Fix for #294, thanks to @francisdb for identifying root cause

parser-typechecker/src/Unison/Codebase/Branch.hs

@@ -600,7 +600,11 @@ intersectWithFile branch file =
 
 
 modify :: (Branch0 -> Branch0) -> Branch -> Branch
-modify f (Branch b) = Branch $ Causal.step f b
+modify f b@(Branch causal) = let
+  b0 = head b
+  b1 = f b0
+  in if b1 == b0 then b
+     else Branch $ Causal.cons b1 causal
 
 append :: Branch0 -> Branch -> Branch
 append b0 = modify (<> b0)

parser-typechecker/src/Unison/Codebase/CommandLine2.hs

@@ -516,9 +516,6 @@ notifyUser dir o = case o of
 allow :: FilePath -> Bool
 allow = (||) <$> (".u" `isSuffixOf`) <*> (".uu" `isSuffixOf`)
 
--- TODO: Return all of these thread IDs so we can throw async exceptions at
--- them when we need to quit.
-
 watchFileSystem :: TQueue Event -> FilePath -> IO (IO ())
 watchFileSystem q dir = do
   (cancel, watcher) <- Watch.watchDirectory dir allow
@@ -527,13 +524,19 @@ watchFileSystem q dir = do
     atomically . Q.enqueue q $ UnisonFileChanged (Text.pack filePath) text
   pure (cancel >> killThread t)
 
-watchBranchUpdates :: TQueue Event -> Codebase IO v a -> IO (IO ())
-watchBranchUpdates q codebase = do
+watchBranchUpdates :: IO (Branch, BranchName) -> TQueue Event -> Codebase IO v a -> IO (IO ())
+watchBranchUpdates currentBranch q codebase = do
   (cancelExternalBranchUpdates, externalBranchUpdates) <-
     Codebase.branchUpdates codebase
   thread <- forkIO . forever $ do
     updatedBranches <- externalBranchUpdates
-    atomically . Q.enqueue q . UnisonBranchChanged $ updatedBranches
+    (b, bname) <- currentBranch
+    b' <- Codebase.getBranch codebase bname
+    -- We only issue the event if the branch is different than what's already
+    -- in memory. This skips over file events triggered by saving to disk what's
+    -- already in memory.
+    when (b' /= Just b) $
+      atomically . Q.enqueue q . UnisonBranchChanged $ updatedBranches
   pure (cancelExternalBranchUpdates >> killThread thread)
 
 warnNote :: String -> String
@@ -899,7 +902,7 @@ main dir currentBranchName _initialFile startRuntime codebase = do
     runtime                  <- startRuntime
     branchRef                <- newIORef (currentBranch, currentBranchName)
     cancelFileSystemWatch    <- watchFileSystem eventQueue dir
-    cancelWatchBranchUpdates <- watchBranchUpdates eventQueue codebase
+    cancelWatchBranchUpdates <- watchBranchUpdates (readIORef branchRef) eventQueue codebase
     let patternMap =
           Map.fromList
             $   validInputs

parser-typechecker/src/Unison/Codebase/Editor/Actions.hs

@@ -196,7 +196,13 @@ loop s = Free.unfold' go s
                   else Editor.addCollisionHandler
             updateo <- Free.eval $ SlurpFile collisionHandler (currentBranch s) uf'
             let branch' = updatedBranch updateo
-            doMerge (currentBranchName s) branch'
+            -- Don't bother doing anything if the branch is unchanged by the slurping
+            when (branch' /= currentBranch s) $ do
+              -- This order is important - we tell the app state about the
+              -- branch change before doing the merge, so it knows to ignore
+              -- the file system event that is triggered by `doMerge`
+              Free.eval $ SwitchBranch branch' (currentBranchName s)
+              doMerge (currentBranchName s) branch'
             Free.eval . Notify $ SlurpOutput updateo
             pure . Right $ s { currentBranch = branch' }
         ListBranchesI ->