Atomic write to addrbook and privvalidator

common/os.go

@@ -2,6 +2,7 @@ package common
 
 import (
 	"fmt"
+	"io/ioutil"
 	"os"
 	"os/signal"
 )
@@ -26,3 +27,26 @@ func Exit(s string) {
 	fmt.Printf(s + "\n")
 	os.Exit(1)
 }
+
+// Writes to newBytes to filePath.
+// Guaranteed not to lose *both* oldBytes and newBytes,
+// (assuming that the OS is perfect)
+func AtomicWriteFile(filePath string, newBytes []byte) error {
+	// If a file already exists there, copy to filePath+".bak" (overwrite anything)
+	if _, err := os.Stat(filePath); !os.IsNotExist(err) {
+		fileBytes, err := ioutil.ReadFile(filePath)
+		if err != nil {
+			return fmt.Errorf("Failed to read file %v. %v", filePath, err)
+		}
+		err = ioutil.WriteFile(filePath+".bak", fileBytes, 0600)
+		if err != nil {
+			return fmt.Errorf("Failed to write file %v. %v", filePath+".bak", err)
+		}
+	}
+	// Write newBytes to filePath.
+	err := ioutil.WriteFile(filePath, newBytes, 0600)
+	if err != nil {
+		return fmt.Errorf("Failed to write file %v. %v", filePath, err)
+	}
+	return nil
+}