make state update atomic and clean up some code

1 files changed, 43 insertions, 9 deletions

diff --git a/core/update b/core/update
index 04fd29c..fbe0e86 100644
--- a/core/update
+++ b/core/update
@@ -1,13 +1,12 @@
 #!/bin/sh
 export now="$(date +%s.%N)"
 
-update_time=0
-
+# load current state
 mkdir -p "$POMODORO_STATE_PATH"
 load_or_init() {
 	property="$1"
 	default_value="$2"
-	property_path="$POMODORO_STATE_PATH/$property"
+	property_path="$POMODORO_STATE_PATH/current/$property"
 	if [ -f "$property_path" ] ; then
 		eval $property='"$(cat "$property_path")"'
 	else
@@ -17,7 +16,13 @@ load_or_init() {
 [ -z "$lap" ] && load_or_init lap 0
 [ -z "$state" ] && load_or_init state 'reset'
 [ -z "$time" ] && load_or_init time 0.0
-[ "$time" = "0.0" ] && update_time=1
+
+# save state (for diff)
+diff_state() { printf '%s:' "$lap" "$state" "$time" ; }
+original_state="$(diff_state)"
+
+# allow overriding this value manually
+[ -z "$update_time" ] && update_time=0
 
 # calculate remaining time on timer
 if [ -z "$remaining" ] ; then
@@ -34,6 +39,7 @@ fi
 
 # update remaining time if in reset
 [ "$state" = 'reset' ] && update_time=1
+[ "$time" = "0.0" ] && update_time=1
 
 if [ $update_time -eq 1 ] ; then
 	time="$POMODORO_NORMAL_DURATION"
@@ -42,12 +48,40 @@ if [ $update_time -eq 1 ] ; then
 	remaining="$time"
 fi
 
-# save state
 save_state() {
-	mkdir -p "$POMODORO_STATE_PATH"
-	echo "$lap" > "$POMODORO_STATE_PATH/lap"
-	echo "$state" > "$POMODORO_STATE_PATH/state"
-	echo "$time" > "$POMODORO_STATE_PATH/time"
+	# do not save files if state didn't change
+	[ "$original_state" = "$(diff_state)" ] && return
+
+	# The state update needs to be atomic since other instances may access the
+	# state during an update. This is accomplished by replacing a single symlink,
+	# which should be atomic on most POSIX-compatible platforms, and is less
+	# complicated than a lockfile. $POMODORO_STATE_PATH contains three items:
+	#
+	# 1. current -- a symlink to `primary` or `secondary`
+	# 2. primary -- folder containing current state*
+	# 3. secondary -- temporary folder used write new state
+	#
+	# * `primary` will only contain stale state during an update (i.e. while
+	# `current` is pointing to `secondary`)
+
+	mkdir -p "$POMODORO_STATE_PATH/primary" "$POMODORO_STATE_PATH/secondary"
+
+	echo "$lap" > "$POMODORO_STATE_PATH/secondary/lap"
+	echo "$state" > "$POMODORO_STATE_PATH/secondary/state"
+	echo "$time" > "$POMODORO_STATE_PATH/secondary/time"
+
+	ln -sf "secondary" "$POMODORO_STATE_PATH/secondary/current"
+
+	# this is the atomic update that replaces the `current` symlink with the new
+	# state
+	mv "$POMODORO_STATE_PATH/secondary/current" "$POMODORO_STATE_PATH"
+
+	# next, replace the old `primary` state with the new state from `secondary`,
+	# and update the symlink a second time to now point to `primary` again (this
+	# does not change the state)
+	cp -r "$POMODORO_STATE_PATH/secondary/." "$POMODORO_STATE_PATH/primary/"
+	ln -sf "primary" "$POMODORO_STATE_PATH/primary/current"
+	mv "$POMODORO_STATE_PATH/primary/current" "$POMODORO_STATE_PATH"
 }
 
 # export state variables