agent/proxy: improve comments on snapshotting

agent/proxy/manager.go

@@ -96,8 +96,14 @@ type Manager struct {
 	// for changes to this value.
 	runState managerRunState
 
-	proxies      map[string]Proxy
+	// lastSnapshot stores a pointer to the last snapshot that successfully
+	// wrote to disk. This is used for dup detection to prevent rewriting
+	// the same snapshot multiple times. snapshots should never be that
+	// large so keeping it in-memory should be cheap even for thousands of
+	// proxies (unlikely scenario).
 	lastSnapshot *snapshot
+
+	proxies map[string]Proxy
 }
 
 // NewManager initializes a Manager. After initialization, the exported

agent/proxy/snapshot.go

@@ -101,7 +101,10 @@ func (m *Manager) snapshot(path string, checkDup bool) error {
 
 	// Write the file
 	err = file.WriteAtomic(path, encoded)
-	if err == nil && checkDup {
+
+	// If we are checking for dups and we had a successful write, store
+	// it so we don't rewrite the same value.
+	if checkDup && err == nil {
 		m.lastSnapshot = &s
 	}
 	return err