ZSQLCatalog.SQLCatalog: Stop updating _max_uid .

Even if it does not lead to ConflictError being raised, updating _max_uid
has a cost and a transaction-serialising effect (most noticeable with NEO).
This value is not used (outside of a bootstrap value for portal_ids
generator), so stop doing this unnecessary work.
Also, fold produceUid into its only caller.
Also, change the folded pattern to react to an empty buffer rather than
test for emptiness before each access.

product/ZSQLCatalog/SQLCatalog.py

@@ -616,9 +616,6 @@ class Catalog(Folder,
   # These are ZODB variables, so shared by multiple Zope instances.
   # This is set to the last logical time when clearReserved is called.
   _last_clear_reserved_time = 0
-  # This is to record the maximum value of uids. Because this uses the class Length
-  # in BTrees.Length, this does not generate conflict errors.
-  _max_uid = None
 
   # This is an instance id which specifies who owns which reserved uids.
   _instance_id = getattr(getConfiguration(), 'instance_id', None)
@@ -1072,34 +1069,6 @@ class Catalog(Folder,
     return uid_buffer_dict[thread_key]
 
   # the cataloging API
-  security.declarePrivate('produceUid')
-  def produceUid(self):
-    """
-      Produces reserved uids in advance
-    """
-    global global_clear_reserved_time
-    assert global_reserved_uid_lock.locked()
-    # This checks if the list of local reserved uids was cleared after clearReserved
-    # had been called.
-    force_new_buffer = (global_clear_reserved_time != self._last_clear_reserved_time)
-    uid_buffer = self.getUIDBuffer(force_new_buffer=force_new_buffer)
-    global_clear_reserved_time = self._last_clear_reserved_time
-    if len(uid_buffer) == 0:
-      if self._max_uid is None:
-        self._max_uid = Length(1)
-      # TODO: if this method is kept and former uid allocation code is
-      # discarded, self._max_uid duplicates work done by portal_ids: it
-      # already keeps track of the highest allocated number for all id
-      # generator groups.
-      uid_buffer.extend(
-        self.getPortalObject().portal_ids.generateNewIdList(
-          id_generator='uid',
-          id_group='catalog_uid',
-          id_count=UID_BUFFER_SIZE,
-          default=self._max_uid(),
-        ),
-      )
-
   security.declarePrivate('isIndexable')
   def isIndexable(self):
     """
@@ -1157,21 +1126,31 @@ class Catalog(Folder,
       Similar problems may happen with relations and acquisition of uid values (ex. order_uid)
       with the risk of graph loops
     """
+    global global_clear_reserved_time
+
     if not self.getPortalObject().isIndexable():
       return None
 
     with global_reserved_uid_lock:
-      self.produceUid()
-      uid_buffer = self.getUIDBuffer()
-      if len(uid_buffer) > 0:
-        uid = uid_buffer.pop()
-        if self._max_uid is None:
-          self._max_uid = Length(1)
-        if uid > self._max_uid():
-          self._max_uid.set(uid)
-        return long(uid)
-      else:
-        raise CatalogError("Could not retrieve new uid")
+      uid_buffer = self.getUIDBuffer(
+        force_new_buffer=global_clear_reserved_time != self._last_clear_reserved_time,
+      )
+      global_clear_reserved_time = self._last_clear_reserved_time
+      try:
+        return long(uid_buffer.pop())
+      except IndexError:
+        uid_buffer.extend(
+          self.getPortalObject().portal_ids.generateNewIdList(
+            id_generator='uid',
+            id_group='catalog_uid',
+            id_count=UID_BUFFER_SIZE,
+            default=getattr(self, '_max_uid', lambda: 1)(),
+          ),
+        )
+        try:
+          return long(uid_buffer.pop())
+        except IndexError:
+          raise CatalogError("Could not retrieve new uid")
 
   security.declareProtected(manage_zcatalog_entries, 'manage_catalogObject')
   def manage_catalogObject(self, REQUEST, RESPONSE, URL1, urls=None):