Cursor input skeleton (elastic#19378)

This PR is part of introducing a new input architecture in filebeat. The current state of the full implementation can be seen [here](https://github.com/urso/beats/tree/fb-input-v2-combined/filebeat/input/v2) and [sample inputs based on the new API](https://github.com/urso/beats/tree/fb-input-v2-combined/filebeat/features/input).

The full list of changes will include:
- Introduce v2 API interfaces
- Introduce [compatibility layer](https://github.com/urso/beats/tree/fb-input-v2-combined/filebeat/input/v2/compat) to integrate API with existing functionality
- Introduce helpers for writing [stateless](https://github.com/urso/beats/blob/fb-input-v2-combined/filebeat/input/v2/input-stateless/stateless.go) inputs.
- Introduce helpers for writing [inputs that store a state](https://github.com/urso/beats/tree/fb-input-v2-combined/filebeat/input/v2/input-cursor) between restarts.
- Integrate new API with [existing inputs and modules](https://github.com/urso/beats/blob/fb-input-v2-combined/filebeat/beater/filebeat.go#L301) in filebeat.

The change introduces the skeleton and documentation with details for
cursor based inputs. Future updates will add the actual implementation
and tests.

filebeat/input/v2/input-cursor/clean.go

@@ -0,0 +1,45 @@
+// 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 cursor
+
+import (
+	"time"
+
+	"github.com/elastic/beats/v7/libbeat/logp"
+	"github.com/elastic/go-concert/unison"
+)
+
+// cleaner removes finished entries from the registry file.
+type cleaner struct {
+	log *logp.Logger
+}
+
+// run starts a loop that tries to clean entries from the registry.
+// The cleaner locks the store, such that no new states can be created
+// during the cleanup phase. Only resources that are finished and whos TTL
+// (clean_timeout setting) has expired will be removed.
+//
+// Resources are considered "Finished" if they do not have a current owner (active input), and
+// if they have no pending updates that still need to be written to the registry file after associated
+// events have been ACKed by the outputs.
+// The event acquisition timestamp is used as reference to clean resources. If a resources was blocked
+// for a long time, and the life time has been exhausted, then the resource will be removed immediately
+// once the last event has been ACKed.
+func (c *cleaner) run(canceler unison.Canceler, store *store, interval time.Duration) {
+	panic("TODO: implement me")
+}

filebeat/input/v2/input-cursor/cursor.go

@@ -0,0 +1,43 @@
+// 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 cursor
+
+// Cursor allows the input to check if cursor status has been stored
+// in the past and unpack the status into a custom structure.
+type Cursor struct {
+	store    *store
+	resource *resource
+}
+
+func makeCursor(store *store, res *resource) Cursor {
+	return Cursor{store: store, resource: res}
+}
+
+// IsNew returns true if no cursor information has been stored
+// for the current Source.
+func (c Cursor) IsNew() bool { return c.resource.IsNew() }
+
+// Unpack deserialized the cursor state into to. Unpack fails if no pointer is
+// given, or if the structure to points to is not compatible with the document
+// stored.
+func (c Cursor) Unpack(to interface{}) error {
+	if c.IsNew() {
+		return nil
+	}
+	return c.resource.UnpackCursor(to)
+}

filebeat/input/v2/input-cursor/doc.go

@@ -0,0 +1,58 @@
+// 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 cursor provides an InputManager for use with the v2 API, that is
+// capable of storing an internal cursor state between restarts.
+//
+// The InputManager requires authors to Implement a configuration function and
+// the cursor.Input interface. The configuration function returns a slice of
+// sources ([]Source) that it has read from the configuration object, and the
+// actual Input that will be used to collect events from each configured
+// source.
+// When Run a go-routine will be started per configured source. If two inputs have
+// configured the same source, only one will be active, while the other waits
+// for the resource to become free.
+// The manager keeps track of the state per source. When publishing an event a
+// new cursor value can be passed as well. Future instance of the input can
+// read the last published cursor state.
+//
+// For each source an in-memory and a persitent state are tracked. Internal
+// meta updates by the input manager can not be read by Inputs, and will be
+// written to the persistent store immediately. Cursor state updates are read
+// and update by the input. Cursor updates are written to the persistent store
+// only after the events have been ACKed by the output. Internally the input
+// manager keeps track of already ACKed updates and pending ACKs.
+// In order to guarantee progress even if the pbulishing is slow or blocked, all cursor
+// updates are written to the in-memory state immediately. Source without any
+// pending updates are in-sync (in-memory state == persistet state). All
+// updates are ordered, but we allow the in-memory state to be ahead of the
+// persistent state.
+// When an input is started, the cursor state is read from the in-memory state.
+// This way a new input instance can continue where other inputs have been
+// stopped, even if we still have in-flight events from older input instances.
+// The coordination between inputs guarantees that all updates are always in
+// order.
+//
+// When a shutdown signal is received, the publisher is directly disconnected
+// from the outputs. As all coordination is directly handled by the
+// InputManager, shutdown will be immediate (once the input itself has
+// returned), and can not be blocked by the outputs.
+//
+// An input that is about to collect a source that is already collected by
+// another input will wait until the other input has returned or the current
+// input did receive a shutdown signal.
+package cursor

filebeat/input/v2/input-cursor/input.go

@@ -0,0 +1,84 @@
+// 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 cursor
+
+import (
+	"fmt"
+	"time"
+
+	input "github.com/elastic/beats/v7/filebeat/input/v2"
+	"github.com/elastic/beats/v7/libbeat/beat"
+)
+
+// Input interface for cursor based inputs. This interface must be implemented
+// by inputs that with to use the InputManager in order to implement a stateful
+// input that can store state between restarts.
+type Input interface {
+	Name() string
+
+	// Test checks the configuaration and runs additional checks if the Input can
+	// actually collect data for the given configuration (e.g. check if host/port or files are
+	// accessible).
+	// The input manager will call Test per configured source.
+	Test(Source, input.TestContext) error
+
+	// Run starts the data collection. Run must return an error only if the
+	// error is fatal making it impossible for the input to recover.
+	// The input run a go-routine can call Run per configured Source.
+	Run(input.Context, Source, Cursor, Publisher) error
+}
+
+// managedInput implements the v2.Input interface, integrating cursor Inputs
+// with the v2 input API.
+// The managedInput starts go-routines per configured source.
+// If a Run returns the error is 'remembered', but active data collecting
+// continues. Only after all Run calls have returned will the managedInput be
+// done.
+type managedInput struct {
+	manager      *InputManager
+	userID       string
+	sources      []Source
+	input        Input
+	cleanTimeout time.Duration
+}
+
+// Name is required to implement the v2.Input interface
+func (inp *managedInput) Name() string { return inp.input.Name() }
+
+// Test runs the Test method for each configured source.
+func (inp *managedInput) Test(ctx input.TestContext) error {
+	panic("TODO: implement me")
+}
+
+// Run creates a go-routine per source, waiting until all go-routines have
+// returned, either by error, or by shutdown signal.
+// If an input panics, we create an error value with stack trace to report the
+// issue, but not crash the whole process.
+func (inp *managedInput) Run(
+	ctx input.Context,
+	pipeline beat.PipelineConnector,
+) (err error) {
+	panic("TODO: implement me")
+}
+
+func (inp *managedInput) createSourceID(s Source) string {
+	if inp.userID != "" {
+		return fmt.Sprintf("%v::%v::%v", inp.manager.Type, inp.userID, s.Name())
+	}
+	return fmt.Sprintf("%v::%v", inp.manager.Type, s.Name())
+}

filebeat/input/v2/input-cursor/manager.go

@@ -0,0 +1,106 @@
+// 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 cursor
+
+import (
+	"time"
+
+	input "github.com/elastic/beats/v7/filebeat/input/v2"
+	v2 "github.com/elastic/beats/v7/filebeat/input/v2"
+	"github.com/elastic/beats/v7/libbeat/common"
+	"github.com/elastic/beats/v7/libbeat/logp"
+	"github.com/elastic/beats/v7/libbeat/statestore"
+
+	"github.com/elastic/go-concert/unison"
+)
+
+// InputManager is used to create, manage, and coordinate stateful inputs and
+// their persistent state.
+// The InputManager ensures that only one input can be active for a unique source.
+// If two inputs have overlapping sources, both can still collect data, but
+// only one input will collect from the common source.
+//
+// The InputManager automatically cleans up old entries without an active
+// input, and without any pending update operations for the persistent store.
+//
+// The Type field is used to create the key name in the persistent store. Users
+// are allowed to add a custome per input configuration ID using the `id`
+// setting, to collect the same source multiple times, but with different
+// state. The key name in the persistent store becomes <Type>-[<ID>]-<Source Name>
+type InputManager struct {
+	Logger *logp.Logger
+
+	// StateStore gives the InputManager access to the persitent key value store.
+	StateStore StateStore
+
+	// Type must contain the name of the input type. It is used to create the key name
+	// for all sources the inputs collect from.
+	Type string
+
+	// DefaultCleanTimeout configures the key/value garbage collection interval.
+	// The InputManager will only collect keys for the configured 'Type'
+	DefaultCleanTimeout time.Duration
+
+	// Configure returns an array of Sources, and a configured Input instances
+	// that will be used to collect events from each source.
+	Configure func(cfg *common.Config) ([]Source, Input, error)
+
+	store *store
+}
+
+// Source describe a source the input can collect data from.
+// The `Name` method must return an unique name, that will be used to identify
+// the source in the persistent state store.
+type Source interface {
+	Name() string
+}
+
+// StateStore interface and configurations used to give the Manager access to the persistent store.
+type StateStore interface {
+	Access() (*statestore.Store, error)
+	CleanupInterval() time.Duration
+}
+
+// Init starts background processes for deleting old entries from the
+// persistent store if mode is ModeRun.
+func (cim *InputManager) Init(group unison.Group, mode v2.Mode) error {
+	panic("TODO: implement me")
+}
+
+// Create builds a new v2.Input using the provided Configure function.
+// The Input will run a go-routine per source that has been configured.
+func (cim *InputManager) Create(config *common.Config) (input.Input, error) {
+	panic("TODO: implement me")
+}
+
+func lockResource(log *logp.Logger, resource *resource, canceler input.Canceler) error {
+	if !resource.lock.TryLock() {
+		log.Infof("Resource '%v' currently in use, waiting...", resource.key)
+		err := resource.lock.LockContext(canceler)
+		if err != nil {
+			log.Infof("Input for resource '%v' has been stopped while waiting", resource.key)
+			return err
+		}
+	}
+	return nil
+}
+
+func releaseResource(resource *resource) {
+	resource.lock.Unlock()
+	resource.Release()
+}