Index: includes/database.mysql.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/database.mysql.inc,v
retrieving revision 1.56
diff -u -r1.56 database.mysql.inc
--- includes/database.mysql.inc	31 Jul 2006 19:24:16 -0000	1.56
+++ includes/database.mysql.inc	6 Aug 2006 11:23:13 -0000
@@ -209,15 +209,51 @@
  * database tables. Instead, this function is used to return a new unique ID
  * of the type requested. If necessary, a new sequence with the given name
  * will be created.
+ *
+ * Usage example:
+ * @code
+ *    // Attempt to get the next id
+ *    $id = db_next_id('{example}_xid');
+ *    if (!$id) {
+ *       // Failed to get an ID, so take a suitable error action
+ *       return false;
+ *    }
+ *    // Got the id OK, so insert the record
+ *    db_query("INSERT INTO {example} (xid, data) VALUES (%d, '%s')", $id, $data);
+ * @endcode
+ *
+ * Drupal convention is to prefix the table names using the '{'...'}' curly brackets,
+ * although since the sequence table itself is prefixed this is not strictly necessary.
+ *
+ * @param $name
+ *   Sequence name to use. Will be prefixed using db_prefix_tables()
+ * @return
+ *   The next unique ID, or FALSE if error
  */
 function db_next_id($name) {
+  // Prefix any curly-bracket "{table}" names
   $name = db_prefix_tables($name);
-  db_query('LOCK TABLES {sequences} WRITE');
-  $id = db_result(db_query("SELECT id FROM {sequences} WHERE name = '%s'", $name)) + 1;
-  db_query("REPLACE INTO {sequences} VALUES ('%s', %d)", $name, $id);
-  db_query('UNLOCK TABLES');
+  // Assume initially that the sequence already exists.  Attempt to update it atomically.
+  // LAST_INSERT_ID lets us get its value later. IGNORE suppresses update failures.
+  db_query("UPDATE IGNORE {sequences} SET id=LAST_INSERT_ID(id+1) WHERE name = '%s'", $name);
+  // Check whether that worked
+  if (!db_affected_rows()) {
+    // Updated failed, so (a) sequence doesn't exist yet, or (b) some database error
+    // In either case, try to create a new sequence starting from zero
+    // IGNORE suppresses insert error if e.g. a concurrent process got there first.
+    db_query("INSERT IGNORE INTO {sequences} VALUES ('%s', %d)", $name, 0);
+    // Sequence now exists (unless some database error occurred to prevent it)
+    // Retry update, allowing for possible concurrent process. It should work this time.
+    $ok = db_query("UPDATE {sequences} SET id=LAST_INSERT_ID(id+1) WHERE name = '%s'", $name);
+    // Make sure it worked.
+    if (!$ok) {
+      // Update failed, so we failed to get an id. Tell the caller something's wrong.
+      return false;
+    } // end if [failed to get a sequence id]
+  } // end if [sequence needs creating]
 
-  return $id;
+  // If we get here, the increment using LAST_INSERT_ID was successful, so return its value.
+  return mysql_insert_id();
 }
 
 /**