[BUGFIX][Processing] Infinite loop in refresh algorithms tree

When an algorithm provider is refresh, all algorithm providers are reloaded.
But after each reload,the providerReloaded event is emitted and the provider
algorithms tree is refreshed, and all the algorithm providers are reloaded,
and the infinite loop started.

It fixed #18877 *Maximum recursion depth exceeded after adding a model or
script to the Processing algorithm list (since 2.18.18!)* by removing
`updateAlgsList` when refresh provider algorithms tree item.

python/plugins/processing/core/Processing.py

@@ -67,6 +67,8 @@
 
 class Processing:
 
+    currentlyUpdatingAlgList = False
+
     providers = []
 
     # Same structure as algs in algList
@@ -179,10 +181,14 @@ def updateAlgsList():
         requires the list of algorithms to be created again from
         algorithm providers.
         """
+        if Processing.currentlyUpdatingAlgList:
+            return
+        Processing.currentlyUpdatingAlgList = True
         QApplication.setOverrideCursor(QCursor(Qt.WaitCursor))
         for p in Processing.providers:
             Processing.reloadProvider(p.getName())
         QApplication.restoreOverrideCursor()
+        Processing.currentlyUpdatingAlgList = False
 
     @staticmethod
     def reloadProvider(providerName):

python/plugins/processing/gui/ProcessingToolbox.py

@@ -374,7 +374,6 @@ def __init__(self, providerName, tree, toolbox):
 
     def refresh(self):
         self.takeChildren()
-        Processing.updateAlgsList()
         self.populate()
 
     def populate(self):