MDL-43835 phpunit: Create unique starting id's for sequences.

When all starting autoincrement ids are the same, it's difficult to detect coding and testing
errors that use the incorrect id in test calls.  The classic case is cmid vs instance id.
To reduce the chance of the coding error we start sequences at different values where possible.

OUBlog installation is the case that highlighted this as it inserts a sitewide course module
at install time and cmid <> instance id in most cases.

lib/testing/classes/util.php

@@ -74,6 +74,10 @@ abstract class testing_util {
      */
     private static $originaldatafilesjsonadded = false;
 
+    /**
+     * @var int next sequence value for a single test cycle.
+     */
+    protected static $sequencenextstartingid = null;
     /**
      * Return the name of the JSON file containing the init filenames.
      *
@@ -409,6 +413,27 @@ protected static function guess_unmodified_empty_tables() {
         }
     }
 
+    /**
+     * Determine the next unique starting id sequences.
+     *
+     * @static
+     * @param array $records The records to use to determine the starting value for the table.
+     * @return int The value the sequence should be set to.
+     */
+    private static function get_next_sequence_starting_value($records) {
+        $id = self::$sequencenextstartingid;
+
+        // If there are records, calculate the minimum id we can use.
+        // It must be bigger than the last record's id.
+        if (!empty($records)) {
+            $lastrecord = end($records);
+            $id = max($id, $lastrecord->id + 1);
+        }
+
+        self::$sequencenextstartingid = $id + 1000;
+        return $id;
+    }
+
     /**
      * Reset all database sequences to initial values.
      *
@@ -428,18 +453,20 @@ public static function reset_all_database_sequences(array $empties = null) {
             return;
         }
 
+        // If all starting Id's are the same, it's difficult to detect coding and testing
+        // errors that use the incorrect id in tests.  The classic case is cmid vs instance id.
+        // To reduce the chance of the coding error, we start sequences at different values where possible.
+        // In a attempt to avoid tables with existing id's we start at a high number.
+        // Reset the value each time all database sequences are reset.
+        self::$sequencenextstartingid = 100000;
+
         $dbfamily = $DB->get_dbfamily();
         if ($dbfamily === 'postgres') {
             $queries = array();
             $prefix = $DB->get_prefix();
             foreach ($data as $table => $records) {
                 if (isset($structure[$table]['id']) and $structure[$table]['id']->auto_increment) {
-                    if (empty($records)) {
-                        $nextid = 1;
-                    } else {
-                        $lastrecord = end($records);
-                        $nextid = $lastrecord->id + 1;
-                    }
+                    $nextid = self::get_next_sequence_starting_value($records);
                     $queries[] = "ALTER SEQUENCE {$prefix}{$table}_id_seq RESTART WITH $nextid";
                 }
             }
@@ -467,12 +494,7 @@ public static function reset_all_database_sequences(array $empties = null) {
             foreach ($data as $table => $records) {
                 if (isset($structure[$table]['id']) and $structure[$table]['id']->auto_increment) {
                     if (isset($sequences[$table])) {
-                        if (empty($records)) {
-                            $nextid = 1;
-                        } else {
-                            $lastrecord = end($records);
-                            $nextid = $lastrecord->id + 1;
-                        }
+                        $nextid = self::get_next_sequence_starting_value($records);
                         if ($sequences[$table] != $nextid) {
                             $DB->change_database_structure("ALTER TABLE {$prefix}{$table} AUTO_INCREMENT = $nextid");
                         }
@@ -501,12 +523,7 @@ public static function reset_all_database_sequences(array $empties = null) {
 
             foreach ($data as $table => $records) {
                 if (isset($structure[$table]['id']) and $structure[$table]['id']->auto_increment) {
-                    $lastrecord = end($records);
-                    if ($lastrecord) {
-                        $nextid = $lastrecord->id + 1;
-                    } else {
-                        $nextid = 1;
-                    }
+                    $nextid = self::get_next_sequence_starting_value($records);
                     if (!isset($current[$table])) {
                         $DB->get_manager()->reset_sequence($table);
                     } else if ($nextid == $current[$table]) {
@@ -522,6 +539,7 @@ public static function reset_all_database_sequences(array $empties = null) {
 
         } else {
             // note: does mssql support any kind of faster reset?
+            // This also implies mssql will not use unique sequence values.
             if (is_null($empties)) {
                 $empties = self::guess_unmodified_empty_tables();
             }