From eb62801c3c23f196ddef66c183231906ab4ef51c Mon Sep 17 00:00:00 2001
From: Jasper Van der Jeugt <m@jaspervdj.be>
Date: Tue, 26 Nov 2024 20:43:28 +0100
Subject: [PATCH] Atomically write speaker notes

---
 CHANGELOG.md                       |  8 ++++++++
 README.md                          |  8 +++-----
 lib/Patat/Presentation/Comments.hs | 20 ++++++++++++++++----
 3 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 86e3880..a6a7523 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,13 @@
 # Changelog
 
+## unreleased
+
+ *  Use a temporary file to atomically write speaker notes.
+
+    We weren't writing the file all-at-once before, so if you were using a
+    simple tool like `tail -F` before, this could cause some speaker notes to
+    not be displayed.
+
 ## 0.13.0.0 (2024-10-30)
 
  *  Incrementally display output of `eval` commands (#132)
diff --git a/README.md b/README.md
index f608525..6c81c4e 100644
--- a/README.md
+++ b/README.md
@@ -791,13 +791,11 @@ patat:
 ```
 
 Then, you can display these in a second terminal (presumably on a second
-monitor) by just displaying this file whenever it changes.  [entr] is one
-way to do that:
-
-[entr]: http://eradman.com/entrproject/
+monitor) by just displaying this file whenever it changes. `tail` is a primitive
+way of doing that:
 
 ```bash
-echo /tmp/notes.txt | entr -s 'clear; cat /tmp/notes.txt'
+tail -F /tmp/notes.txt
 ```
 
 Alternatively, just use a second `patat` instance with `--watch` enabled:
diff --git a/lib/Patat/Presentation/Comments.hs b/lib/Patat/Presentation/Comments.hs
index 92dbdcd..f49150c 100644
--- a/lib/Patat/Presentation/Comments.hs
+++ b/lib/Patat/Presentation/Comments.hs
@@ -35,7 +35,8 @@ import qualified Data.Yaml                   as Yaml
 import           Patat.EncodingFallback      (EncodingFallback)
 import qualified Patat.EncodingFallback      as EncodingFallback
 import           Patat.Presentation.Settings
-import           System.Directory            (removeFile)
+import           System.Directory            (removeFile, renameFile)
+import           System.FilePath             (splitFileName, (</>))
 import qualified System.IO                   as IO
 import qualified Text.Pandoc                 as Pandoc
 
@@ -136,9 +137,20 @@ writeSpeakerNotes
     :: SpeakerNotesHandle -> EncodingFallback -> SpeakerNotes -> IO ()
 writeSpeakerNotes h encodingFallback sn = do
     change <- IORef.atomicModifyIORef' (snhActive h) $ \old -> (sn, old /= sn)
-    when change $ IO.withFile (snsFile $ snhSettings h) IO.WriteMode $ \ioh ->
-        EncodingFallback.withHandle ioh encodingFallback $
-        T.hPutStr ioh $ speakerNotesToText sn
+    when change $ do
+        -- Write the file and then rename it to get atomic replace.
+        IO.withFile speakerNotesTmpPath IO.WriteMode $ \ioh ->
+            EncodingFallback.withHandle ioh encodingFallback $
+            T.hPutStr ioh $ speakerNotesToText sn
+        renameFile speakerNotesTmpPath speakerNotesPath
+  where
+    speakerNotesPath = snsFile $ snhSettings h
+    speakerNotesTmpPath =
+        -- We only get atomic rename if we're on the same filesystem.  I think
+        -- we can assume that the directory the speaker notes are in is
+        --  | entr -s 'clear; cat /tmp/notes.txt'writable.
+        let (dir, name) = splitFileName speakerNotesPath in
+        dir </> ".tmp." <> name
 
 
 --------------------------------------------------------------------------------