django: implement SchemaEditor.quote_value()

[timgraham]

For example:

======================================================================
ERROR: test_extra_args (schema.test_logging.SchemaLoggerTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tim/code/django/tests/schema/test_logging.py", line 12, in test_extra_args
    editor.execute(sql, params)
  File "/home/tim/code/django/django/db/backends/base/schema.py", line 132, in execute
    self.collected_sql.append((sql % tuple(map(self.quote_value, params))) + ending)
  File "/home/tim/code/django/django/db/backends/base/schema.py", line 243, in quote_value
    raise NotImplementedError()
NotImplementedError

[timgraham]

The docstring of this method says, "Return a quoted version of the value so it's safe to use in an SQL string. This is not safe against injection from user code; it is intended only for use in making SQL scripts or preparing default values for particularly tricky backends (defaults are not user-defined, though, so this is safe)."

Some implementations on other databases:

SQLite: sqlite3.adapt()
MySQL: self.connection.connection.escape(value, self.connection.connection.encoders)
PostgreSQL: psycopg2.extensions.adapt(value).getquoted().decode()

@odeke-em, do you know if Spanner's Python libraries provides anything similar? If not, we could consider an implementation in Python similar to what the Oracle backend does.

[odeke-em]

Unfortunately, I don't think that the Python client libraries support quoting values, we might have to add that function ourselves. Looking at the Cloud Spanner advisory on string and byte literals, they have a section on quoting https://cloud.google.com/spanner/docs/lexical#string-and-bytes-literals

[timgraham]

It looks like the usage of quote_value() is limited to conditional indexes and constraints which Spanner doesn't support, so I don't think this is high priority.

django/db/models/indexes.py:        return sql % tuple(schema_editor.quote_value(p) for p in params)
django/db/models/constraints.py:        return sql % tuple(schema_editor.quote_value(p) for p in params)