From 86a4634ee9ee5243aed165fcf2b516ac508e7785 Mon Sep 17 00:00:00 2001
From: Seb Patane <seb@libero.com.au>
Date: Mon, 14 Nov 2016 12:41:55 +1000
Subject: [PATCH] Only restore blobs which don't exist locally * If we can,
 calculate the hash of the part of the local file that the blob corresponds to
   * If it matches then we already have the data and can skip fetching it   *
 This should significantly improve the speed of remote restores when a local
 dataset already exists

---
 src/restic/node.go | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/restic/node.go b/src/restic/node.go
index 81f3ef8d5..7ee38dbbf 100644
--- a/src/restic/node.go
+++ b/src/restic/node.go
@@ -192,7 +192,7 @@ func (node Node) createDirAt(path string) error {
 }
 
 func (node Node) createFileAt(path string, repo Repository) error {
-	f, err := fs.OpenFile(path, os.O_CREATE|os.O_WRONLY, 0600)
+	f, err := fs.OpenFile(path, os.O_CREATE|os.O_RDWR, 0600)
 	defer f.Close()
 
 	if err != nil {
@@ -206,6 +206,24 @@ func (node Node) createFileAt(path string, repo Repository) error {
 			return err
 		}
 
+		// Skip loading if the data already exists locally
+		existingData := make([]byte, size)
+
+		readSize, _ := f.Read(existingData)
+		if readSize == int(size) {
+			if Hash(existingData) == id {
+				continue
+			}
+		}
+
+		debug.Log("Fetching blob %v (size: %v)", id, size)
+
+		// Move the write offset back to where it was
+		_, err = f.Seek(int64(-readSize), os.SEEK_CUR)
+		if err != nil {
+			return err
+		}
+
 		buf = buf[:cap(buf)]
 		if uint(len(buf)) < size {
 			buf = make([]byte, size)