Adds a shared flock guard used by all five cron entry points, keyed per

script and per install. Replaces the mail queue's own lock file, which
was not atomic and could be deleted out from under a long run. Bounds
the SMTP conversation so an unresponsive server cannot hold the lock.
Recovers rows left at Sending by a run that died, which nothing
previously picked up.

includes/cron_lock.php

@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * ITFlow - Single-run guard for cron entry points
+ *
+ * Required by each cron script immediately after its CLI check and before config.php,
+ * with the caller setting $cron_lock_script = __FILE__ first.
+ *
+ * Cron work is not safe to run twice at once: autopay charges cards, the mail queue
+ * sends client email, the parser creates tickets. If a run is still going when the next
+ * one fires, those actions can happen twice. The lock name is derived from the calling
+ * script's full path, so it is unique per script and per install - separate ITFlow
+ * instances on one host never block each other, and neither do two different cron
+ * scripts belonging to the same install.
+ *
+ * flock is used rather than a lock file whose presence is checked, because checking for
+ * a file and then creating it is not atomic - two runs starting together can both find
+ * it absent. flock is also released by the kernel however the process exits, so a killed
+ * run leaves nothing stale behind and no age heuristic is needed to clean up after it.
+ *
+ * Contention is not logged: mail_queue and the email parser run every minute, so hitting
+ * a run that is still going is expected, not a fault worth reporting. A lock file that
+ * cannot be opened at all is a real misconfiguration and does report loudly.
+ *
+ * The handle is deliberately left open: the lock is held for the life of the process.
+ */
+
+$cron_lock_file = sys_get_temp_dir() . '/itflow_cron_' . md5($cron_lock_script) . '.lock';
+$cron_lock_handle = fopen($cron_lock_file, 'c');
+if ($cron_lock_handle === false) {
+    die("Cannot open the cron lock file at $cron_lock_file - check permissions and open_basedir.\n");
+}
+if (!flock($cron_lock_handle, LOCK_EX | LOCK_NB)) {
+    // Exit silently. On a per-minute schedule, finding a previous run still going is
+    // normal operation rather than an error, and anything written to stdout here would
+    // be mailed to the crontab owner every single minute for the length of that run.
+    exit(0);
+}