Handle the same file name in the fetchBlob method (#258)

With the suggested change, when a file exists, we'll verify whether it
has the same content before skipping it.

Author: dkovba

Sources/ContainerizationOCI/Client/LocalOCILayoutClient.swift

@@ -35,6 +35,26 @@ package final class LocalOCILayoutClient: ContentClient {
         return c
     }
 
+    private func calculateFileDigest(at url: URL) throws -> SHA256Digest {
+        let fileHandle = try FileHandle(forReadingFrom: url)
+        defer {
+            try? fileHandle.close()
+        }
+
+        var hasher = SHA256()
+        let chunkSize = Int(getpagesize()) * 1024
+
+        while true {
+            let chunk = fileHandle.readData(ofLength: chunkSize)
+            if chunk.isEmpty {
+                break
+            }
+            hasher.update(data: chunk)
+        }
+
+        return hasher.finalize()
+    }
+
     package func fetch<T: Codable>(name: String, descriptor: Descriptor) async throws -> T {
         let c = try await self._fetch(digest: descriptor.digest)
         return try c.decode()
@@ -44,7 +64,8 @@ package final class LocalOCILayoutClient: ContentClient {
         let c = try await self._fetch(digest: descriptor.digest)
         let fileManager = FileManager.default
         let filePath = file.absolutePath()
-        if !fileManager.fileExists(atPath: filePath) {
+
+        do {
             let src = c.path
             try fileManager.copyItem(at: src, to: file)
 
@@ -53,7 +74,33 @@ package final class LocalOCILayoutClient: ContentClient {
                     ProgressEvent(event: "add-size", value: fileSize)
                 ])
             }
+        } catch let error as NSError {
+            guard error.code == NSFileWriteFileExistsError else {
+                throw error
+            }
+
+            do {
+                let expectedDigest = try c.digest()
+                let existingDigest = try calculateFileDigest(at: file)
+
+                guard existingDigest.digestString == expectedDigest.digestString else {
+                    throw ContainerizationError(
+                        .internalError,
+                        message:
+                            "File \(filePath) exists but contains different content. Expected digest: \(expectedDigest.digestString), existing digest: \(existingDigest.digestString)"
+                    )
+                }
+
+                if let progress, let fileSize = fileManager.fileSize(atPath: filePath) {
+                    await progress([
+                        ProgressEvent(event: "add-size", value: fileSize)
+                    ])
+                }
+            } catch {
+                throw error
+            }
         }
+
         let size = try Int64(c.size())
         let digest = try c.digest()
         return (size, digest)