From 8f3a9f1e2f0c9e066d2e3894b9fc2d054d2f148e Mon Sep 17 00:00:00 2001
From: René 'Necoro' Neumann <necoro@necoro.eu>
Date: Sat, 22 Aug 2020 14:26:29 +0200
Subject: Lock cache

---
 internal/feed/cache.go | 68 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 57 insertions(+), 11 deletions(-)

(limited to 'internal/feed/cache.go')

diff --git a/internal/feed/cache.go b/internal/feed/cache.go
index 45cefd0..2d8f9aa 100644
--- a/internal/feed/cache.go
+++ b/internal/feed/cache.go
@@ -8,6 +8,8 @@ import (
 	"os"
 	"time"
 
+	"github.com/nightlyone/lockfile"
+
 	"github.com/Necoro/feed2imap-go/pkg/log"
 )
 
@@ -17,12 +19,18 @@ const (
 	currentVersion Version = 1
 )
 
-type Cache interface {
+type CacheImpl interface {
 	findItem(*Feed) CachedFeed
 	Version() Version
 	Info() string
 	SpecificInfo(interface{}) string
-	transformToCurrent() (Cache, error)
+	transformToCurrent() (CacheImpl, error)
+}
+
+type Cache struct {
+	CacheImpl
+	lock   lockfile.Lockfile
+	locked bool
 }
 
 type CachedFeed interface {
@@ -34,7 +42,7 @@ type CachedFeed interface {
 	Commit()
 }
 
-func cacheForVersion(version Version) (Cache, error) {
+func cacheForVersion(version Version) (CacheImpl, error) {
 	switch version {
 	case v1Version:
 		return newV1Cache(), nil
@@ -43,8 +51,29 @@ func cacheForVersion(version Version) (Cache, error) {
 	}
 }
 
+func lockName(fileName string) string {
+	return fileName + ".lck"
+}
+
+func lock(fileName string) (lock lockfile.Lockfile, err error) {
+	lockFile := lockName(fileName)
+	log.Debugf("Handling lock file '%s'", lockFile)
+
+	if lock, err = lockfile.New(lockFile); err != nil {
+		err = fmt.Errorf("Creating lock file: %w", err)
+		return
+	}
+
+	if err = lock.TryLock(); err != nil {
+		err = fmt.Errorf("Locking: %w", err)
+		return
+	}
+
+	return
+}
+
 func storeCache(cache Cache, fileName string) error {
-	if cache == nil {
+	if cache.CacheImpl == nil {
 		return fmt.Errorf("trying to store nil cache")
 	}
 	if cache.Version() != currentVersion {
@@ -70,11 +99,23 @@ func storeCache(cache Cache, fileName string) error {
 	writer.Flush()
 	log.Printf("Stored cache to '%s'.", fileName)
 
+	if cache.locked {
+		if err = cache.lock.Unlock(); err != nil {
+			return fmt.Errorf("Unlocking cache: %w", err)
+		}
+	}
 	return nil
 }
 
 func newCache() (Cache, error) {
-	return cacheForVersion(currentVersion)
+	cache, err := cacheForVersion(currentVersion)
+	if err != nil {
+		return Cache{}, err
+	}
+	return Cache{
+		CacheImpl: cache,
+		locked:    false,
+	}, nil
 }
 
 func LoadCache(fileName string) (Cache, error) {
@@ -84,33 +125,38 @@ func LoadCache(fileName string) (Cache, error) {
 			// no cache there yet -- make new
 			return newCache()
 		}
-		return nil, fmt.Errorf("opening cache at '%s': %w", fileName, err)
+		return Cache{}, fmt.Errorf("opening cache at '%s': %w", fileName, err)
 	}
 	defer f.Close()
 
+	lock, err := lock(fileName)
+	if err != nil {
+		return Cache{}, err
+	}
+
 	log.Printf("Loading cache from '%s'", fileName)
 
 	reader := bufio.NewReader(f)
 	version, err := reader.ReadByte()
 	if err != nil {
-		return nil, fmt.Errorf("reading from '%s': %w", fileName, err)
+		return Cache{}, fmt.Errorf("reading from '%s': %w", fileName, err)
 	}
 
 	cache, err := cacheForVersion(Version(version))
 	if err != nil {
-		return nil, err
+		return Cache{}, err
 	}
 
 	decoder := gob.NewDecoder(reader)
 	if err = decoder.Decode(cache); err != nil {
-		return nil, fmt.Errorf("decoding for version '%d' from '%s': %w", version, fileName, err)
+		return Cache{}, fmt.Errorf("decoding for version '%d' from '%s': %w", version, fileName, err)
 	}
 
 	if cache, err = cache.transformToCurrent(); err != nil {
-		return nil, fmt.Errorf("cannot transform from version %d to %d: %w", version, currentVersion, err)
+		return Cache{}, fmt.Errorf("cannot transform from version %d to %d: %w", version, currentVersion, err)
 	}
 
 	log.Printf("Loaded cache (version %d), transformed to version %d.", version, currentVersion)
 
-	return cache, nil
+	return Cache{cache, lock, true}, nil
 }
-- 
cgit v1.2.3-54-g00ecf