fix: Add timestamp to process output

CHANGELOG.md

@@ -5,7 +5,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
-Nothing yet
+- Add timestamps to process output (fgrosse/prox#5)
 
 ## [1.1.0] - 2024-07-13
 - Improve file name matching when determining parser for value of --procfile flag (fgrosse/prox#8)

executor.go

@@ -69,7 +69,9 @@ func (e *Executor) Run(ctx context.Context, processes []Process) error {
 	defer logger.Sync()
 	go e.monitorContext(ctx, logger)
 
-	output := e.newOutput(processes)
+	// Add a timestamp to the process output, but not to prox's own log
+	// output.
+	output := newOutput(processes, e.noColors, newTimestampWriter(e.output))
 	pp := make([]process, len(processes))
 	for i, p := range processes {
 		po := output.next(p)

timestamp.go

@@ -0,0 +1,49 @@
+package prox
+
+import (
+	"bytes"
+	"io"
+	"sync"
+	"time"
+)
+
+const timestampFormat = "15:04:05"
+
+// timestampWriter prefixes every complete line written to it with the current
+// time (HH:MM:SS).
+type timestampWriter struct {
+	w   io.Writer
+	mu  sync.Mutex
+	buf []byte
+}
+
+func newTimestampWriter(w io.Writer) *timestampWriter {
+	return &timestampWriter{w: w}
+}
+
+func (t *timestampWriter) Write(p []byte) (int, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	t.buf = append(t.buf, p...)
+
+	for {
+		i := bytes.IndexByte(t.buf, '\n')
+		if i == -1 {
+			break
+		}
+
+		line := t.buf[:i+1]
+		t.buf = t.buf[i+1:]
+
+		stamped := make([]byte, 0, len(line)+9)
+		stamped = append(stamped, time.Now().Format(timestampFormat)+" "...)
+		stamped = append(stamped, line...)
+
+		if _, err := t.w.Write(stamped); err != nil {
+			return 0, err
+		}
+	}
+
+	return len(p), nil
+}