Add QgsMapLayer::isTemporary to test whether a map layer is temporary

These include memory layer provider layers, or layers physically
stored inside a user's temporary folder (e.g. /tmp)

python/core/auto_generated/qgsmaplayer.sip.in

@@ -511,6 +511,16 @@ Returns ``True`` if the layer can be edited.
 Returns ``True`` if the layer is considered a spatial layer, ie it has some form of geometry associated with it.
 
 .. versionadded:: 2.16
+%End
+
+    virtual bool isTemporary() const;
+%Docstring
+Returns ``True`` if the layer is considered a temporary layer.
+
+These include memory-only layers such as those created by the "memory" data provider, or layers
+stored inside a local temporary folder (such as the "/tmp" folder on Linux).
+
+.. versionadded:: 3.10.1
 %End
 
     enum ReadFlag

src/core/qgsmaplayer.cpp

@@ -1727,6 +1727,31 @@ bool QgsMapLayer::isSpatial() const
   return true;
 }
 
+bool QgsMapLayer::isTemporary() const
+{
+  // invalid layers are temporary? -- who knows?!
+  if ( !isValid() )
+    return false;
+
+  if ( mProviderKey == QLatin1String( "memory" ) )
+    return true;
+
+  const QVariantMap sourceParts = QgsProviderRegistry::instance()->decodeUri( mProviderKey, mDataSource );
+  const QString path = sourceParts.value( QStringLiteral( "path" ) ).toString();
+  if ( path.isEmpty() )
+    return false;
+
+  // check if layer path is inside one of the standard temporary file locations for this platform
+  const QStringList tempPaths = QStandardPaths::standardLocations( QStandardPaths::TempLocation );
+  for ( const QString &tempPath : tempPaths )
+  {
+    if ( path.startsWith( tempPath ) )
+      return true;
+  }
+
+  return false;
+}
+
 void QgsMapLayer::setValid( bool valid )
 {
   mValid = valid;

src/core/qgsmaplayer.h

@@ -522,6 +522,16 @@ class CORE_EXPORT QgsMapLayer : public QObject
      */
     virtual bool isSpatial() const;
 
+    /**
+     * Returns TRUE if the layer is considered a temporary layer.
+     *
+     * These include memory-only layers such as those created by the "memory" data provider, or layers
+     * stored inside a local temporary folder (such as the "/tmp" folder on Linux).
+     *
+     * \since QGIS 3.10.1
+     */
+    virtual bool isTemporary() const;
+
     /**
      * Flags which control project read behavior.
      * \since QGIS 3.10

tests/src/python/test_qgsmaplayer.py

@@ -13,9 +13,12 @@ __copyright__ = 'Copyright 2017, The QGIS Project'
 import os
 import qgis  # NOQA
 import tempfile
+import glob
+import shutil
 
 from qgis.core import (QgsReadWriteContext,
                        QgsVectorLayer,
+                       QgsRasterLayer,
                        QgsProject)
 from qgis.testing import start_app, unittest
 from qgis.PyQt.QtXml import QDomDocument
@@ -138,6 +141,50 @@ class TestQgsMapLayer(unittest.TestCase):
         uri = layer.styleURI()
         self.assertEqual(uri, os.path.join(TEST_DATA_DIR, 'delimitedtext', 'test.qml'))
 
+    def testIsTemporary(self):
+        # test if a layer is correctly marked as temporary
+        dir = QTemporaryDir()
+        dir_path = dir.path()
+        for file in glob.glob(os.path.join(TEST_DATA_DIR, 'france_parts.*')):
+            shutil.copy(os.path.join(TEST_DATA_DIR, file), dir_path)
+
+        not_temp_source = os.path.join(TEST_DATA_DIR, 'france_parts.*')
+        temp_source = os.path.join(dir_path, 'france_parts.shp')
+
+        vl = QgsVectorLayer('invalid', 'test')
+        self.assertFalse(vl.isValid())
+        self.assertFalse(vl.isTemporary())
+
+        vl = QgsVectorLayer(os.path.join(TEST_DATA_DIR, 'france_parts.shp'), 'test')
+        self.assertTrue(vl.isValid())
+        self.assertFalse(vl.isTemporary())
+
+        vl = QgsVectorLayer(temp_source, 'test')
+        self.assertTrue(vl.isValid())
+        self.assertTrue(vl.isTemporary())
+
+        # memory layers are temp
+        layer = QgsVectorLayer("Point?field=fldtxt:string",
+                               "layer", "memory")
+        self.assertTrue(vl.isValid())
+        self.assertTrue(vl.isTemporary())
+
+        rl = QgsRasterLayer('invalid', 'test')
+        self.assertFalse(rl.isValid())
+        self.assertFalse(rl.isTemporary())
+
+        not_temp_source = os.path.join(TEST_DATA_DIR, 'float1-16.tif')
+        shutil.copy(not_temp_source, dir_path)
+        temp_source = os.path.join(dir_path, 'float1-16.tif')
+
+        rl = QgsRasterLayer(not_temp_source, 'test')
+        self.assertTrue(rl.isValid())
+        self.assertFalse(rl.isTemporary())
+
+        rl = QgsRasterLayer(temp_source, 'test')
+        self.assertTrue(rl.isValid())
+        self.assertTrue(rl.isTemporary())
+
 
 if __name__ == '__main__':
     unittest.main()