os.Rename fails on windows if the file exists. I can find no way to

invoke ReplaceFile via Go, so lumberjack's registrar updates will not
be atomic on Windows platform. On windows, the "old" registrar db is
saved as ".lumberjack.old"

harvester.go

@@ -34,6 +34,8 @@ func (h *Harvester) Harvest(output chan *FileEvent) {
   // get current offset in file
   offset, _ := h.file.Seek(0, os.SEEK_CUR)
 
+  log.Printf("Current file offset: %d\n", offset)
+
   // TODO(sissel): Make the buffer size tunable at start-time
   reader := bufio.NewReaderSize(h.file, 16<<10) // 16kb buffer by default
 

registrar.go

@@ -2,8 +2,6 @@ package main
 
 import (
   "log"
-  "os"
-  "encoding/json"
 )
 
 func Registrar(input chan []*FileEvent) {
@@ -28,25 +26,12 @@ func Registrar(input chan []*FileEvent) {
         Inode: ino,
         Device: dev,
       }
+      log.Printf("State %s: %d\n", *event.Source, event.Offset)
     }
 
     if len(state) > 0 {
-      write(state)
-      os.Rename(".lumberjack.new", ".lumberjack")
+      WriteRegistry(state, ".lumberjack")
     }
   }
 }
 
-func write(state map[string]*FileState) {
-  log.Printf("Saving registrar state.\n")
-  // Open tmp file, write, flush, rename
-  file, err := os.Create(".lumberjack.new")
-  if err != nil {
-    log.Printf("Failed to open .lumberjack.new for writing: %s\n", err)
-    return
-  }
-
-  encoder := json.NewEncoder(file)
-  encoder.Encode(state)
-  file.Close()
-}

registrar_other.go

+// +build !windows
+
+package main
+import (
+  "encoding/json"
+  "os"
+  "log"
+)
+
+func WriteRegistry(state map[string]*FileState, path string) {
+  // Open tmp file, write, flush, rename
+  file, err := os.Create(".lumberjack.new")
+  if err != nil {
+    log.Printf("Failed to open .lumberjack.new for writing: %s\n", err)
+    return
+  }
+  defer file.Close()
+
+  encoder := json.NewEncoder(file)
+  encoder.Encode(state)
+
+  os.Rename(".lumberjack.new", path)
+}

registrar_windows.go

+package main
+
+import (
+  "encoding/json"
+  "os"
+  "log"
+)
+
+func WriteRegistry(state map[string]*FileState, path string) {
+  tmp := path + ".new"
+  file, err := os.Create(tmp)
+  if err != nil {
+    log.Printf("Failed to open .lumberjack.new for writing: %s\n", err)
+    return
+  }
+
+  encoder := json.NewEncoder(file)
+  encoder.Encode(state)
+  file.Close()
+
+  old := path + ".old"
+  os.Rename(path, old)
+  os.Rename(tmp, path)
+}