Add zapio.Writer

This adds a new zapio package that provides a `Writer`. The writer
implements `io.WriteCloser` and `zapcore.WriteSyncer`.

It works by splitting the input on newlines, flushing to the logger as
new lines are encountered, and buffering input otherwise.

So for example, if write "foobar\n" is split across multiple Write calls
"foo" and "bar\n", instead of emitting two separate logs for "foo"
and "bar", the Writer will buffer the input until the newline is
encountered and write a single log for "foobar".

Resolves #929

zapio/writer.go

@@ -0,0 +1,88 @@
+package zapio
+
+import (
+	"bytes"
+	"io"
+
+	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
+)
+
+// Writer is an io.Writer that writes to the provided Zap logger, splitting log
+// messages on line boundaries.
+//
+// Writer must be closed when finished to flush buffered data to the logger.
+type Writer struct {
+	Log   *zap.Logger   // log to write to
+	Level zapcore.Level // log level to write at
+
+	buff bytes.Buffer
+}
+
+var (
+	_ zapcore.WriteSyncer = (*Writer)(nil)
+	_ io.Closer           = (*Writer)(nil)
+)
+
+// Write writes the provided bytes to the underlying logger at the configured
+// log level and returns the length of the bytes.
+func (w *Writer) Write(bs []byte) (n int, err error) {
+	// Skip all checks if the level isn't enabled.
+	if !w.Log.Core().Enabled(w.Level) {
+		return len(bs), nil
+	}
+
+	n = len(bs)
+	for len(bs) > 0 {
+		bs = w.writeLine(bs)
+	}
+
+	return n, nil
+}
+
+// writeLine writes a single line from the input, returning the remaining,
+// unconsumed bytes.
+func (w *Writer) writeLine(line []byte) (remaining []byte) {
+	idx := bytes.IndexByte(line, '\n')
+	if idx < 0 {
+		// If there are no newlines, buffer the entire string.
+		w.buff.Write(line)
+		return nil
+	}
+
+	// Split on the newline, buffer and flush the left.
+	line, remaining = line[:idx], line[idx+1:]
+	w.buff.Write(line)
+
+	// Log empty messages in the middle of the stream so that we don't lose
+	// information when the user writes "foo\n\nbar".
+	w.flush(true /* allowEmpty */)
+
+	return remaining
+}
+
+// Close closes the writer, flushing any buffered data in the process.
+func (w *Writer) Close() error {
+	return w.Sync()
+}
+
+// Sync flushes the buffered data from the writer, even if it doesn't end with
+// a newline.
+func (w *Writer) Sync() error {
+	// Don't allow empty messages on explicit Sync calls or on Close
+	// because we don't want an extraneous empty message at the end of the
+	// stream -- it's common for files to end with a newline.
+	w.flush(false /* allowEmpty */)
+	return nil
+}
+
+// flush flushes the buffered data to the logger, allowing empty messages only
+// if the bool is set.
+func (w *Writer) flush(allowEmpty bool) {
+	if allowEmpty || w.buff.Len() > 0 {
+		if ce := w.Log.Check(w.Level, w.buff.String()); ce != nil {
+			ce.Write()
+		}
+	}
+	w.buff.Reset()
+}

zapio/writer_test.go

@@ -0,0 +1,138 @@
+package zapio
+
+import (
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
+	"go.uber.org/zap/zaptest/observer"
+)
+
+func TestWriter(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		desc   string
+		level  zapcore.Level // defaults to info
+		writes []string
+		want   []zapcore.Entry
+	}{
+		{
+			desc: "simple",
+			writes: []string{
+				"foo\n",
+				"bar\n",
+				"baz\n",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.InfoLevel, Message: "foo"},
+				{Level: zap.InfoLevel, Message: "bar"},
+				{Level: zap.InfoLevel, Message: "baz"},
+			},
+		},
+		{
+			desc:  "level too low",
+			level: zap.DebugLevel,
+			writes: []string{
+				"foo\n",
+				"bar\n",
+			},
+			want: []zapcore.Entry{},
+		},
+		{
+			desc:  "multiple newlines in a message",
+			level: zap.WarnLevel,
+			writes: []string{
+				"foo\nbar\n",
+				"baz\n",
+				"qux\nquux\n",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.WarnLevel, Message: "foo"},
+				{Level: zap.WarnLevel, Message: "bar"},
+				{Level: zap.WarnLevel, Message: "baz"},
+				{Level: zap.WarnLevel, Message: "qux"},
+				{Level: zap.WarnLevel, Message: "quux"},
+			},
+		},
+		{
+			desc:  "message split across multiple writes",
+			level: zap.ErrorLevel,
+			writes: []string{
+				"foo",
+				"bar\nbaz",
+				"qux",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.ErrorLevel, Message: "foobar"},
+				{Level: zap.ErrorLevel, Message: "bazqux"},
+			},
+		},
+		{
+			desc: "blank lines in the middle",
+			writes: []string{
+				"foo\n\nbar\nbaz",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.InfoLevel, Message: "foo"},
+				{Level: zap.InfoLevel, Message: ""},
+				{Level: zap.InfoLevel, Message: "bar"},
+				{Level: zap.InfoLevel, Message: "baz"},
+			},
+		},
+		{
+			desc: "blank line at the end",
+			writes: []string{
+				"foo\nbar\nbaz\n",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.InfoLevel, Message: "foo"},
+				{Level: zap.InfoLevel, Message: "bar"},
+				{Level: zap.InfoLevel, Message: "baz"},
+			},
+		},
+		{
+			desc: "multiple blank line at the end",
+			writes: []string{
+				"foo\nbar\nbaz\n\n",
+			},
+			want: []zapcore.Entry{
+				{Level: zap.InfoLevel, Message: "foo"},
+				{Level: zap.InfoLevel, Message: "bar"},
+				{Level: zap.InfoLevel, Message: "baz"},
+				{Level: zap.InfoLevel, Message: ""},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt // for t.Parallel
+		t.Run(tt.desc, func(t *testing.T) {
+			t.Parallel()
+
+			core, observed := observer.New(zap.InfoLevel)
+
+			w := Writer{
+				Log:   zap.New(core),
+				Level: tt.level,
+			}
+
+			for _, s := range tt.writes {
+				_, err := io.WriteString(&w, s)
+				require.NoError(t, err, "Writer.Write failed.")
+			}
+
+			assert.NoError(t, w.Close(), "Writer.Close failed.")
+
+			// Turn []observer.LoggedEntry => []zapcore.Entry
+			got := make([]zapcore.Entry, observed.Len())
+			for i, ent := range observed.AllUntimed() {
+				got[i] = ent.Entry
+			}
+			assert.Equal(t, tt.want, got, "Logged entries do not match.")
+		})
+	}
+}