bpo-27645: Add support for native backup facility of SQLite (GH-4238)

Author: lelit

Doc/library/sqlite3.rst

@@ -532,6 +532,56 @@ Connection Objects
                  f.write('%s\n' % line)
 
 
+   .. method:: backup(target, *, pages=0, progress=None, name="main", sleep=0.250)
+
+      This method makes a backup of a SQLite database even while it's being accessed
+      by other clients, or concurrently by the same connection.  The copy will be
+      written into the mandatory argument *target*, that must be another
+      :class:`Connection` instance.
+
+      By default, or when *pages* is either ``0`` or a negative integer, the entire
+      database is copied in a single step; otherwise the method performs a loop
+      copying up to *pages* pages at a time.
+
+      If *progress* is specified, it must either be ``None`` or a callable object that
+      will be executed at each iteration with three integer arguments, respectively
+      the *status* of the last iteration, the *remaining* number of pages still to be
+      copied and the *total* number of pages.
+
+      The *name* argument specifies the database name that will be copied: it must be
+      a string containing either ``"main"``, the default, to indicate the main
+      database, ``"temp"`` to indicate the temporary database or the name specified
+      after the ``AS`` keyword in an ``ATTACH DATABASE`` statement for an attached
+      database.
+
+      The *sleep* argument specifies the number of seconds to sleep by between
+      successive attempts to backup remaining pages, can be specified either as an
+      integer or a floating point value.
+
+      Example 1, copy an existing database into another::
+
+         import sqlite3
+
+         def progress(status, remaining, total):
+             print(f'Copied {total-remaining} of {total} pages...')
+
+         con = sqlite3.connect('existing_db.db')
+         with sqlite3.connect('backup.db') as bck:
+             con.backup(bck, pages=1, progress=progress)
+
+      Example 2, copy an existing database into a transient copy::
+
+         import sqlite3
+
+         source = sqlite3.connect('existing_db.db')
+         dest = sqlite3.connect(':memory:')
+         source.backup(dest)
+
+      Availability: SQLite 3.6.11 or higher
+
+      .. versionadded:: 3.7
+
+
 .. _sqlite3-cursor-objects:
 
 Cursor Objects

Doc/whatsnew/3.7.rst

@@ -630,6 +630,15 @@ can be set within the scope of a group.
 ``'^$'`` or ``(?=-)`` that matches an empty string.
 (Contributed by Serhiy Storchaka in :issue:`25054`.)
 
+
+sqlite3
+-------
+
+:class:`sqlite3.Connection` now exposes a :class:`~sqlite3.Connection.backup`
+method, if the underlying SQLite library is at version 3.6.11 or higher.
+(Contributed by Lele Gaifax in :issue:`27645`.)
+
+
 ssl
 ---
 

Lib/sqlite3/test/backup.py

@@ -0,0 +1,162 @@
+import sqlite3 as sqlite
+import unittest
+
+
+@unittest.skipIf(sqlite.sqlite_version_info < (3, 6, 11), "Backup API not supported")
+class BackupTests(unittest.TestCase):
+    def setUp(self):
+        cx = self.cx = sqlite.connect(":memory:")
+        cx.execute('CREATE TABLE foo (key INTEGER)')
+        cx.executemany('INSERT INTO foo (key) VALUES (?)', [(3,), (4,)])
+        cx.commit()
+
+    def tearDown(self):
+        self.cx.close()
+
+    def verify_backup(self, bckcx):
+        result = bckcx.execute("SELECT key FROM foo ORDER BY key").fetchall()
+        self.assertEqual(result[0][0], 3)
+        self.assertEqual(result[1][0], 4)
+
+    def test_bad_target_none(self):
+        with self.assertRaises(TypeError):
+            self.cx.backup(None)
+
+    def test_bad_target_filename(self):
+        with self.assertRaises(TypeError):
+            self.cx.backup('some_file_name.db')
+
+    def test_bad_target_same_connection(self):
+        with self.assertRaises(ValueError):
+            self.cx.backup(self.cx)
+
+    def test_bad_target_closed_connection(self):
+        bck = sqlite.connect(':memory:')
+        bck.close()
+        with self.assertRaises(sqlite.ProgrammingError):
+            self.cx.backup(bck)
+
+    def test_bad_target_in_transaction(self):
+        bck = sqlite.connect(':memory:')
+        bck.execute('CREATE TABLE bar (key INTEGER)')
+        bck.executemany('INSERT INTO bar (key) VALUES (?)', [(3,), (4,)])
+        with self.assertRaises(sqlite.OperationalError) as cm:
+            self.cx.backup(bck)
+        if sqlite.sqlite_version_info < (3, 8, 7):
+            self.assertEqual(str(cm.exception), 'target is in transaction')
+
+    def test_keyword_only_args(self):
+        with self.assertRaises(TypeError):
+            with sqlite.connect(':memory:') as bck:
+                self.cx.backup(bck, 1)
+
+    def test_simple(self):
+        with sqlite.connect(':memory:') as bck:
+            self.cx.backup(bck)
+            self.verify_backup(bck)
+
+    def test_progress(self):
+        journal = []
+
+        def progress(status, remaining, total):
+            journal.append(status)
+
+        with sqlite.connect(':memory:') as bck:
+            self.cx.backup(bck, pages=1, progress=progress)
+            self.verify_backup(bck)
+
+        self.assertEqual(len(journal), 2)
+        self.assertEqual(journal[0], sqlite.SQLITE_OK)
+        self.assertEqual(journal[1], sqlite.SQLITE_DONE)
+
+    def test_progress_all_pages_at_once_1(self):
+        journal = []
+
+        def progress(status, remaining, total):
+            journal.append(remaining)
+
+        with sqlite.connect(':memory:') as bck:
+            self.cx.backup(bck, progress=progress)
+            self.verify_backup(bck)
+
+        self.assertEqual(len(journal), 1)
+        self.assertEqual(journal[0], 0)
+
+    def test_progress_all_pages_at_once_2(self):
+        journal = []
+
+        def progress(status, remaining, total):
+            journal.append(remaining)
+
+        with sqlite.connect(':memory:') as bck:
+            self.cx.backup(bck, pages=-1, progress=progress)
+            self.verify_backup(bck)
+
+        self.assertEqual(len(journal), 1)
+        self.assertEqual(journal[0], 0)
+
+    def test_non_callable_progress(self):
+        with self.assertRaises(TypeError) as cm:
+            with sqlite.connect(':memory:') as bck:
+                self.cx.backup(bck, pages=1, progress='bar')
+        self.assertEqual(str(cm.exception), 'progress argument must be a callable')
+
+    def test_modifying_progress(self):
+        journal = []
+
+        def progress(status, remaining, total):
+            if not journal:
+                self.cx.execute('INSERT INTO foo (key) VALUES (?)', (remaining+1000,))
+                self.cx.commit()
+            journal.append(remaining)
+
+        with sqlite.connect(':memory:') as bck:
+            self.cx.backup(bck, pages=1, progress=progress)
+            self.verify_backup(bck)
+
+            result = bck.execute("SELECT key FROM foo"
+                                 " WHERE key >= 1000"
+                                 " ORDER BY key").fetchall()
+            self.assertEqual(result[0][0], 1001)
+
+        self.assertEqual(len(journal), 3)
+        self.assertEqual(journal[0], 1)
+        self.assertEqual(journal[1], 1)
+        self.assertEqual(journal[2], 0)
+
+    def test_failing_progress(self):
+        def progress(status, remaining, total):
+            raise SystemError('nearly out of space')
+
+        with self.assertRaises(SystemError) as err:
+            with sqlite.connect(':memory:') as bck:
+                self.cx.backup(bck, progress=progress)
+        self.assertEqual(str(err.exception), 'nearly out of space')
+
+    def test_database_source_name(self):
+        with sqlite.connect(':memory:') as bck:
+            self.cx.backup(bck, name='main')
+        with sqlite.connect(':memory:') as bck:
+            self.cx.backup(bck, name='temp')
+        with self.assertRaises(sqlite.OperationalError) as cm:
+            with sqlite.connect(':memory:') as bck:
+                self.cx.backup(bck, name='non-existing')
+        self.assertIn(
+            str(cm.exception),
+            ['SQL logic error', 'SQL logic error or missing database']
+        )
+
+        self.cx.execute("ATTACH DATABASE ':memory:' AS attached_db")
+        self.cx.execute('CREATE TABLE attached_db.foo (key INTEGER)')
+        self.cx.executemany('INSERT INTO attached_db.foo (key) VALUES (?)', [(3,), (4,)])
+        self.cx.commit()
+        with sqlite.connect(':memory:') as bck:
+            self.cx.backup(bck, name='attached_db')
+            self.verify_backup(bck)
+
+
+def suite():
+    return unittest.makeSuite(BackupTests)
+
+if __name__ == "__main__":
+    unittest.main()

Lib/test/test_sqlite.py

@@ -7,7 +7,7 @@
 import sqlite3
 from sqlite3.test import (dbapi, types, userfunctions,
                                 factory, transactions, hooks, regression,
-                                dump)
+                                dump, backup)
 
 def load_tests(*args):
     if test.support.verbose:
@@ -18,7 +18,8 @@ def load_tests(*args):
                                userfunctions.suite(),
                                factory.suite(), transactions.suite(),
                                hooks.suite(), regression.suite(),
-                               dump.suite()])
+                               dump.suite(),
+                               backup.suite()])
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2017-10-05-20-41-48.bpo-27645.1Y_Wag.rst

@@ -0,0 +1,3 @@
+:class:`sqlite3.Connection` now exposes a :class:`~sqlite3.Connection.backup`
+method, if the underlying SQLite library is at version 3.6.11
+or higher.  Patch by Lele Gaifax.