mail: Add checkmail routine

Added script to call for checking mail. Will run individual pre- and
post-hooks. (By default, imapfilter and notmuch update.)

Simple options can be changed through environment variables:

Hooks can be set to different scripts with MBSYNC_PRE and
MBSYNC_POST, respectively.

Will also run mbsync repeatedly, until it exits without error. By
default it will try running it 3 times before giving up, this can be set
with MBSYNC_MAX_TRIES.

No functional imapfilters have been added yet.

mbsync and imapfilter both adhere to the XDG base directory layout
(somewhat), checking for their option files in .config/{isync,imapfilter}
respectively.

Everything still pending changes, the mail eco-system is hard to
wrap one's head around.

mail/.local/bin/checkmail

@@ -0,0 +1,57 @@
+#!/usr/bin/env sh
+# runs mbsync, with pre-hooks and post-hooks
+# by default, first runs imapfilter
+# and after indexes with notmuch
+#
+# To run without invoking any hooks or retry attempts,
+# run `checkmail raw`
+# this will simply invoke a single isync run on all
+# boxes with the correct config file set.
+#
+# for more advanced, per-account/channel hooks, see
+# https://sourceforge.net/p/isync/feature-requests/8/
+
+MBSYNC_MAX_TRIES=3
+MBSYNC_PRE="imapfilter"
+MBSYNC_POST="notmuch new"
+
+success="false"
+tries=0
+
+# fail the routine and optionally send a message why
+fail() {
+  [ -n "$1" ] && echo "$1"
+  exit 1
+}
+
+checkmail() {
+  mbsync -c "$XDG_CONFIG_HOME/isync/mbsyncrc" -a
+}
+
+checkonline() {
+  # Ping 1.1.1.1 to confirm that we are on the internet
+  ping -c 1 "1.1.1.1" >/dev/null 2>/dev/null || fail "checkmail can not access the internet."
+}
+
+# Routine start
+
+# check
+
+# skip any retries and pre/post hooks, just run mbsync
+if [ "$1" = "raw" ]; then
+  checkmail
+  exit
+fi
+
+$MBSYNC_PRE
+while true; do
+  if checkmail; then
+    success="true"
+  fi
+
+  if [ $success = "true" ] || [ $tries -gt $MBSYNC_MAX_TRIES ]; then
+    fail "Maximum retries reached unsuccessfully."
+  fi
+  tries=$((tries + 1))
+done
+$MBSYNC_POST