[processing] fixes for SAGA folder configuration

This tries to address some problem with previously set SAGA paths, by always showing the SAGA path config value, and by trying to ruin a built in copy of SAGA if the current SAGA folder is wrong (as it usually happens when updating QGIS)

python/plugins/processing/algs/saga/SagaAlgorithmProvider.py

@@ -57,7 +57,7 @@ def __init__(self):
         self.activate = True
 
     def initializeSettings(self):
-        if (isWindows() or isMac()) and SagaUtils.findSagaFolder() is None:
+        if (isWindows() or isMac()):
             ProcessingConfig.addSetting(Setting("SAGA",
                                                 SagaUtils.SAGA_FOLDER, self.tr('SAGA folder'),
                                                 '',
@@ -77,7 +77,7 @@ def initializeSettings(self):
 
     def unload(self):
         AlgorithmProvider.unload(self)
-        if (isWindows() or isMac()) and SagaUtils.findSagaFolder() is None:
+        if (isWindows() or isMac()):
             ProcessingConfig.removeSetting(SagaUtils.SAGA_FOLDER)
 
         ProcessingConfig.removeSetting(SagaUtils.SAGA_LOG_CONSOLE)

python/plugins/processing/algs/saga/SagaUtils.py

@@ -72,10 +72,13 @@ def findSagaFolder():
 
 def sagaPath():
     folder = ProcessingConfig.getSetting(SAGA_FOLDER)
+    if not os.path.isdir(folder):
+        folder = None
+        ProcessingLog.addToLog(ProcessingLog.LOG_WARNING,
+                               'Specified SAGA folder does not exist. Will try to find built-in binaries.')
     if folder is None or folder == '':
         folder = findSagaFolder()
-        if folder is not None:
-            ProcessingConfig.setSettingValue(SAGA_FOLDER, folder)
+
     return folder or ''