Add tests of reader of filestream input (#21814)

## What does this PR do?

This PR adds tests for `logFile` in the `filestream` input. This element of the architecture is responsible for reading directly from the disk and closing the reader if the state or the position meets the configured criteria.

Conditions tested in the PR:
- file is removed
- file is renamed
- file is truncated
- file is inactive for a time
- file reader reaches EOF
- timeout of the file reader is reached

filebeat/input/filestream/filestream.go

@@ -138,20 +138,16 @@ func (f *logFile) Read(buf []byte) (int, error) {
 }
 
 func (f *logFile) startFileMonitoringIfNeeded() {
-	if f.closeInactive == 0 && f.closeAfterInterval == 0 {
-		return
-	}
-
-	if f.closeInactive > 0 {
+	if f.closeInactive > 0 || f.closeRemoved || f.closeRenamed {
 		f.tg.Go(func(ctx unison.Canceler) error {
-			f.closeIfTimeout(ctx)
+			f.periodicStateCheck(ctx)
 			return nil
 		})
 	}
 
 	if f.closeAfterInterval > 0 {
 		f.tg.Go(func(ctx unison.Canceler) error {
-			f.periodicStateCheck(ctx)
+			f.closeIfTimeout(ctx)
 			return nil
 		})
 	}

filebeat/input/filestream/filestream_test.go

@@ -0,0 +1,136 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package filestream
+
+import (
+	"context"
+	"io"
+	"io/ioutil"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/elastic/beats/v7/libbeat/logp"
+)
+
+func TestLogFileTimedClosing(t *testing.T) {
+	testCases := map[string]struct {
+		inactive      time.Duration
+		closeEOF      bool
+		afterInterval time.Duration
+		expectedErr   error
+	}{
+		"read from file and close inactive": {
+			inactive:    2 * time.Second,
+			expectedErr: ErrClosed,
+		},
+		"read from file and close after interval": {
+			afterInterval: 3 * time.Second,
+			expectedErr:   ErrClosed,
+		},
+		"read from file and close on EOF": {
+			closeEOF:    true,
+			expectedErr: io.EOF,
+		},
+	}
+
+	for name, test := range testCases {
+		test := test
+
+		f := createTestLogFile()
+		defer f.Close()
+		defer os.Remove(f.Name())
+
+		t.Run(name, func(t *testing.T) {
+			reader, err := newFileReader(
+				logp.L(),
+				context.TODO(),
+				f,
+				readerConfig{},
+				closerConfig{
+					OnStateChange: stateChangeCloserConfig{
+						CheckInterval: 1 * time.Second,
+						Inactive:      test.inactive,
+					},
+					Reader: readerCloserConfig{
+						OnEOF:         test.closeEOF,
+						AfterInterval: test.afterInterval,
+					},
+				},
+			)
+			if err != nil {
+				t.Fatalf("error while creating logReader: %+v", err)
+			}
+
+			err = readUntilError(reader)
+
+			assert.Equal(t, test.expectedErr, err)
+		})
+	}
+}
+
+func TestLogFileTruncated(t *testing.T) {
+	f := createTestLogFile()
+	defer f.Close()
+	defer os.Remove(f.Name())
+
+	reader, err := newFileReader(logp.L(), context.TODO(), f, readerConfig{}, closerConfig{})
+	if err != nil {
+		t.Fatalf("error while creating logReader: %+v", err)
+	}
+
+	buf := make([]byte, 1024)
+	_, err = reader.Read(buf)
+	assert.Nil(t, err)
+
+	err = f.Truncate(0)
+	if err != nil {
+		t.Fatalf("error while truncating file: %+v", err)
+	}
+
+	err = readUntilError(reader)
+
+	assert.Equal(t, ErrFileTruncate, err)
+}
+
+func createTestLogFile() *os.File {
+	f, err := ioutil.TempFile("", "filestream_reader_test")
+	if err != nil {
+		panic(err)
+	}
+	content := []byte("first log line\nanother interesting line\na third log message\n")
+	if _, err := f.Write(content); err != nil {
+		panic(err)
+	}
+	if _, err := f.Seek(0, io.SeekStart); err != nil {
+		panic(err)
+	}
+	return f
+}
+
+func readUntilError(reader *logFile) error {
+	buf := make([]byte, 1024)
+	_, err := reader.Read(buf)
+	for err == nil {
+		buf := make([]byte, 1024)
+		_, err = reader.Read(buf)
+	}
+	return err
+}

filebeat/input/filestream/filestream_test_non_windows.go

@@ -0,0 +1,104 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+// +build !windows
+
+package filestream
+
+import (
+	"context"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/elastic/beats/v7/libbeat/logp"
+)
+
+// these tests are separated as one cannot delete/rename files
+// while another process is working with it on Windows
+func TestLogFileRenamed(t *testing.T) {
+	f := createTestLogFile()
+	defer f.Close()
+
+	renamedFile := f.Name() + ".renamed"
+
+	reader, err := newFileReader(
+		logp.L(),
+		context.TODO(),
+		f,
+		readerConfig{},
+		closerConfig{
+			OnStateChange: stateChangeCloserConfig{
+				CheckInterval: 1 * time.Second,
+				Renamed:       true,
+			},
+		},
+	)
+	if err != nil {
+		t.Fatalf("error while creating logReader: %+v", err)
+	}
+
+	buf := make([]byte, 1024)
+	_, err = reader.Read(buf)
+	assert.Nil(t, err)
+
+	err = os.Rename(f.Name(), renamedFile)
+	if err != nil {
+		t.Fatalf("error while renaming file: %+v", err)
+	}
+
+	err = readUntilError(reader)
+	os.Remove(renamedFile)
+
+	assert.Equal(t, ErrClosed, err)
+}
+
+func TestLogFileRemoved(t *testing.T) {
+	f := createTestLogFile()
+	defer f.Close()
+
+	reader, err := newFileReader(
+		logp.L(),
+		context.TODO(),
+		f,
+		readerConfig{},
+		closerConfig{
+			OnStateChange: stateChangeCloserConfig{
+				CheckInterval: 1 * time.Second,
+				Removed:       true,
+			},
+		},
+	)
+	if err != nil {
+		t.Fatalf("error while creating logReader: %+v", err)
+	}
+
+	buf := make([]byte, 1024)
+	_, err = reader.Read(buf)
+	assert.Nil(t, err)
+
+	err = os.Remove(f.Name())
+	if err != nil {
+		t.Fatalf("error while remove file: %+v", err)
+	}
+
+	err = readUntilError(reader)
+
+	assert.Equal(t, ErrClosed, err)
+}