Add mutex on LinkFromPool to fix #1449

This fixes the race condition that happens when you call publish
concurrently. It adds a valuable test that reproduces the error almost
deterministically, it's hard to say always but I have run this in loop
100 times and it reproduces the error consistently without the patch and
after the patch it works consistently.

files/public.go

@@ -22,6 +22,26 @@ type PublishedStorage struct {
 	verifyMethod uint
 }
 
+// Global mutex map to prevent concurrent access to the same destinationPath in LinkFromPool
+var (
+	fileLockMutex sync.Mutex
+	fileLocks = make(map[string]*sync.Mutex)
+)
+
+// getFileLock returns a mutex for a specific file path to prevent concurrent modifications
+func getFileLock(filePath string) *sync.Mutex {
+	fileLockMutex.Lock()
+	defer fileLockMutex.Unlock()
+
+	if mutex, exists := fileLocks[filePath]; exists {
+		return mutex
+	}
+
+	mutex := &sync.Mutex{}
+	fileLocks[filePath] = mutex
+	return mutex
+}
+
 // Check interfaces
 var (
 	_ aptly.PublishedStorage           = (*PublishedStorage)(nil)
@@ -136,6 +156,12 @@ func (storage *PublishedStorage) LinkFromPool(publishedPrefix, publishedRelPath,
 
 	baseName := filepath.Base(fileName)
 	poolPath := filepath.Join(storage.rootPath, publishedPrefix, publishedRelPath, filepath.Dir(fileName))
+	destinationPath := filepath.Join(poolPath, baseName)
+
+	// Acquire file-specific lock to prevent concurrent access to the same file
+	fileLock := getFileLock(destinationPath)
+	fileLock.Lock()
+	defer fileLock.Unlock()
 
 	var localSourcePool aptly.LocalPackagePool
 	if storage.linkMethod != LinkMethodCopy {
@@ -154,7 +180,7 @@ func (storage *PublishedStorage) LinkFromPool(publishedPrefix, publishedRelPath,
 
 	var dstStat os.FileInfo
 
-	dstStat, err = os.Stat(filepath.Join(poolPath, baseName))
+	dstStat, err = os.Stat(destinationPath)
 	if err == nil {
 		// already exists, check source file
 
@@ -173,7 +199,7 @@ func (storage *PublishedStorage) LinkFromPool(publishedPrefix, publishedRelPath,
 			} else {
 				// if source and destination have the same checksums, no need to copy
 				var dstMD5 string
-				dstMD5, err = utils.MD5ChecksumForFile(filepath.Join(poolPath, baseName))
+				dstMD5, err = utils.MD5ChecksumForFile(destinationPath)
 
 				if err != nil {
 					return err
@@ -204,11 +230,11 @@ func (storage *PublishedStorage) LinkFromPool(publishedPrefix, publishedRelPath,
 
 		// source and destination have different inodes, if !forced, this is fatal error
 		if !force {
-			return fmt.Errorf("error linking file to %s: file already exists and is different", filepath.Join(poolPath, baseName))
+			return fmt.Errorf("error linking file to %s: file already exists and is different", destinationPath)
 		}
 
 		// forced, so remove destination
-		err = os.Remove(filepath.Join(poolPath, baseName))
+		err = os.Remove(destinationPath)
 		if err != nil {
 			return err
 		}
@@ -223,7 +249,7 @@ func (storage *PublishedStorage) LinkFromPool(publishedPrefix, publishedRelPath,
 		}
 
 		var dst *os.File
-		dst, err = os.Create(filepath.Join(poolPath, baseName))
+		dst, err = os.Create(destinationPath)
 		if err != nil {
 			_ = r.Close()
 			return err
@@ -244,9 +270,9 @@ func (storage *PublishedStorage) LinkFromPool(publishedPrefix, publishedRelPath,
 
 		err = dst.Close()
 	} else if storage.linkMethod == LinkMethodSymLink {
-		err = localSourcePool.Symlink(sourcePath, filepath.Join(poolPath, baseName))
+		err = localSourcePool.Symlink(sourcePath, destinationPath)
 	} else {
-		err = localSourcePool.Link(sourcePath, filepath.Join(poolPath, baseName))
+		err = localSourcePool.Link(sourcePath, destinationPath)
 	}
 
 	return err