(args []string) int {
 		return 1
 	}
 	if c.list {
-		ps, err := process.List()
+		ps, err := process.FindAll()
 		if err != nil {
-			fmt.Fprintf(c.stderr, "failed to list processes: %w", err)
+			fmt.Fprintf(c.stderr, "failed to list processes: %w\n", err)
 			return 1
 		}
 		fmt.Fprintf(c.stderr, "%v", ps)
 		return 0
 	}
 
-	// Try to run diagnoser
-	var addr *net.TCPAddr
+	var pid string
 	if len(args) == 0 {
-		// TODO: Make sure to use the process where the agent runs on.
-		addr = &net.TCPAddr{}
-	} else {
-		var err error
-		addr, err = targetToAddr(args[0])
+		// Automatically finds the process where the agent runs on if no args given.
+		p, err := process.FindOne()
 		if err != nil {
-			fmt.Fprintf(c.stderr, "failed to convert args into addresses: %v\n", err)
+			fmt.Fprintln(c.stderr, err)
 			return 1
 		}
+		pid = strconv.Itoa(p.PID)
+	} else {
+		pid = args[0]
+	}
+	addr, err := targetToAddr(pid)
+	if err != nil {
+		fmt.Fprintf(c.stderr, "failed to convert args into addresses: %v\n", err)
+		return 1
 	}
 	if err := diagnoser.Run(addr, c.scrapeInterval); err != nil {
 		fmt.Fprintf(c.stderr, "failed to start diagnoser: %s\n", err.Error())

process/process.go

@@ -55,39 +55,60 @@ func (ps Processes) String() string {
 	return b.String()
 }
 
-// List gives back all processes where the agent runs on.
-func List() (Processes, error) {
+// FindAll gives back all processes where the agent runs on.
+func FindAll() (Processes, error) {
 	processes := make(Processes, 0)
 	ps, err := ps.Processes()
 	if err != nil {
 		return nil, err
 	}
 	for _, p := range ps {
-		pid := p.Pid()
-		if pid == 0 {
-			// Ignore system process.
-			continue
-		}
-		pidfile, err := PIDFile(pid)
+		process, err := newProcess(p)
 		if err != nil {
-			return nil, fmt.Errorf("failed to find pid file: %w", err)
-		}
-		if _, err := os.Stat(pidfile); err != nil {
-			// Ignore ps where the agent doesn't run on.
 			continue
 		}
-		path, err := p.Path()
-		if err != nil {
-			return nil, fmt.Errorf("failed to detect full path to the executable: %w", err)
+		processes = append(processes, *process)
+	}
+	return processes, nil
+}
+
+// FindOne finds processes where the agent runs on and gives back the first one it found.
+func FindOne() (*Process, error) {
+	ps, err := ps.Processes()
+	if err != nil {
+		return nil, err
+	}
+	for _, p := range ps {
+		process, err := newProcess(p)
+		if err == nil {
+			return process, nil
 		}
+	}
+	return nil, fmt.Errorf("no process where the agent runs found")
+}
 
-		processes = append(processes, Process{
-			PID:        pid,
-			Executable: p.Executable(),
-			Path:       path,
-		})
+func newProcess(p ps.Process) (*Process, error) {
+	pid := p.Pid()
+	if pid == 0 {
+		return nil, fmt.Errorf("system process given")
 	}
-	return processes, nil
+	pidfile, err := PIDFile(pid)
+	if err != nil {
+		return nil, fmt.Errorf("failed to find pid file: %w", err)
+	}
+	if _, err := os.Stat(pidfile); err != nil {
+		return nil, fmt.Errorf("given process isn't the process where the agent doesn't run on")
+	}
+	path, err := p.Path()
+	if err != nil {
+		return nil, fmt.Errorf("failed to detect full path to the executable: %w", err)
+	}
+	return &Process{
+		PID:        pid,
+		Executable: p.Executable(),
+		Path:       path,
+	}, nil
+
 }
 
 func pad(s string, total int) string {