Implement tail for docker logs

Fixes #4330
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)

tailfile/tailfile.go

@@ -0,0 +1,61 @@
+package tailfile
+
+import (
+	"bytes"
+	"errors"
+	"os"
+)
+
+const blockSize = 1024
+
+var eol = []byte("\n")
+var ErrNonPositiveLinesNumber = errors.New("Lines number must be positive")
+
+//TailFile returns last n lines of file f
+func TailFile(f *os.File, n int) ([][]byte, error) {
+	if n <= 0 {
+		return nil, ErrNonPositiveLinesNumber
+	}
+	size, err := f.Seek(0, os.SEEK_END)
+	if err != nil {
+		return nil, err
+	}
+	block := -1
+	var data []byte
+	var cnt int
+	for {
+		var b []byte
+		step := int64(block * blockSize)
+		left := size + step // how many bytes to beginning
+		if left < 0 {
+			if _, err := f.Seek(0, os.SEEK_SET); err != nil {
+				return nil, err
+			}
+			b = make([]byte, blockSize+left)
+			if _, err := f.Read(b); err != nil {
+				return nil, err
+			}
+			data = append(b, data...)
+			break
+		} else {
+			b = make([]byte, blockSize)
+			if _, err := f.Seek(step, os.SEEK_END); err != nil {
+				return nil, err
+			}
+			if _, err := f.Read(b); err != nil {
+				return nil, err
+			}
+			data = append(b, data...)
+		}
+		cnt += bytes.Count(b, eol)
+		if cnt > n {
+			break
+		}
+		block--
+	}
+	lines := bytes.Split(data, eol)
+	if n < len(lines) {
+		return lines[len(lines)-n-1 : len(lines)-1], nil
+	}
+	return lines[:len(lines)-1], nil
+}

tailfile/tailfile_test.go

@@ -0,0 +1,148 @@
+package tailfile
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestTailFile(t *testing.T) {
+	f, err := ioutil.TempFile("", "tail-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	defer os.RemoveAll(f.Name())
+	testFile := []byte(`first line
+second line
+third line
+fourth line
+fifth line
+next first line
+next second line
+next third line
+next fourth line
+next fifth line
+last first line
+next first line
+next second line
+next third line
+next fourth line
+next fifth line
+next first line
+next second line
+next third line
+next fourth line
+next fifth line
+last second line
+last third line
+last fourth line
+last fifth line
+truncated line`)
+	if _, err := f.Write(testFile); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.Seek(0, os.SEEK_SET); err != nil {
+		t.Fatal(err)
+	}
+	expected := []string{"last fourth line", "last fifth line"}
+	res, err := TailFile(f, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, l := range res {
+		t.Logf("%s", l)
+		if expected[i] != string(l) {
+			t.Fatalf("Expected line %s, got %s", expected[i], l)
+		}
+	}
+}
+
+func TestTailFileManyLines(t *testing.T) {
+	f, err := ioutil.TempFile("", "tail-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	defer os.RemoveAll(f.Name())
+	testFile := []byte(`first line
+second line
+truncated line`)
+	if _, err := f.Write(testFile); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.Seek(0, os.SEEK_SET); err != nil {
+		t.Fatal(err)
+	}
+	expected := []string{"first line", "second line"}
+	res, err := TailFile(f, 10000)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, l := range res {
+		t.Logf("%s", l)
+		if expected[i] != string(l) {
+			t.Fatalf("Expected line %s, got %s", expected[i], l)
+		}
+	}
+}
+
+func TestTailEmptyFile(t *testing.T) {
+	f, err := ioutil.TempFile("", "tail-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	defer os.RemoveAll(f.Name())
+	res, err := TailFile(f, 10000)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res) != 0 {
+		t.Fatal("Must be empty slice from empty file")
+	}
+}
+
+func TestTailNegativeN(t *testing.T) {
+	f, err := ioutil.TempFile("", "tail-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	defer os.RemoveAll(f.Name())
+	testFile := []byte(`first line
+second line
+truncated line`)
+	if _, err := f.Write(testFile); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.Seek(0, os.SEEK_SET); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := TailFile(f, -1); err != ErrNonPositiveLinesNumber {
+		t.Fatalf("Expected ErrNonPositiveLinesNumber, got %s", err)
+	}
+	if _, err := TailFile(f, 0); err != ErrNonPositiveLinesNumber {
+		t.Fatalf("Expected ErrNonPositiveLinesNumber, got %s", err)
+	}
+}
+
+func BenchmarkTail(b *testing.B) {
+	f, err := ioutil.TempFile("", "tail-test")
+	if err != nil {
+		b.Fatal(err)
+	}
+	defer f.Close()
+	defer os.RemoveAll(f.Name())
+	for i := 0; i < 10000; i++ {
+		if _, err := f.Write([]byte("tailfile pretty interesting line\n")); err != nil {
+			b.Fatal(err)
+		}
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if _, err := TailFile(f, 1000); err != nil {
+			b.Fatal(err)
+		}
+	}
+}