=== modified file 'includes/bootstrap.inc'
--- includes/bootstrap.inc	
+++ includes/bootstrap.inc	
@@ -286,10 +286,10 @@ function variable_get($name, $default) {
 function variable_set($name, $value) {
   global $conf;
 
-  db_lock_table('variable');
-  db_query("DELETE FROM {variable} WHERE name = '%s'", $name);
-  db_query("INSERT INTO {variable} (name, value) VALUES ('%s', '%s')", $name, serialize($value));
-  db_unlock_tables();
+  db_query_replace(
+    array("DELETE FROM {variable} WHERE name = '%s'", $name),
+    array("INSERT INTO {variable} (name, value) VALUES ('%s', '%s')", $name, serialize($value))
+  );
 
   cache_clear_all('variables');
 
=== modified file 'includes/cache.inc'
--- includes/cache.inc	
+++ includes/cache.inc	
@@ -63,12 +63,10 @@ function cache_get($key) {
  *   A string containing HTTP header information for cached pages.
  */
 function cache_set($cid, $data, $expire = CACHE_PERMANENT, $headers = NULL) {
-  db_lock_table('cache');
-  db_query("UPDATE {cache} SET data = %b, created = %d, expire = %d, headers = '%s' WHERE cid = '%s'", $data, time(), $expire, $headers, $cid);
-  if (!db_affected_rows()) {
-    @db_query("INSERT INTO {cache} (cid, data, created, expire, headers) VALUES ('%s', %b, %d, %d, '%s')", $cid, $data, time(), $expire, $headers);
-  }
-  db_unlock_tables();
+  db_query_replace(
+    array("DELETE FROM {cache} WHERE cid = '%s'", $cid),
+    array("INSERT INTO {cache} (cid, data, created, expire, headers) VALUES ('%s', %b, %d, %d, '%s')", $cid, $data, time(), $expire, $headers)
+  );
 }
 
 /**
=== modified file 'includes/database.mysql.inc'
--- includes/database.mysql.inc	
+++ includes/database.mysql.inc	
@@ -203,21 +203,25 @@ function db_error() {
 }
 
 /**
- * Return a new unique ID in the given sequence.
- *
- * For compatibility reasons, Drupal does not use auto-numbered fields in its
- * 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.
+ * Return a new unique ID.
  */
 function db_next_id($name) {
-  $name = db_prefix_tables($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');
-
-  return $id;
+  // Prefix any curly-bracket "{table}" names
+   $name = db_prefix_tables($name);
+  // 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 the sequence doesn't exist yet.
+    // 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.
+    db_query("UPDATE {sequences} SET id = LAST_INSERT_ID(id + 1) WHERE name = '%s'", $name);
+  }
+  return mysql_insert_id();
 }
 
 /**
@@ -379,7 +383,20 @@ function db_table_exists($table) {
 }
 
 /**
+ * Perform a replace query.
+ *
+ * @param $delete
+ *   An array, where the first value is a query which deletes one row,
+ *   subsequent values are arguments. Same syntax as of db_query.
+ * @param
+ *   An array, where the first value is a query which inserts the new row,
+ *   subsequent values are arguments. Same syntax as of db_query.
+ */
+function db_query_replace($delete, $insert) {
+  $insert_query = array_shift($insert);
+  db_query(preg_replace('/^INSERT /', 'REPLACE ', $insert_query), $insert);
+}
+
+/**
  * @} End of "ingroup database".
  */
-
-
=== modified file 'includes/database.mysqli.inc'
--- includes/database.mysqli.inc	
+++ includes/database.mysqli.inc	
@@ -198,21 +198,29 @@ function db_error() {
 }
 
 /**
- * Return a new unique ID in the given sequence.
- *
- * For compatibility reasons, Drupal does not use auto-numbered fields in its
- * 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.
+ * Return a new unique ID.
+ */
+function db_next_id() {
+/**
+ * Return a new unique ID.
  */
 function db_next_id($name) {
-  $name = db_prefix_tables($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');
-
-  return $id;
+  // Prefix any curly-bracket "{table}" names
+   $name = db_prefix_tables($name);
+  // 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 the sequence doesn't exist yet.
+    // 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.
+    db_query("UPDATE {sequences} SET id = LAST_INSERT_ID(id + 1) WHERE name = '%s'", $name);
+  }
+  return db_result(db_query("SELECT LAST_INSERT_ID() FROM {sequences} WHERE name = '%s'", $name));
 }
 
 /**
@@ -374,6 +382,21 @@ function db_table_exists($table) {
 }
 
 /**
+ * Perform a replace query.
+ *
+ * @param $delete
+ *   An array, where the first value is a query which deletes one row,
+ *   subsequent values are arguments. Same syntax as of db_query.
+ * @param
+ *   An array, where the first value is a query which inserts the new row,
+ *   subsequent values are arguments. Same syntax as of db_query.
+ */
+function db_query_replace($delete, $insert) {
+  $insert_query = array_shift($insert);
+  db_query(preg_replace('/^INSERT /', 'REPLACE ', $insert_query), $insert);
+}
+
+/**
  * @} End of "ingroup database".
  */
 
=== modified file 'includes/database.pgsql.inc'
--- includes/database.pgsql.inc	
+++ includes/database.pgsql.inc	
@@ -361,6 +361,26 @@ function db_check_setup() {
 }
 
 /**
+ * Perform a replace query.
+ *
+ * @param $delete
+ *   An array, where the first value is a query which deletes one row,
+ *   subsequent values are arguments. Same syntax as of db_query.
+ * @param
+ *   An array, where the first value is a query which inserts the new row,
+ *   subsequent values are arguments. Same syntax as of db_query.
+ */
+function db_query_replace($delete, $insert) {
+  $delete_query = array_shift($delete);
+  db_query('BEGIN');
+  db_query(preg_replace('/^DELETE /', 'SELECT ', $delete_query .' FOR UPDATE'), $delete);
+  db_query($delete_query, $delete);
+  $insert_query = array_shift($insert);
+  db_query($insert_query, $insert);
+  db_query('COMMIT');
+}
+
+/**
  * @} End of "ingroup database".
  */