clipmenud: Allow disable with USR1 and enable with USR2

This allows avoiding having to delete after the fact for things like
issues #57 and #98.

Why have this over just stopping clipmenud? Well:

1. Stopping clipmenud should usually be an init system action, but we
   are init-system agnostic. If we just exit, we don't have a way of
   reliably starting again.
2. Even if we *do* do it using the init system, we don't want some
   things (like a lingering xsel which owns the selection for
   CM_OWN_CLIPBOARD) being killed as well.
3. This is a nicer interface for things like password managers to stop
   clipmenu rather than stopping clipmenu entirely.

.travis.yml

@@ -3,7 +3,7 @@ language: bash
 dist: xenial
 
 script:
-  - shellcheck -s bash clipmenu clipmenud clipdel clipfsck
+  - shellcheck -s bash clipmenu clipmenud clipdel clipfsck clipctl
   - tests/test-clipmenu
 
 matrix:

README.md

@@ -68,6 +68,7 @@ The behavior of `clipmenud` can be customized through environment variables. Fea
 
  * Customizing max number of clips (Default: 1000)
  * Choosing which selections to manage
+ * Disabling clip collection temporarily with `clipctl disable`, reenabling with `clipctl enable`
  * Ignoring certain windows, like password managers
  * Enabling debugging
  * Customizing the cache dir location

clipctl

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+if [[ -z $1 ]] || [[ $1 == --help ]] || [[ $1 == -h ]]; then
+    cat << 'EOF'
+clipctl provides controls for the clipmenud daemon.
+
+You can temporarily disable clip collection without stopping clipmenud entirely
+by running "clipctl disable". You can then reenable with "clipctl enable".
+EOF
+    exit 0
+fi
+
+_CLIPMENUD_PID=$(pgrep -u "$(id -u)" -nf '.*/?clipmenud$')
+
+if [[ -z "$_CLIPMENUD_PID" ]]; then
+    echo "clipmenud is not running"
+    exit 2
+fi
+
+case $1 in
+    enable) kill -USR2 "$_CLIPMENUD_PID" ;;
+    disable) kill -USR1 "$_CLIPMENUD_PID" ;;
+    *)
+        printf 'Unknown command: %s\n' "$1"
+        exit 1
+    ;;
+esac

clipmenud

@@ -55,9 +55,26 @@ get_first_line() {
 
 debug() { (( CM_DEBUG )) && printf '%s\n' "$@" >&2; }
 
+sig_disable() {
+    info "Received disable signal, suspending clipboard capture"
+    _CM_DISABLED=1
+    _CM_FIRST_DISABLE=1
+    [[ -v _CM_CLIPNOTIFY_PID ]] && kill "$_CM_CLIPNOTIFY_PID"
+}
+
+sig_enable() {
+    if ! (( _CM_DISABLED )); then
+        info "Received enable signal but we're not disabled, so doing nothing"
+        return
+    fi
+    info "Received enable signal, resuming clipboard capture"
+    _CM_DISABLED=0
+}
+
 if [[ $1 == --help ]] || [[ $1 == -h ]]; then
     cat << 'EOF'
-clipmenud collects and caches what's on the clipboard.
+clipmenud collects and caches what's on the clipboard. You can manage its
+operation with clipctl.
 
 Environment variables:
 
@@ -92,8 +109,29 @@ fi
 
 exec {lock_fd}> "$lock_file"
 
+trap sig_disable USR1
+trap sig_enable USR2
+
+# Kill all background processes on exit
+trap 'trap - TERM; kill -- -$$' INT TERM EXIT
+
 while true; do
-    (( CM_ONESHOT )) || clipnotify
+    if ! (( CM_ONESHOT )); then
+        # Make sure we're interruptible for the sig_{en,dis}able traps
+        clipnotify &
+        _CM_CLIPNOTIFY_PID="$!"
+        wait "$_CM_CLIPNOTIFY_PID"
+    fi
+
+    if (( _CM_DISABLED )); then
+        # The first one will just be from interrupting `wait`, so don't print
+        if (( _CM_FIRST_DISABLE )); then
+            unset _CM_FIRST_DISABLE
+        else
+            info "Got a clipboard notification, but we are disabled, skipping"
+        fi
+        continue
+    fi
 
     if [[ $CM_IGNORE_WINDOW ]] && (( has_xdotool )); then
         windowname="$(xdotool getactivewindow getwindowname)"