internal/pkg: optionally colour output

This increases readability in some cases.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec.go

@@ -444,7 +444,7 @@ func scanVerbose(
 	msg message.Msg,
 	cancel context.CancelFunc,
 	done chan<- struct{},
-	prefix string,
+	prefix, suffix string,
 	r io.Reader,
 ) {
 	defer close(done)
@@ -454,7 +454,7 @@ func scanVerbose(
 		bufio.MaxScanTokenSize<<12,
 	)
 	for s.Scan() {
-		msg.Verbose(prefix, s.Text())
+		msg.Verbose(prefix, s.Text()+suffix)
 	}
 	if err := s.Err(); err != nil && !errors.Is(err, os.ErrClosed) {
 		cancel()
@@ -728,14 +728,25 @@ func (a *execArtifact) cure(f *FContext, hostNet bool) (err error) {
 
 		brStdout, brStderr := f.cache.getReader(stdout), f.cache.getReader(stderr)
 		stdoutDone, stderrDone := make(chan struct{}), make(chan struct{})
+
+		var suffix, prefixO, prefixE string
+		if f.cache.attr.Flags&CColourOutput != 0 {
+			suffix = "\x1b[0m"
+			prefixO = "\x1b[1;37m(" + a.name + ")\x1b[0m"
+			prefixE = "\x1b[1;97m(" + a.name + ")\x1b[0m"
+		} else {
+			prefixO = "(" + a.name + ":1)"
+			prefixE = "(" + a.name + ":2)"
+		}
+
 		go scanVerbose(
 			msg, cancel, stdoutDone,
-			"("+a.name+":1)",
+			prefixO, suffix,
 			io.TeeReader(brStdout, status),
 		)
 		go scanVerbose(
 			msg, cancel, stderrDone,
-			"("+a.name+":2)",
+			prefixE, suffix,
 			io.TeeReader(brStderr, status),
 		)
 		defer func() {

internal/pkg/pkg.go

@@ -721,6 +721,9 @@ const (
 	// CExternShallow arranges for only non-flood inputs to be fetched when
 	// curing an [Artifact] available via the external cache.
 	CExternShallow
+
+	// CColourOutput enables output colouring via ANSI control sequences.
+	CColourOutput
 )
 
 // toplevel holds [context.WithCancel] over caller-supplied context, where all