Added support for chunk read in Filestore

ambry-server/src/main/java/com/github/ambry/server/AmbryServer.java

@@ -100,6 +100,8 @@
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashSet;
@@ -504,11 +506,10 @@ public void startup() throws InstantiationException {
         System.err.println("An error occurred: " + e.getMessage());
       }
 
-      FileInputStream inputStream = fileStore.getStreamForFileRead("/tmp/0/", "0_log");
-      long fileSize = inputStream.available();
-      byte[] content = new byte[(int) fileSize]; // Read the content of the source file into a byte array
-      inputStream.read(content); // Read bytes into the array
-      System.out.println("Parsed log file contents read: " + new String(content));
+      int offset = 10;
+      int size = 10;
+      ByteBuffer byteBuffer = fileStore.getStreamForFileRead("/tmp/0/", "0_log", offset, size);
+      System.out.println("Parsed log file contents read for offset=" + offset + ", size=" + size + " is: " + StandardCharsets.UTF_8.decode(byteBuffer));
 
 
     } catch (Exception e) {

ambry-store/src/main/java/com/github/ambry/store/FileStore.java

@@ -22,10 +22,13 @@
 import java.io.DataOutputStream;
 import java.io.File;
 import java.io.FileInputStream;
+import java.io.FileNotFoundException;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.io.RandomAccessFile;
+import java.nio.ByteBuffer;
 import java.nio.file.Files;
 import java.nio.file.Paths;
 import java.nio.file.StandardOpenOption;
@@ -61,19 +64,20 @@ public void stop() {
   }
 
 
-  public FileInputStream getStreamForFileRead(String mountPath, String fileName)
+  public ByteBuffer getStreamForFileRead(String mountPath, String fileName, int offset, int size)
       throws IOException {
     if(!isRunning){
       throw new FileStoreException("FileStore is not running", FileStoreErrorCode.FileStoreRunningFailure);
     }
     // TODO: Handle edge cases and validations
     String filePath = mountPath + "/" + fileName;
     File file = new File(filePath);
-    // Check if file exists and is readable
-    if (!file.exists() || !file.canRead()) {
-      throw new IOException("File doesn't exist or cannot be read: " + filePath);
-    }
-    return new FileInputStream(file);
+    RandomAccessFile randomAccessFile = new RandomAccessFile(file, "r");
+    randomAccessFile.seek(offset);
+    ByteBuffer buf = ByteBuffer.allocate(size);
+    randomAccessFile.getChannel().read(buf);
+    buf.flip();
+    return buf;
   }
 
   public void putChunkToFile(String outputFilePath, FileInputStream fileInputStream)