Unsubscribe markers when destroying display buffer

Previously the display buffer markers would still fire events
even after a display buffer was destroyed since they were not
being unsubscribed from their underlying buffer marker which
could still fire events in the case of an editor split on the
same buffer.

Closes #593

spec/app/display-buffer-spec.coffee

@@ -532,6 +532,13 @@ describe "DisplayBuffer", ->
       buffer.delete([[6, 0], [6, 65]])
       expect(displayBuffer.getMaxLineLength()).toBe 62
 
+  describe ".destroy()", ->
+    it "unsubscribes all display buffer markers from their underlying buffer marker (regression)", ->
+      marker = displayBuffer.markBufferPosition([12, 2])
+      displayBuffer.destroy()
+      expect(marker.bufferMarker.subscriptionCount()).toBe 0
+      expect( -> buffer.insert([12, 2], '\n')).not.toThrow()
+
   describe "markers", ->
     beforeEach ->
       displayBuffer.createFold(4, 7)

src/app/display-buffer-marker.coffee

@@ -1,6 +1,7 @@
 Range = require 'range'
 _ = require 'underscore'
 EventEmitter = require 'event-emitter'
+Subscriber = require 'subscriber'
 
 module.exports =
 class DisplayBufferMarker
@@ -131,6 +132,7 @@ class DisplayBufferMarker
   # Destroys the marker
   destroy: ->
     @bufferMarker.destroy()
+    @unsubscribe()
 
   # Returns a {String} representation of the marker
   inspect: ->
@@ -143,11 +145,11 @@ class DisplayBufferMarker
     @trigger 'destroyed'
 
   observeBufferMarker: ->
-    @bufferMarker.on 'destroyed', => @destroyed()
+    @subscribe @bufferMarker, 'destroyed', => @destroyed()
 
     @getHeadScreenPosition() # memoize current value
     @getTailScreenPosition() # memoize current value
-    @bufferMarker.on 'changed', ({oldHeadPosition, newHeadPosition, oldTailPosition, newTailPosition, bufferChanged, valid}) =>
+    @subscribe @bufferMarker, 'changed', ({oldHeadPosition, newHeadPosition, oldTailPosition, newTailPosition, bufferChanged, valid}) =>
       @notifyObservers
         oldHeadBufferPosition: oldHeadPosition
         newHeadBufferPosition: newHeadPosition
@@ -192,3 +194,4 @@ class DisplayBufferMarker
     }
 
 _.extend DisplayBufferMarker.prototype, EventEmitter
+_.extend DisplayBufferMarker.prototype, Subscriber

src/app/display-buffer.coffee

@@ -393,9 +393,12 @@ class DisplayBuffer
   #
   # Returns the {DisplayBufferMarker} (if it exists).
   getMarker: (id) ->
-    @markers[id] ?= do =>
+    marker = @markers[id]
+    unless marker?
       if bufferMarker = @buffer.getMarker(id)
-        new DisplayBufferMarker({bufferMarker, displayBuffer: this})
+        marker = new DisplayBufferMarker({bufferMarker, displayBuffer: this})
+        @markers[id] = marker
+    marker
 
   # Retrieves the active markers in the buffer.
   #
@@ -492,6 +495,7 @@ class DisplayBuffer
       marker.notifyObservers(bufferChanged: false)
 
   destroy: ->
+    marker.unsubscribe() for marker in @getMarkers()
     @tokenizedBuffer.destroy()
     @unsubscribe()