Implement PrefixWriter

PrefixWriter is a passthrough io.Writer that prepends a prefix to the
beginning of each line.

src/gopheros/kernel/hal/hal.go

@@ -51,16 +51,18 @@ func probe(hwProbeFns []device.ProbeFn) []device.Driver {
 			continue
 		}
 
+		strBuf.Reset()
 		major, minor, patch := drv.DriverVersion()
+		kfmt.Fprintf(&strBuf, "[hal] %s(%d.%d.%d): ", drv.DriverName(), major, minor, patch)
+		w.prefix = strBuf.Bytes()
 
-		kfmt.Printf("[hal] %s(%d.%d.%d): ", drv.DriverName(), major, minor, patch)
+		if err := drv.DriverInit(&w); err != nil {
-		if err := drv.DriverInit(); err != nil {
+			kfmt.Fprintf(&w, "init failed: %s\n", err.Message)
-			kfmt.Printf("init failed: %s\n", err.Message)
 			continue
 		}
 
+		kfmt.Fprintf(&w, "initialized\n")
 		drivers = append(drivers, drv)
-		kfmt.Printf("initialized\n")
 	}
 
 	return drivers

src/gopheros/kernel/kfmt/prefix_writer.go

@@ -0,0 +1,57 @@
+package kfmt
+
+import "io"
+
+// PrefixWriter is an io.Writer that wraps another io.Writer and injects a
+// prefix at the beginning of each line.
+type PrefixWriter struct {
+	// A writer where all writes get sent to.
+	Sink io.Writer
+
+	// The prefix injected at the beginning of each line.
+	Prefix []byte
+
+	bytesAfterPrefix int
+}
+
+// Write writes len(p) bytes from p to the underlying data stream and returns
+// back the number of bytes written. The PrefixWriter keeps track of the
+// beginning of new lines and injects the configured prefix at each new line.
+// The injected prefix is not included in the number of written bytes returned
+// by this method.
+func (w *PrefixWriter) Write(p []byte) (int, error) {
+	var (
+		written              int
+		startIndex, curIndex int
+	)
+
+	if w.bytesAfterPrefix == 0 && len(p) != 0 {
+		w.Sink.Write(w.Prefix)
+	}
+
+	for ; curIndex < len(p); curIndex++ {
+		if p[curIndex] == '\n' {
+			n, err := w.Sink.Write(p[startIndex : curIndex+1])
+			if curIndex+1 != len(p) {
+				w.Sink.Write(w.Prefix)
+			}
+			written += n
+			if err != nil {
+				return written, err
+			}
+			w.bytesAfterPrefix = 0
+			startIndex = curIndex + 1
+		}
+	}
+
+	if startIndex < curIndex {
+		n, err := w.Sink.Write(p[startIndex:curIndex])
+		written += n
+		w.bytesAfterPrefix = n
+		if err != nil {
+			return written, err
+		}
+	}
+
+	return written, nil
+}

src/gopheros/kernel/kfmt/prefix_writer_test.go

@@ -0,0 +1,92 @@
+package kfmt
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+)
+
+func TestPrefixWriter(t *testing.T) {
+	specs := []struct {
+		input string
+		exp   string
+	}{
+		{
+			"",
+			"",
+		},
+		{
+			"\n",
+			"prefix: \n",
+		},
+		{
+			"no line break anywhere",
+			"prefix: no line break anywhere",
+		},
+		{
+			"line feed at the end\n",
+			"prefix: line feed at the end\n",
+		},
+		{
+			"\nthe big brown\nfog jumped\nover the lazy\ndog",
+			"prefix: \nprefix: the big brown\nprefix: fog jumped\nprefix: over the lazy\nprefix: dog",
+		},
+	}
+
+	var (
+		buf bytes.Buffer
+		w   = PrefixWriter{
+			Sink:   &buf,
+			Prefix: []byte("prefix: "),
+		}
+	)
+
+	for specIndex, spec := range specs {
+		buf.Reset()
+		w.bytesAfterPrefix = 0
+
+		wrote, err := w.Write([]byte(spec.input))
+		if err != nil {
+			t.Errorf("[spec %d] unexpected error: %v", specIndex, err)
+		}
+
+		if expLen := len(spec.input); expLen != wrote {
+			t.Errorf("[spec %d] expected writer to write %d bytes; wrote %d", specIndex, expLen, wrote)
+		}
+
+		if got := buf.String(); got != spec.exp {
+			t.Errorf("[spec %d] expected output:\n%q\ngot:\n%q", specIndex, spec.exp, got)
+		}
+	}
+}
+
+func TestPrefixWriterErrors(t *testing.T) {
+	specs := []string{
+		"no line break anywhere",
+		"\nthe big brown\nfog jumped\nover the lazy\ndog",
+	}
+
+	var (
+		expErr = errors.New("write failed")
+		w      = PrefixWriter{
+			Sink:   writerThatAlwaysErrors{expErr},
+			Prefix: []byte("prefix: "),
+		}
+	)
+
+	for specIndex, spec := range specs {
+		w.bytesAfterPrefix = 0
+		_, err := w.Write([]byte(spec))
+		if err != expErr {
+			t.Errorf("[spec %d] expected error: %v; got %v", specIndex, expErr, err)
+		}
+	}
+}
+
+type writerThatAlwaysErrors struct {
+	err error
+}
+
+func (w writerThatAlwaysErrors) Write(_ []byte) (int, error) {
+	return 0, w.err
+}