[processing][grass] Fix use of v.external with ogr layers which

do not support random read

These layers are not compatible with v.external, so we have to
use v.in.ogr for them

python/plugins/processing/algs/grass7/Grass7Algorithm.py

@@ -63,6 +63,7 @@ from qgis.core import (Qgis,
                        QgsProcessingUtils,
                        QgsVectorLayer)
 from qgis.utils import iface
+from osgeo import ogr
 
 from processing.core.ProcessingConfig import ProcessingConfig
 
@@ -786,6 +787,17 @@ class Grass7Algorithm(QgsProcessingAlgorithm):
         if external is None:
             external = ProcessingConfig.getSetting(
                 Grass7Utils.GRASS_USE_VEXTERNAL)
+
+        # safety check: we can only use external for ogr layers which support random read
+        if external:
+            ds = ogr.Open(layer.source())
+            if ds is not None:
+                ogr_layer = ds.GetLayer()
+                if ogr_layer is None or not ogr_layer.TestCapability(ogr.OLCRandomRead):
+                    external = False
+            else:
+                external = False
+
         self.inputLayers.append(layer)
         self.setSessionProjectionFromLayer(layer)
         destFilename = 'vector_{}'.format(os.path.basename(getTempFilename()))

python/plugins/processing/tests/testdata/expected/grass7/buffer_lines.prj

@@ -0,0 +1 @@
+GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]

python/plugins/processing/tests/testdata/grass7_algorithms_vector_tests.yaml

@@ -3,18 +3,33 @@
 tests:
 
 # v.* modules
-#  - algorithm: grass7:r.plane
-#    name: GRASS7 r.plane
-#    params:
-#      GRASS_REGION_PARAMETER: 344500.0,358400.0,6682800.0,6693700.0
-#      azimuth: 125
-#      dip: 45
-#      easting: 351610
-#      elevation: 50
-#      northing: 6688312
-#      type: 1
-#    results:
-#      output:
-#        hash: a9326678c39b6f925e7f22f6e79a48217100071cc8af85d675f28462
-#        type: rasterhash
+  - algorithm: grass7:v.buffer
+    name: Buffer vectors
+    params:
+      -c: false
+      -s: false
+      -t: false
+      GRASS_MIN_AREA_PARAMETER: 0.0001
+      GRASS_OUTPUT_TYPE_PARAMETER: 0
+      GRASS_SNAP_TOLERANCE_PARAMETER: -1.0
+      angle: 0.0
+      cats: ''
+      distance: 0.1
+      input:
+        name: lines.gml
+        type: vector
+      scale: 1.0
+      tolerance: 0.01
+      type:
+      - 0
+      - 1
+      - 4
+      where: ''
+    results:
+      output:
+        name: expected/grass7/buffer_lines.shp
+        type: vector
+        compare:
+          geometry:
+            precision: 7