rpc: Re-factor ReadFile behavior client <--> server.

Current code did not marshal/unmarshal buffers properly from
the server, The reason being buffers have to be allocated and
sent properly back to client to be consumable.

cmd/storage-rpc-client.go

@@ -49,6 +49,9 @@ func splitNetPath(networkPath string) (netAddr, netPath string) {
 // written so that the storageAPI errors are consistent across network
 // disks as well.
 func toStorageErr(err error) error {
+	if err == nil {
+		return nil
+	}
 	switch err.Error() {
 	case io.EOF.Error():
 		return io.EOF
@@ -240,16 +243,18 @@ func (n networkStorage) ReadFile(volume string, path string, offset int64, buffe
 	if n.rpcClient == nil {
 		return 0, errVolumeBusy
 	}
-	if err = n.rpcClient.Call("Storage.ReadFileHandler", ReadFileArgs{
+	var result []byte
+	err = n.rpcClient.Call("Storage.ReadFileHandler", ReadFileArgs{
 		Token:  n.rpcToken,
 		Vol:    volume,
 		Path:   path,
 		Offset: offset,
-		Buffer: buffer,
-	}, &m); err != nil {
-		return 0, toStorageErr(err)
-	}
-	return m, nil
+		Size:   len(buffer),
+	}, &result)
+	// Copy results to buffer.
+	copy(buffer, result)
+	// Return length of result, err if any.
+	return int64(len(result)), toStorageErr(err)
 }
 
 // ListDir - list all entries at prefix.

cmd/storage-rpc-server-datatypes.go

@@ -76,8 +76,8 @@ type ReadFileArgs struct {
 	// Starting offset to start reading into Buffer.
 	Offset int64
 
-	// Data buffer read from the path at offset.
-	Buffer []byte
+	// Data size read from the path at offset.
+	Size int
 }
 
 // AppendFileArgs represents append file RPC arguments.

cmd/storage-rpc-server.go

@@ -17,8 +17,10 @@
 package main
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
+	"io"
 	"net/rpc"
 	"path"
 	"strings"
@@ -162,16 +164,29 @@ func (s *storageServer) ReadAllHandler(args *ReadFileArgs, reply *[]byte) error
 }
 
 // ReadFileHandler - read file handler is rpc wrapper to read file.
-func (s *storageServer) ReadFileHandler(args *ReadFileArgs, reply *int64) error {
+func (s *storageServer) ReadFileHandler(args *ReadFileArgs, reply *[]byte) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			// Recover any panic and return ErrCacheFull.
+			err = bytes.ErrTooLarge
+		}
+	}() // Do not crash the server.
 	if !isRPCTokenValid(args.Token) {
 		return errors.New("Invalid token")
 	}
-	n, err := s.storage.ReadFile(args.Vol, args.Path, args.Offset, args.Buffer)
-	if err != nil {
-		return err
+	// Allocate the requested buffer from the client.
+	*reply = make([]byte, args.Size)
+	var n int64
+	n, err = s.storage.ReadFile(args.Vol, args.Path, args.Offset, *reply)
+	// Sending an error over the rpc layer, would cause unmarshalling to fail. In situations
+	// when we have short read i.e `io.ErrUnexpectedEOF` treat it as good condition and copy
+	// the buffer properly.
+	if err == io.ErrUnexpectedEOF {
+		// Reset to nil as good condition.
+		err = nil
 	}
-	*reply = n
-	return nil
+	*reply = (*reply)[0:n]
+	return err
 }
 
 // AppendFileHandler - append file handler is rpc wrapper to append file.