correctly quote strings when copying in data

Encapsulate the copy string in a class that ensures that
copy lines are written with correct quoting.

nominatim/db/utils.py

@@ -4,6 +4,7 @@ Helper functions for handling DB accesses.
 import subprocess
 import logging
 import gzip
+import io
 
 from nominatim.db.connection import get_pg_env
 from nominatim.errors import UsageError
@@ -57,3 +58,49 @@ def execute_file(dsn, fname, ignore_errors=False, pre_code=None, post_code=None)
 
     if ret != 0 or remain > 0:
         raise UsageError("Failed to execute SQL file.")
+
+
+# List of characters that need to be quoted for the copy command.
+_SQL_TRANSLATION = {ord(u'\\') : u'\\\\',
+                    ord(u'\t') : u'\\t',
+                    ord(u'\n') : u'\\n'}
+
+class CopyBuffer:
+    """ Data collector for the copy_from command.
+    """
+
+    def __init__(self):
+        self.buffer = io.StringIO()
+
+
+    def __enter__(self):
+        return self
+
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        if self.buffer is not None:
+            self.buffer.close()
+
+
+    def add(self, *data):
+        """ Add another row of data to the copy buffer.
+        """
+        first = True
+        for column in data:
+            if first:
+                first = False
+            else:
+                self.buffer.write('\t')
+            if column is None:
+                self.buffer.write('\\N')
+            else:
+                self.buffer.write(str(column).translate(_SQL_TRANSLATION))
+        self.buffer.write('\n')
+
+
+    def copy_out(self, cur, table, columns=None):
+        """ Copy all collected data into the given table.
+        """
+        if self.buffer.tell() > 0:
+            self.buffer.seek(0)
+            cur.copy_from(self.buffer, table, columns=columns)