From 82a13b897f279d9ba5882fb181cc912bbd51b482 Mon Sep 17 00:00:00 2001
From: Sean Whitton <spwhitton@spwhitton.name>
Date: Mon, 24 Oct 2022 13:53:47 -0700
Subject: emacsclient wrapper: add locking around starting up daemons

---
 bin/emacsclient | 36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

(limited to 'bin/emacsclient')

diff --git a/bin/emacsclient b/bin/emacsclient
index 20e243c7..ed1c2215 100755
--- a/bin/emacsclient
+++ b/bin/emacsclient
@@ -33,7 +33,9 @@ status=
 daemon_name=
 gdbmacs=
 args=
-socket_dir="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}/emacs/"
+xdg="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}"
+socket_dir="$xdg/emacs/"
+locks_dir="$xdg/spw_emacsclient/"
 
 get_listener () {
     local socket="$1"
@@ -53,6 +55,15 @@ maybe_notify () {
 	&& notify-send --urgency=low --expire-time=10000 "$1"
 }
 
+spw_flock () {
+    mkdir -p "$locks_dir"
+    eval "exec $1<>${locks_dir}$2"
+    if ! flock --wait 15 "$1"; then
+	maybe_notify "couldn't lock starting Emacs daemon named $2"
+	exit 1
+    fi
+}
+
 die_wedged () {
     maybe_notify "in-tree Emacs appears to be wedged"
     # Only exit non-zero if we were expected to start the daemon.
@@ -92,6 +103,8 @@ installed_emacsclient=$(PATH=$(echo "$PATH" \
 
 command -v ss >/dev/null || exec "$installed_emacsclient" -a "" "$@"
 
+$want_version || spw_flock 3 "${daemon_name:-server}"
+
 socket="$socket_dir${daemon_name:-server}"
 listener="$(get_listener $socket)"
 if [ -n "$listener" ]; then
@@ -138,6 +151,7 @@ if ! $want_installed && ! $installed_running \
 
     # ---- Two special cases for primary session that's always under gdb
     if [ -z "$daemon_name" ] && ! $want_version; then
+	spw_flock 4 gdbmacs
 	# If devel Emacs is stopped or gdbmacs can't start it, ask gdbmacs to
 	# handle the request: it's probs. C-i e or editing a file via EDITOR.
 
@@ -162,8 +176,9 @@ if ! $want_installed && ! $installed_running \
 			--exclude "$(egrep_negate ${socket_dir}server)" &
 	    inotifywait=$!
 
-	    if "$installed_emacsclient" -a '' -sgdbmacs \
-					--eval '(spw/gdbmacs-attach)'; then
+	    if "$installed_emacsclient" \
+		   -a '' -sgdbmacs \
+		   --eval '(spw/gdbmacs-attach)' 3>&- 4>&-; then
 		wait $inotifywait
 		listener=true
 	    elif $want_eval || $want_update; then
@@ -178,9 +193,11 @@ if ! $want_installed && ! $installed_running \
 	if $pass_to_gdbmacs; then
 	    emacs="$installed_emacs"
 	    emacsclient="$installed_emacsclient"
+	    socket="${socket_dir}gdbmacs"
 	    listener="$gdbmacs"
 	    set -- "$@" -sgdbmacs
 	fi
+	exec 4>&-
     fi
     # ---- End two special cases for primary session that's always under gdb
 else
@@ -210,12 +227,21 @@ if $want_update && [ -n "$listener" ]; then
 			  --eval "(spw/update-environment$args))"
 fi
 
+maybe_hold_lock () {
+    # Hold on to the lock until daemon has started up.
+    $want_version || [ -n "$listener" ] \
+	|| inotifywait -qq -e create "${socket_dir}" \
+		       --exclude "$(egrep_negate $socket)" &
+}
+
 # emacsclient(1) requires an argument.  As a special case, if there are no
 # arguments except -s/--socket-name, and no daemon is running, just start one.
 # And if there were no arguments but we were requested just to update env
 # vars, exit peacefully.
 if [ "$#" -eq "$min_arg" -a -z "$listener" ]; then
-    exec "$emacs" --daemon"${daemon_name:+=$daemon_name}"
+    maybe_hold_lock
+    exec "$emacs" --daemon"${daemon_name:+=$daemon_name}" 3>&-
 elif ! ( [ "$#" -eq "$min_arg" ] && $want_update ); then
-    exec "$emacsclient" -a "" "$@"
+    maybe_hold_lock
+    exec "$emacsclient" -a "" "$@" 3>&-
 fi
-- 
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