grass_dbmidriver: Support no locking for SQLite driver over NFS

[metzm]

From https://sqlite.org/faq.html: "file locking is broken on many NFS implementations"
This causes db operations to be extremely slow or not working at all if the GRASS SQLite db is located on a NFS drive.

This PR attempts to solve this issue for affected NFS implementations.

The NFS implementations I have access to are all working fine wrt SQLite, therefore additional testing is needed.

This is not a bug in GRASS, but a bug in certain combinations of SQLite + NFS.

[metzm]

How to test:

• have a GRASS db on some NFS drive where db operations are slow (e.g. import of vector points, v.db.update)
• set the env var GRASS_SQLITE_NOLOCK to some number not 0 (zero)
• look for the message "Disabling SQLite locking" and check if db operations are now faster

[neteler]

Would it make sense to add this test?

The message Disabling SQLite locking appears:

diff --git a/scripts/v.db.update/testsuite/test_v_db_update.py b/scripts/v.db.update/testsuite/test_v_db_update.py
index 243ea71a3a..9357377a0e 100644
--- a/scripts/v.db.update/testsuite/test_v_db_update.py
+++ b/scripts/v.db.update/testsuite/test_v_db_update.py
@@ -4,6 +4,7 @@ Created on Sun Jun 08 22:14:26 2018
 @author: Sanjeet Bhatti
 """
 
+import os
 from grass.gunittest.case import TestCase
 from grass.gunittest.main import test
 from grass.gunittest.gmodules import SimpleModule
@@ -40,6 +41,19 @@ class TestVDbUpdate(TestCase):
         )
         self.assertModule(module)
 
+    def test_update_no_locking(self):
+        """update value test with GRASS_SQLITE_NOLOCK (https://github.com/OSGeo/grass/pull/1667)"""
+        run_command("v.db.addcolumn", map=self.mapName, column="zval2 double precision")
+        os.environ["GRASS_SQLITE_NOLOCK"] = "1"
+        module = SimpleModule(
+            "v.db.update",
+            map=self.mapName,
+            column="zval2",
+            query_column="CAST(z_value AS double precision)",
+            where="z_value <> 'N/A'",
+        )
+        self.assertModule(module)
+
 
 if __name__ == "__main__":
     test()

[neteler]

Would it make sense to add this test?

The message Disabling SQLite locking appears:

diff --git a/scripts/v.db.update/testsuite/test_v_db_update.py b/scripts/v.db.update/testsuite/test_v_db_update.py
index 243ea71a3a..9357377a0e 100644
--- a/scripts/v.db.update/testsuite/test_v_db_update.py
+++ b/scripts/v.db.update/testsuite/test_v_db_update.py
@@ -4,6 +4,7 @@ Created on Sun Jun 08 22:14:26 2018
 @author: Sanjeet Bhatti
 """
 
+import os
 from grass.gunittest.case import TestCase
 from grass.gunittest.main import test
 from grass.gunittest.gmodules import SimpleModule
@@ -40,6 +41,19 @@ class TestVDbUpdate(TestCase):
         )
         self.assertModule(module)
 
+    def test_update_no_locking(self):
+        """update value test with GRASS_SQLITE_NOLOCK (https://github.com/OSGeo/grass/pull/1667)"""
+        run_command("v.db.addcolumn", map=self.mapName, column="zval2 double precision")
+        os.environ["GRASS_SQLITE_NOLOCK"] = "1"
+        module = SimpleModule(
+            "v.db.update",
+            map=self.mapName,
+            column="zval2",
+            query_column="CAST(z_value AS double precision)",
+            where="z_value <> 'N/A'",
+        )
+        self.assertModule(module)
+
 
 if __name__ == "__main__":
     test()