First step implementing "prefixed" output option

Ref #104

internal/output/group.go

@@ -7,7 +7,7 @@ import (
 
 type Group struct{}
 
-func (Group) WrapWriter(w io.Writer) io.WriteCloser {
+func (Group) WrapWriter(w io.Writer, _ string) io.WriteCloser {
 	return &groupWriter{writer: w}
 }
 

internal/output/interleaved.go

@@ -6,7 +6,7 @@ import (
 
 type Interleaved struct{}
 
-func (Interleaved) WrapWriter(w io.Writer) io.WriteCloser {
+func (Interleaved) WrapWriter(w io.Writer, _ string) io.WriteCloser {
 	return nopWriterCloser{w: w}
 }
 

internal/output/output.go

@@ -5,5 +5,5 @@ import (
 )
 
 type Output interface {
-	WrapWriter(io.Writer) io.WriteCloser
+	WrapWriter(w io.Writer, prefix string) io.WriteCloser
 }

internal/output/prefixed.go

@@ -0,0 +1,65 @@
+package output
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"strings"
+)
+
+type Prefixed struct{}
+
+func (Prefixed) WrapWriter(w io.Writer, prefix string) io.WriteCloser {
+	return &prefixWriter{writer: w, prefix: prefix}
+}
+
+type prefixWriter struct {
+	writer io.Writer
+	prefix string
+	buff   bytes.Buffer
+}
+
+func (pw *prefixWriter) Write(p []byte) (int, error) {
+	n, err := pw.buff.Write(p)
+	if err != nil {
+		return n, err
+	}
+
+	return n, pw.writeOutputLines(false)
+}
+
+func (pw *prefixWriter) Close() error {
+	return pw.writeOutputLines(true)
+}
+
+func (pw *prefixWriter) writeOutputLines(force bool) error {
+	for {
+		line, err := pw.buff.ReadString('\n')
+		if err == nil {
+			if err = pw.writeLine(line); err != nil {
+				return err
+			}
+		} else if err == io.EOF {
+			// if this line was not a complete line, re-add to the buffer
+			if !force && !strings.HasSuffix(line, "\n") {
+				_, err = pw.buff.WriteString(line)
+				return err
+			}
+
+			return pw.writeLine(line)
+		} else {
+			return err
+		}
+	}
+}
+
+func (pw *prefixWriter) writeLine(line string) error {
+	if line == "" {
+		return nil
+	}
+	if !strings.HasSuffix(line, "\n") {
+		line += "\n"
+	}
+	_, err := fmt.Fprintf(pw.writer, "[%s] %s", pw.prefix, line)
+	return err
+}

internal/taskfile/task.go

@@ -17,4 +17,5 @@ type Task struct {
 	Env       Vars
 	Silent    bool
 	Method    string
+	Prefix    string
 }