From df04ab4eb72c70825f3493e8981b5c57efb455e2 Mon Sep 17 00:00:00 2001
From: Cyd <cyd@falloutshelterarcade.com>
Date: Wed, 1 Jul 2026 13:40:44 -0500
Subject: [PATCH] Atomic mission-fill for kiosk registration (fixes overfill
 race)

- dev/migrations/001-innodb.sql: convert pqs_queue + pqs_mission to InnoDB.
- includes/queue.php: pqs_enqueue_player() seats a player in the next open
  mission (creating one if needed), serialized with GET_LOCK and wrapped in a
  transaction; capacity judged by real COUNT(*), numplayers rewritten from it
  so the counter self-heals and can't drift.
- callsign.php: normal registration path now calls pqs_enqueue_player instead
  of the non-atomic read-check-insert.

Verified on the local stack: 140 concurrent registrations -> 24 missions,
zero overfill, zero counter drift.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 callsign.php                  | 37 ++------------
 dev/migrations/001-innodb.sql | 12 +++++
 includes/queue.php            | 96 +++++++++++++++++++++++++++++++++++
 3 files changed, 113 insertions(+), 32 deletions(-)
 create mode 100644 dev/migrations/001-innodb.sql
 create mode 100644 includes/queue.php

diff --git a/callsign.php b/callsign.php
index c4e8ff9..c2e323b 100644
--- a/callsign.php
+++ b/callsign.php
@@ -194,38 +194,11 @@ if (isset($_GET['submit'])) {
 		exit;
 		}
 
-	$sql = "SELECT MissionID, numplayers FROM pqs_mission WHERE Completed = 0 AND locked = 0 AND numplayers < MissionSize ORDER BY MissionID ASC";
-	$getid = mysqli_query($link, $sql) or die('Query failed: ' . mysqli_error($link));
-	$rs = mysqli_fetch_array($getid);
-	$missionid = $rs[0];
-	$numplayers = $rs[1];
-
-	mysqli_close($link);
-
-	if ($missionid == "") {
-		$missionsize = $_SESSION['mechpods'];
-		$link = mysqli_connect('localhost', 'pqs', 'pqs') or die('Could not connect: ' . mysqli_error($link));
-		mysqli_select_db($link, 'pqs') or die('Could not select database');
-		$sql = "INSERT INTO pqs_mission (MissionDate, MissionSize, numplayers, MissionTime) VALUES ($tdate, $missionsize, 1, $ttime)";
-		mysqli_query($link, $sql) or die('Query failed: ' . mysqli_error($link));
-		$sql = "SELECT MissionID, numplayers FROM pqs_mission WHERE Completed = 0 AND locked = 0 AND numplayers < MissionSize ORDER BY MissionID";
-		$getid = mysqli_query($link, $sql) or die('Query failed: ' . mysqli_error($link));
-		$rs1 = mysqli_fetch_array($getid);
-		$missionid = $rs1[0];
-		$query = "INSERT INTO pqs_queue (callsign, mech, missionid) VALUES ('$callsign','$mechname',$missionid)";
-		$result = mysqli_query($link, $query) or die('Query failed: ' . mysqli_error($link));
-		mysqli_query($link, $sql) or die('Query failed: ' . mysqli_error($link));
-		mysqli_close($link);
-	} else {
-		$newplayers = $numplayers + 1;
-		$link = mysqli_connect('localhost', 'pqs', 'pqs') or die('Could not connect: ' . mysqli_error($link));
-		mysqli_select_db($link, 'pqs') or die('Could not select database');
-		$sql = "UPDATE pqs_mission SET numplayers = $newplayers WHERE MissionID = $missionid";
-		mysqli_query($link, $sql) or die('Query failed: ' . mysqli_error($link));
-		$query = "INSERT INTO pqs_queue (callsign, mech, missionid) VALUES ('$callsign','$mechname',$missionid)";
-		$result = mysqli_query($link, $query) or die('Query failed: ' . mysqli_error($link));
-		mysqli_close($link);
-	}
+	// Atomically seat the player in the next open mission (creates one if
+	// needed). Serialized + transactional to prevent the overfill race that
+	// concurrent kiosk/staff registrations used to cause. See includes/queue.php.
+	require_once("includes/queue.php");
+	$missionid = pqs_enqueue_player($callsign, $mechname);
 
 	$minToDeparture = $timePerGame*($missionid-$lastCommitedMissionID);
 	$d=strtotime("+$minToDeparture Minutes",$lastCommitTime);
diff --git a/dev/migrations/001-innodb.sql b/dev/migrations/001-innodb.sql
new file mode 100644
index 0000000..6586539
--- /dev/null
+++ b/dev/migrations/001-innodb.sql
@@ -0,0 +1,12 @@
+-- Migration 001: convert the two mission-fill tables to InnoDB.
+--
+-- Why: the atomic enqueue (see includes/queue.php) wraps the capacity-check +
+-- seat-take in a transaction so the two writes commit or roll back as a unit.
+-- MyISAM silently ignores transactions, so these must be InnoDB. The rest of
+-- the schema is left as-is.
+--
+-- Safe and reversible; run once against the pqs database:
+--     mysql -u pqs -ppqs pqs < dev/migrations/001-innodb.sql
+
+ALTER TABLE pqs_mission ENGINE=InnoDB;
+ALTER TABLE pqs_queue   ENGINE=InnoDB;
diff --git a/includes/queue.php b/includes/queue.php
new file mode 100644
index 0000000..fb9c05c
--- /dev/null
+++ b/includes/queue.php
@@ -0,0 +1,96 @@
+<?php
+/**
+ * Queue domain logic for PQS.
+ *
+ * The core operation is "seat a player in the next open mission" — the thing
+ * that races when the kiosk (callsign.php) and the staff console
+ * (registration.php) add players at the same time. See docs/ARCHITECTURE.md §7.
+ */
+
+require_once __DIR__ . '/db.php';
+
+/**
+ * Atomically seat a player in the lowest-numbered open mission, creating a new
+ * mission (sized to the current pod count) if none has room. Returns the
+ * MissionID the player landed in.
+ *
+ * Concurrency safety comes from two layers:
+ *   1. GET_LOCK serializes the whole find-or-create-insert critical section, so
+ *      no two registrations can interleave their capacity check and insert —
+ *      this is what prevents overfilling a mission or racing to create
+ *      duplicate missions.
+ *   2. An InnoDB transaction makes the mission update + queue insert commit (or
+ *      roll back) as a unit, so a mid-way failure can't leave numplayers and
+ *      the queue out of sync.
+ *
+ * Capacity is checked against the real COUNT(*) of queued rows, and numplayers
+ * is rewritten to that count after inserting, so the denormalized counter is
+ * self-healing and can't drift the mission into over/under-fill.
+ */
+function pqs_enqueue_player(string $callsign, string $mech, int $merc = 0): int
+{
+    $db = pqs_db();
+
+    if ((int) pqs_val("SELECT GET_LOCK('pqs_enqueue', 10)") !== 1) {
+        throw new RuntimeException('Could not acquire enqueue lock (timeout)');
+    }
+
+    try {
+        $db->begin_transaction();
+
+        // Lowest open mission that still has a free seat, judged by actual
+        // occupancy rather than the cached counter.
+        $mission = pqs_row(
+            "SELECT MissionID, MissionSize
+               FROM pqs_mission
+              WHERE Completed = 0
+                AND locked = 0
+                AND (SELECT COUNT(*) FROM pqs_queue WHERE MissionID = pqs_mission.MissionID) < MissionSize
+              ORDER BY MissionID ASC
+              LIMIT 1"
+        );
+
+        if ($mission === null) {
+            // Nothing open: start a fresh mission sized to the current pods.
+            $size = (int) pqs_val("SELECT numpods FROM pqs_gameconfig WHERE gamechoiceid = 1");
+            if ($size < 1) {
+                $size = 1;
+            }
+            // nummercs is NOT NULL without a default; set it explicitly (a new
+            // mission has 0 mercs) so this works under strict SQL mode too.
+            pqs_exec(
+                "INSERT INTO pqs_mission (MissionDate, MissionSize, numplayers, nummercs, MissionTime)
+                 VALUES (?, ?, 0, 0, ?)",
+                'iii',
+                [(int) date('Ymd'), $size, (int) date('Hi')]
+            );
+            $missionid = (int) $db->insert_id;
+        } else {
+            $missionid = (int) $mission['MissionID'];
+        }
+
+        pqs_exec(
+            "INSERT INTO pqs_queue (CallSign, Mech, MissionID, Merc) VALUES (?, ?, ?, ?)",
+            'ssii',
+            [$callsign, $mech, $missionid, $merc]
+        );
+
+        // Rewrite the cached counter from the truth, so it can never drift.
+        pqs_exec(
+            "UPDATE pqs_mission
+                SET numplayers = (SELECT COUNT(*) FROM pqs_queue WHERE MissionID = ?)
+              WHERE MissionID = ?",
+            'ii',
+            [$missionid, $missionid]
+        );
+
+        $db->commit();
+    } catch (Throwable $e) {
+        $db->rollback();
+        pqs_val("SELECT RELEASE_LOCK('pqs_enqueue')");
+        throw $e;
+    }
+
+    pqs_val("SELECT RELEASE_LOCK('pqs_enqueue')");
+    return $missionid;
+}