Pull request: 2499 merge rewrites vol.1

Merge in DNS/adguard-home from 2499-merge-rewrites-vol.1 to master

Updates #2499.

Squashed commit of the following:

commit 6b308bc2b360cee8c22e506f31d62bacb4bf8fb3
Merge: f49e9186 2b635bf6
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Oct 14 19:23:07 2021 +0300

    Merge branch 'master' into 2499-merge-rewrites-vol.1

commit f49e9186ffc8b7074d03c6721ee56cdb09243684
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Oct 14 18:50:49 2021 +0300

    aghos: fix fs events filtering

commit 567dd646556606212af5dab60e3ecbb8fff22c25
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Oct 14 16:50:37 2021 +0300

    all: imp code, docs, fix windows

commit 140c8bf519345eb54d0e7500a996fcf465353d71
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Oct 13 19:41:53 2021 +0300

    aghnet: use const

commit bebf3f76bd394a498ccad812c57d4507c69529ba
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Oct 13 19:32:37 2021 +0300

    all: imp tests, docs

commit 9bfdbb6eb454833135d616e208e82699f98e2562
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Oct 13 18:42:20 2021 +0300

    all: imp path more, imp docs

commit ee9ea4c132a6b17787d150bf2bee703abaa57be3
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Oct 13 16:09:46 2021 +0300

    all: fix windows, imp paths

commit 6fac8338a81e9ecfebfc23a1adcb964e89f6aee6
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Oct 11 19:53:35 2021 +0300

    all: imp code, docs

commit da1ce1a2a3dd2be3fdff2412a6dbd596859dc249
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Oct 11 18:22:50 2021 +0300

    aghnet: fix windows tests

commit d29de359ed68118d71efb226a8433fac15ff5c66
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Oct 8 21:02:14 2021 +0300

    all: repl & imp

commit 1356c08944cdbb85ce5532d90fe5b077219ce5ff
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Oct 8 01:41:19 2021 +0300

    all: add tests, mv logic, added tmpfs

commit f4b11adf8998bc8d9d955c5ac9f386f671bd5213
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Oct 7 14:26:30 2021 +0300

    all: imp filewalker, refactor hosts container

internal/aghos/fswatcher.go

@@ -0,0 +1,131 @@
+package aghos
+
+import (
+	"fmt"
+	"io"
+	"io/fs"
+	"path/filepath"
+
+	"github.com/AdguardTeam/golibs/errors"
+	"github.com/AdguardTeam/golibs/log"
+	"github.com/fsnotify/fsnotify"
+)
+
+// event is a convenient alias for an empty struct to signal that watching
+// event happened.
+type event = struct{}
+
+// FSWatcher tracks all the fyle system events and notifies about those.
+//
+// TODO(e.burkov, a.garipov): Move into another package like aghfs.
+type FSWatcher interface {
+	io.Closer
+
+	// Events should return a read-only channel which notifies about events.
+	Events() (e <-chan event)
+
+	// Add should check if the file named name is accessible and starts tracking
+	// it.
+	Add(name string) (err error)
+}
+
+// osWatcher tracks the file system provided by the OS.
+type osWatcher struct {
+	// w is the actual notifier that is handled by osWatcher.
+	w *fsnotify.Watcher
+
+	// events is the channel to notify.
+	events chan event
+}
+
+const (
+	// osWatcherPref is a prefix for logging and wrapping errors in osWathcer's
+	// methods.
+	osWatcherPref = "os watcher"
+)
+
+// NewOSWritesWatcher creates FSWatcher that tracks the real file system of the
+// OS and notifies only about writing events.
+func NewOSWritesWatcher() (w FSWatcher, err error) {
+	defer func() { err = errors.Annotate(err, "%s: %w", osWatcherPref) }()
+
+	var watcher *fsnotify.Watcher
+	watcher, err = fsnotify.NewWatcher()
+	if err != nil {
+		return nil, fmt.Errorf("creating watcher: %w", err)
+	}
+
+	fsw := &osWatcher{
+		w:      watcher,
+		events: make(chan event, 1),
+	}
+
+	go fsw.handleErrors()
+	go fsw.handleEvents()
+
+	return fsw, nil
+}
+
+// handleErrors handles accompanying errors.  It used to be called in a separate
+// goroutine.
+func (w *osWatcher) handleErrors() {
+	defer log.OnPanic(fmt.Sprintf("%s: handling errors", osWatcherPref))
+
+	for err := range w.w.Errors {
+		log.Error("%s: %s", osWatcherPref, err)
+	}
+}
+
+// Events implements the FSWatcher interface for *osWatcher.
+func (w *osWatcher) Events() (e <-chan event) {
+	return w.events
+}
+
+// Add implements the FSWatcher interface for *osWatcher.
+func (w *osWatcher) Add(name string) (err error) {
+	defer func() { err = errors.Annotate(err, "%s: %w", osWatcherPref) }()
+
+	if _, err = fs.Stat(RootDirFS(), name); err != nil {
+		return fmt.Errorf("checking file %q: %w", name, err)
+	}
+
+	return w.w.Add(filepath.Join("/", name))
+}
+
+// Close implements the FSWatcher interface for *osWatcher.
+func (w *osWatcher) Close() (err error) {
+	return w.w.Close()
+}
+
+// handleEvents notifies about the received file system's event if needed.  It
+// used to be called in a separate goroutine.
+func (w *osWatcher) handleEvents() {
+	defer log.OnPanic(fmt.Sprintf("%s: handling events", osWatcherPref))
+
+	defer close(w.events)
+
+	ch := w.w.Events
+	for e := range ch {
+		if e.Op&fsnotify.Write == 0 {
+			continue
+		}
+
+		// Skip the following events assuming that sometimes the same event
+		// occurrs several times.
+		for ok := true; ok; {
+			select {
+			case _, ok = <-ch:
+				// Go on.
+			default:
+				ok = false
+			}
+		}
+
+		select {
+		case w.events <- event{}:
+			// Go on.
+		default:
+			log.Debug("%s: events buffer is full", osWatcherPref)
+		}
+	}
+}