Fix duplicate daemon spawn on login via atomic PID claim

Replace the check-then-write singleton guard in run_loop with an atomic
noclobber PID-file claim. The old guard read the PID file then wrote it
in two steps; two starts racing (e.g. a fast double-login) could both
read a stale PID and spawn duplicate daemons. With noclobber only the
first writer creates the file; the loser aborts, and a stale PID from a
previous session is taken over rather than duplicated.

Also document install (~/bin copy) and autostart in CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

2 files changed, 29 insertions, 7 deletions

diff --git a/CLAUDE.md b/CLAUDE.md
index f47ab25..fd8523d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -6,6 +6,23 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 `breaktimer` is a Bash break-reminder daemon for a Linux/Wayland desktop (dunst + pipewire + Waybar). No build, no package manager, no tests — it's a single shell script plus Waybar integration files. Comments and notification strings are in Italian; keep that convention when editing.
 
+## Install
+
+The runtime copy lives at `~/bin/breaktimer.sh` (a plain copy, not a symlink). After editing the repo, install and reload:
+
+```bash
+cp breaktimer.sh ~/bin/breaktimer.sh
+~/bin/breaktimer.sh restart
+```
+
+Autostart is a Hyprland exec in `~/.config/hypr/sections/autostart.lua`:
+
+```lua
+hl.exec_cmd("/home/danix/bin/breaktimer.sh start")
+```
+
+`start` is idempotent: an atomic `noclobber` PID-file claim in `run_loop` means a second `start` (or a fast double-login) can't spawn a duplicate daemon — the loser aborts. A stale PID file from a previous session is taken over, not duplicated.
+
 ## Running
 
 ```bash
diff --git a/breaktimer.sh b/breaktimer.sh
index 4ce5a42..9a9c5fa 100755
--- a/breaktimer.sh
+++ b/breaktimer.sh
@@ -107,15 +107,20 @@ notify_back() {
 
 # ---------- Loop principale ----------
 run_loop() {
-    # singleton: rifiuta se un altro daemon vivo possiede gia' il PID file.
-    # Evita due loop che si calpestano gli stessi file di stato.
+    # singleton: rivendica il PID file in modo atomico con noclobber.
+    # check-then-write avrebbe una finestra di race: due loop avviati insieme
+    # (es. doppio login) leggono entrambi un PID file stantio e partono. Con
+    # noclobber solo il primo crea il file; il secondo fallisce e poi decide.
     local owner
-    owner="$(cat "$PID_FILE" 2>/dev/null)"
-    if [ -n "$owner" ] && [ "$owner" != "$$" ] && kill -0 "$owner" 2>/dev/null; then
-        echo "breaktimer: gia' in esecuzione (PID $owner), run annullato" >&2
-        exit 1
+    if ! ( set -o noclobber; echo "$$" > "$PID_FILE" ) 2>/dev/null; then
+        owner="$(cat "$PID_FILE" 2>/dev/null)"
+        if [ -n "$owner" ] && [ "$owner" != "$$" ] && kill -0 "$owner" 2>/dev/null; then
+            echo "breaktimer: gia' in esecuzione (PID $owner), run annullato" >&2
+            exit 1
+        fi
+        # PID file stantio (owner morto): rivendicalo.
+        echo "$$" > "$PID_FILE"
     fi
-    echo "$$" > "$PID_FILE"
     echo "running" > "$STATE_FILE"
     echo "working" > "$PHASE_FILE"
     local count=0