Magento: log the SQL that was buffering when fetchAll() hits memory_limit
=========================================================================

Package:  magento/zend-db (tested on 1.16.3)
Author:   Paxento, https://paxento.com/
Article:  https://paxento.com/magento-checkout-memory-limit-stripe-invoice-scan/
License:  provided as-is, without warranty. Test on staging before production.

Diagnostic only, no behaviour change. When a request dies with a fatal error while
Zend_Db_Statement_Pdo::fetchAll() is buffering rows, writes the SQL and the peak
memory to the PHP error log. Apply while investigating, remove afterwards.

Apply with cweagans/composer-patches or vaimo/composer-patches, e.g.:
  "patches": {
    "magento/zend-db": {
      "Log SQL on memory exhaustion in fetchAll": "patches/magento-log-sql-on-oom-in-fetchall.patch"
    }
  }

--- a/library/Zend/Db/Statement/Pdo.php
+++ b/library/Zend/Db/Statement/Pdo.php
@@ -268,6 +268,18 @@
     {
         return new IteratorIterator($this->_stmt);
     }
+
+    /**
+     * SQL of the fetchAll() that is currently buffering rows, so a fatal shutdown can name it.
+     *
+     * @var string|null
+     */
+    private static $_bufferingSql = null;
+
+    /**
+     * @var bool
+     */
+    private static $_fatalReporterRegistered = false;
 
     /**
      * Returns an array containing all of the result set rows.
@@ -281,23 +293,60 @@
     {
         if ($style === null) {
             $style = $this->_fetchMode;
+        }
+        if (!self::$_fatalReporterRegistered) {
+            self::$_fatalReporterRegistered = true;
+            register_shutdown_function([__CLASS__, 'reportFatalWhileBuffering']);
         }
+        self::$_bufferingSql = $this->_stmt->queryString;
         try {
             if ($style == PDO::FETCH_COLUMN) {
                 if ($col === null) {
                     $col = 0;
                 }
-                return $this->_stmt->fetchAll($style, $col);
+                $result = $this->_stmt->fetchAll($style, $col);
             } else {
-                return $this->_stmt->fetchAll($style);
+                $result = $this->_stmt->fetchAll($style);
             }
         } catch (PDOException $e) {
+            self::$_bufferingSql = null;
             #require_once 'Zend/Db/Statement/Exception.php';
             throw new Zend_Db_Statement_Exception($e->getMessage(), $e->getCode(), $e);
         }
+        self::$_bufferingSql = null;
+        return $result;
     }
 
     /**
+     * Names the query that was buffering rows when the request died, for memory-limit diagnostics.
+     *
+     * A memory exhaustion inside fetchAll() is reported by PHP against whichever code the last
+     * allocation happened in, and the session save handler may mask it entirely, so the query
+     * itself never reaches the log without this.
+     *
+     * @return void
+     */
+    public static function reportFatalWhileBuffering()
+    {
+        if (self::$_bufferingSql === null) {
+            return;
+        }
+
+        $error = error_get_last();
+        if ($error === null || $error['type'] !== E_ERROR) {
+            return;
+        }
+
+        ini_set('memory_limit', '-1');
+        error_log(sprintf(
+            'Fatal while buffering a result set (peak %d MB): %s -- SQL: %s',
+            memory_get_peak_usage(true) / 1048576,
+            $error['message'],
+            self::$_bufferingSql
+        ));
+    }
+
+    /**
      * Returns a single column from the next row of a result set.
      *
      * @param int $col OPTIONAL Position of the column to fetch.