Update First(n int) to read no more data than necessary (#199)

First(N) now closes its output after it has read N lines, terminating the pipe 
without consuming unnecessary further input (apart from the 4KiB scanner 
buffer).

Co-authored-by: John Arundel <john@bitfieldconsulting.com>

script.go

@@ -514,21 +514,24 @@ func (p *Pipe) FilterScan(filter func(string, io.Writer)) *Pipe {
 
 // First produces only the first n lines of the pipe's contents, or all the
 // lines if there are less than n. If n is zero or negative, there is no output
-// at all.
+// at all. When n lines have been produced, First stops reading its input and
+// sends EOF to its output.
 func (p *Pipe) First(n int) *Pipe {
 	if p.Error() != nil {
 		return p
 	}
 	if n <= 0 {
 		return NewPipe()
 	}
-	i := 0
-	return p.FilterScan(func(line string, w io.Writer) {
-		if i >= n {
-			return
+	return p.Filter(func(r io.Reader, w io.Writer) error {
+		scanner := newScanner(r)
+		for i := 0; i < n && scanner.Scan(); i++ {
+			_, err := fmt.Fprintln(w, scanner.Text())
+			if err != nil {
+				return err
+			}
 		}
-		fmt.Fprintln(w, line)
-		i++
+		return scanner.Err()
 	})
 }
 

script_test.go

@@ -573,6 +573,24 @@ func TestFirstHasNoEffectGivenLessThanNInputLines(t *testing.T) {
 	}
 }
 
+func TestFirstDoesNotConsumeUnnecessaryData(t *testing.T) {
+	t.Parallel()
+	// First uses a 4096-byte buffer, so will always read at least
+	// that much, but no more (once N lines have been read).
+	r := strings.NewReader(strings.Repeat("line\n", 1000))
+	got, err := script.NewPipe().WithReader(r).First(1).String()
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "line\n"
+	if want != got {
+		t.Errorf("want output %q, got %q", want, got)
+	}
+	if r.Len() == 0 {
+		t.Errorf("no data left in reader")
+	}
+}
+
 func TestFreqHandlesLongLines(t *testing.T) {
 	t.Parallel()
 	got, err := script.Echo(longLine).Freq().Slice()