eth: fix filter map data race

This commit also documents (but doesn't enforce) that filters
are immutable while they're installed. This required a minor API
change.

ethereum.go

@@ -95,6 +95,8 @@ type Ethereum struct {
 
 	isUpToDate bool
 
+	filterMu sync.RWMutex
+	filterId int
 	filters  map[int]*ethchain.Filter
 }
 
@@ -594,22 +596,29 @@ out:
 	}
 }
 
-var filterId = 0
-
-func (self *Ethereum) InstallFilter(object map[string]interface{}) (*ethchain.Filter, int) {
-	defer func() { filterId++ }()
-
-	filter := ethchain.NewFilterFromMap(object, self)
-	self.filters[filterId] = filter
-
-	return filter, filterId
+// InstallFilter adds filter for blockchain events.
+// The filter's callbacks will run for matching blocks and messages.
+// The filter should not be modified after it has been installed.
+func (self *Ethereum) InstallFilter(filter *ethchain.Filter) (id int) {
+	self.filterMu.Lock()
+	id = self.filterId
+	self.filters[id] = filter
+	self.filterId++
+	self.filterMu.Unlock()
+	return id
 }
 
 func (self *Ethereum) UninstallFilter(id int) {
+	self.filterMu.Lock()
 	delete(self.filters, id)
+	self.filterMu.Unlock()
 }
 
+// GetFilter retrieves a filter installed using InstallFilter.
+// The filter may not be modified.
 func (self *Ethereum) GetFilter(id int) *ethchain.Filter {
+	self.filterMu.RLock()
+	defer self.filterMu.RUnlock()
 	return self.filters[id]
 }
 
@@ -627,14 +636,17 @@ out:
 			break out
 		case block := <-blockChan:
 			if block, ok := block.Resource.(*ethchain.Block); ok {
+				self.filterMu.RLock()
 				for _, filter := range self.filters {
 					if filter.BlockCallback != nil {
 						filter.BlockCallback(block)
 					}
 				}
+				self.filterMu.RUnlock()
 			}
 		case msg := <-messageChan:
 			if messages, ok := msg.Resource.(ethstate.Messages); ok {
+				self.filterMu.RLock()
 				for _, filter := range self.filters {
 					if filter.MessageCallback != nil {
 						msgs := filter.FilterMessages(messages)
@@ -643,6 +655,7 @@ out:
 						}
 					}
 				}
+				self.filterMu.RUnlock()
 			}
 		}
 	}