Implementation of fileProspector (elastic#21479)

## What does this PR do?

This PR adds the implementation of `fileProspector`. The prospector listens for events from the `FSWatcher` and processes them depending on the type of the event. Possible actions are starting a new Harvester to read from a file, removing an entry from the registry, etc.

(cherry picked from commit d987d10)

filebeat/input/filestream/identifier.go

@@ -0,0 +1,139 @@
+// 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 (
+	"fmt"
+	"os"
+
+	loginp "github.com/elastic/beats/v7/filebeat/input/filestream/internal/input-logfile"
+	"github.com/elastic/beats/v7/libbeat/common"
+	"github.com/elastic/beats/v7/libbeat/common/file"
+)
+
+const (
+	nativeName      = "native"
+	pathName        = "path"
+	inodeMarkerName = "inode_marker"
+
+	DefaultIdentifierName = nativeName
+	identitySep           = "::"
+)
+
+var (
+	identifierFactories = map[string]identifierFactory{
+		nativeName:      newINodeDeviceIdentifier,
+		pathName:        newPathIdentifier,
+		inodeMarkerName: newINodeMarkerIdentifier,
+	}
+)
+
+type identifierFactory func(*common.Config) (fileIdentifier, error)
+
+type fileIdentifier interface {
+	GetSource(loginp.FSEvent) fileSource
+	Name() string
+}
+
+// fileSource implements the Source interface
+// It is required to identify and manage file sources.
+type fileSource struct {
+	info    os.FileInfo
+	newPath string
+	oldPath string
+
+	name                string
+	identifierGenerator string
+}
+
+// Name returns the registry identifier of the file.
+func (f fileSource) Name() string {
+	return f.name
+}
+
+// newFileIdentifier creates a new state identifier for a log input.
+func newFileIdentifier(ns *common.ConfigNamespace) (fileIdentifier, error) {
+	if ns == nil {
+		return newINodeDeviceIdentifier(nil)
+	}
+
+	identifierType := ns.Name()
+	f, ok := identifierFactories[identifierType]
+	if !ok {
+		return nil, fmt.Errorf("no such file_identity generator: %s", identifierType)
+	}
+
+	return f(ns.Config())
+}
+
+type inodeDeviceIdentifier struct {
+	name string
+}
+
+func newINodeDeviceIdentifier(_ *common.Config) (fileIdentifier, error) {
+	return &inodeDeviceIdentifier{
+		name: nativeName,
+	}, nil
+}
+
+func (i *inodeDeviceIdentifier) GetSource(e loginp.FSEvent) fileSource {
+	return fileSource{
+		info:                e.Info,
+		newPath:             e.NewPath,
+		oldPath:             e.OldPath,
+		name:                pluginName + identitySep + i.name + identitySep + file.GetOSState(e.Info).String(),
+		identifierGenerator: i.name,
+	}
+}
+
+func (i *inodeDeviceIdentifier) Name() string {
+	return i.name
+}
+
+type pathIdentifier struct {
+	name string
+}
+
+func newPathIdentifier(_ *common.Config) (fileIdentifier, error) {
+	return &pathIdentifier{
+		name: pathName,
+	}, nil
+}
+
+func (p *pathIdentifier) GetSource(e loginp.FSEvent) fileSource {
+	return fileSource{
+		info:                e.Info,
+		newPath:             e.NewPath,
+		oldPath:             e.OldPath,
+		name:                pluginName + identitySep + p.name + identitySep + e.NewPath,
+		identifierGenerator: p.name,
+	}
+}
+
+func (p *pathIdentifier) Name() string {
+	return p.name
+}
+
+// mockIdentifier is used for testing
+type MockIdentifier struct{}
+
+func (m *MockIdentifier) GetSource(e loginp.FSEvent) fileSource {
+	return fileSource{identifierGenerator: "mock"}
+}
+
+func (m *MockIdentifier) Name() string { return "mock" }

filebeat/input/filestream/identifier_inode_deviceid.go

@@ -0,0 +1,108 @@
+// 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 (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"time"
+
+	loginp "github.com/elastic/beats/v7/filebeat/input/filestream/internal/input-logfile"
+	"github.com/elastic/beats/v7/libbeat/common"
+	"github.com/elastic/beats/v7/libbeat/common/file"
+	"github.com/elastic/beats/v7/libbeat/logp"
+)
+
+type inodeMarkerIdentifier struct {
+	log        *logp.Logger
+	name       string
+	markerPath string
+
+	markerFileLastModifitaion time.Time
+	markerTxt                 string
+}
+
+func newINodeMarkerIdentifier(cfg *common.Config) (fileIdentifier, error) {
+	var config struct {
+		MarkerPath string `config:"path" validate:"required"`
+	}
+	err := cfg.Unpack(&config)
+	if err != nil {
+		return nil, fmt.Errorf("error while reading configuration of INode + marker file configuration: %v", err)
+	}
+
+	fi, err := os.Stat(config.MarkerPath)
+	if err != nil {
+		return nil, fmt.Errorf("error while opening marker file at %s: %v", config.MarkerPath, err)
+	}
+	markerContent, err := ioutil.ReadFile(config.MarkerPath)
+	if err != nil {
+		return nil, fmt.Errorf("error while reading marker file at %s: %v", config.MarkerPath, err)
+	}
+	return &inodeMarkerIdentifier{
+		log:                       logp.NewLogger("inode_marker_identifier_" + filepath.Base(config.MarkerPath)),
+		name:                      inodeMarkerName,
+		markerPath:                config.MarkerPath,
+		markerFileLastModifitaion: fi.ModTime(),
+		markerTxt:                 string(markerContent),
+	}, nil
+}
+
+func (i *inodeMarkerIdentifier) markerContents() string {
+	f, err := os.Open(i.markerPath)
+	if err != nil {
+		i.log.Errorf("Failed to open marker file %s: %v", i.markerPath, err)
+		return ""
+	}
+	defer f.Close()
+
+	fi, err := f.Stat()
+	if err != nil {
+		i.log.Errorf("Failed to fetch file information for %s: %v", i.markerPath, err)
+		return ""
+	}
+	if i.markerFileLastModifitaion.Before(fi.ModTime()) {
+		contents, err := ioutil.ReadFile(i.markerPath)
+		if err != nil {
+			i.log.Errorf("Error while reading contents of marker file: %v", err)
+			return ""
+		}
+		i.markerTxt = string(contents)
+	}
+
+	return i.markerTxt
+}
+
+func (i *inodeMarkerIdentifier) GetSource(e loginp.FSEvent) fileSource {
+	osstate := file.GetOSState(e.Info)
+	return fileSource{
+		info:                e.Info,
+		newPath:             e.NewPath,
+		oldPath:             e.OldPath,
+		name:                fmt.Sprintf("%s%s%s-%s", i.name, identitySep, osstate.InodeString(), i.markerContents()),
+		identifierGenerator: i.name,
+	}
+}
+
+func (i *inodeMarkerIdentifier) Name() string {
+	return i.name
+}

filebeat/input/filestream/identifier_inode_deviceid_windows.go

@@ -0,0 +1,30 @@
+// 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 (
+	"fmt"
+
+	"github.com/elastic/beats/v7/libbeat/common"
+)
+
+func newINodeMarkerIdentifier(cfg *common.Config) (fileIdentifier, error) {
+	return nil, fmt.Errorf("inode_deviceid is not supported on Windows")
+}

filebeat/input/filestream/input.go

@@ -29,6 +29,12 @@ import (
 // are actively written by other applications.
 type filestream struct{}
 
+type state struct {
+	Source         string `json:"source" struct:"source"`
+	Offset         int64  `json:"offset" struct:"offset"`
+	IdentifierName string `json:"identifier_name" struct:"identifier_name"`
+}
+
 const pluginName = "filestream"
 
 // Plugin creates a new filestream input plugin for creating a stateful input.