Fix date/time handling when string values are not the same (fixes #143)

This fixes the issue when applying a diff with date/time values,
but the representation of date/time values as strings is slightly
different even though they refer to the same date/time.

Fixes both the case with update and the case with delete.
Insert does not need fixing as there is no comparison with previous values.

geodiff/src/drivers/sqlitedriver.cpp

@@ -629,6 +629,12 @@ static std::string sqlForUpdate( const std::string &tableName, const TableSchema
       sql += " AND ";
     if ( tbl.columns[i].isPrimaryKey )
       sql += sqlitePrintf( " \"%w\" = ?%d ", tbl.columns[i].name.c_str(), i * 3 + 1 );
+    else if ( tbl.columns[i].type.baseType == TableColumnType::DATETIME )
+    {
+      // compare date/time values using datetime() because they may have
+      // multiple equivalent string representations (see #143)
+      sql += sqlitePrintf( " ( ?%d = 0 OR datetime(\"%w\") IS datetime(?%d) ) ", i * 3 + 2, tbl.columns[i].name.c_str(), i * 3 + 1 );
+    }
     else
       sql += sqlitePrintf( " ( ?%d = 0 OR \"%w\" IS ?%d ) ", i * 3 + 2, tbl.columns[i].name.c_str(), i * 3 + 1 );
   }
@@ -652,6 +658,12 @@ static std::string sqlForDelete( const std::string &tableName, const TableSchema
       sql += " AND ";
     if ( tbl.columns[i].isPrimaryKey )
       sql += sqlitePrintf( "\"%w\" = ?", tbl.columns[i].name.c_str() );
+    else if ( tbl.columns[i].type.baseType == TableColumnType::DATETIME )
+    {
+      // compare date/time values using datetime() because they may have
+      // multiple equivalent string representations (see #143)
+      sql += sqlitePrintf( "datetime(\"%w\") IS datetime(?)", tbl.columns[i].name.c_str() );
+    }
     else
       sql += sqlitePrintf( "\"%w\" IS ?", tbl.columns[i].name.c_str() );
   }

geodiff/tests/test_driver_sqlite.cpp

@@ -293,6 +293,33 @@ TEST( SqliteDriverTest, create_changeset_datetime )
                      );
 }
 
+
+TEST( SqliteDriverTest, apply_changeset_datetime )
+{
+  // check that the datetime handling is robust - a single datetime may have
+  // multiple representations, e.g. '2021-03-16T00:00:00' and '2021-03-16T00:00:00Z'
+  // represent the same datetime, so things should work fine even if database
+  // contains '2021-03-16T00:00:00' but changeset refers to '2021-03-16T00:00:00Z'
+
+  // datetime1-3a.diff refers to date/time value in deleted row as '2021-04-01T15:00:00Z'
+  // while datetime1.gpkg has the value stored as '2021-04-01 15:00:00'
+  testApplyChangeset( "test_apply_changeset_datetime_delete",
+                      pathjoin( testdir(), "datetime", "datetime1.gpkg" ),
+                      pathjoin( testdir(), "datetime", "datetime1-3a.diff" ),
+                      pathjoin( testdir(), "datetime", "datetime3.gpkg" )
+                    );
+
+  // datetime1a.gpkg has the same content as datetime1, but date/time values
+  // are represented as '2021-04-01T15:00:00Z' instead of '2021-04-01 15:00:00'
+  // (which is used in datetime1.gpkg and in datetime1-2.diff)
+  testApplyChangeset( "test_apply_changeset_datetime_update",
+                      pathjoin( testdir(), "datetime", "datetime1a.gpkg" ),
+                      pathjoin( testdir(), "datetime", "datetime1-2.diff" ),
+                      pathjoin( testdir(), "datetime", "datetime2.gpkg" )
+                    );
+}
+
+
 TEST( SqliteDriverTest, apply_with_gpkg_contents )
 {
   // In geodiff >= 1.0 we ignore gpkg_* metadata tables. However older geodiff