[sqlite3] add support for changing connection limits

[erlend-aasland]

BPO	45243
Nosy	@tiran, @berkerpeksag, @serhiy-storchaka, @zooba, @pablogsal, @erlend-aasland
PRs	bpo-45243: Add support for setting/getting sqlite3 connection limits #28463 bpo-45243: Expose SQLite connection limits as sqlite3.Connection attributes #28790 bpo-45243: Use connection limits to simplify sqlite3 tests #29356

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/erlend-aasland'
closed_at = <Date 2021-11-13.21:17:03.972>
created_at = <Date 2021-09-19.22:35:54.394>
labels = ['extension-modules', 'type-feature']
title = '[sqlite3] add support for changing connection limits'
updated_at = <Date 2021-11-13.21:17:03.972>
user = 'https://github.com/erlend-aasland'

bugs.python.org fields:

activity = <Date 2021-11-13.21:17:03.972>
actor = 'erlendaasland'
assignee = 'erlendaasland'
closed = True
closed_date = <Date 2021-11-13.21:17:03.972>
closer = 'erlendaasland'
components = ['Extension Modules']
creation = <Date 2021-09-19.22:35:54.394>
creator = 'erlendaasland'
dependencies = []
files = []
hgrepos = []
issue_num = 45243
keywords = ['patch']
message_count = 5.0
messages = ['402176', '402186', '405474', '405566', '405811']
nosy_count = 6.0
nosy_names = ['christian.heimes', 'berker.peksag', 'serhiy.storchaka', 'steve.dower', 'pablogsal', 'erlendaasland']
pr_nums = ['28463', '28790', '29356']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45243'
versions = []

[erlend-aasland]

I propose to add wrappers for the SQLite sqlite3_limit() C API. Using this API, it is possible to query and set limits on a connection basis. This will make it easier (and faster) to test various corner cases in the test suite without relying on test.support.bigmemtest.

Quoting from the SQLite sqlite3_limit() docs:

Run-time limits are intended for use in applications that manage both their
own internal database and also databases that are controlled by untrusted
external sources. An example application might be a web browser that has its
own databases for storing history and separate databases controlled by
JavaScript applications downloaded off the Internet. The internal databases
can be given the large, default limits. Databases managed by external
sources can be given much smaller limits designed to prevent a denial of
service attack.

See also:

• https://sqlite.org/c3ref/limit.html
• https://sqlite.org/c3ref/c_limit_attached.html
• https://sqlite.org/limits.html

Limit categories (C&P from SQLite docs)
---------------------------------------

SQLITE_LIMIT_LENGTH
The maximum size of any string or BLOB or table row, in bytes.

SQLITE_LIMIT_SQL_LENGTH
The maximum length of an SQL statement, in bytes.

SQLITE_LIMIT_COLUMN
The maximum number of columns in a table definition or in the result set of a SELECT or the maximum number of columns in an index or in an ORDER BY or GROUP BY clause.

SQLITE_LIMIT_EXPR_DEPTH
The maximum depth of the parse tree on any expression.

SQLITE_LIMIT_COMPOUND_SELECT
The maximum number of terms in a compound SELECT statement.

SQLITE_LIMIT_VDBE_OP
The maximum number of instructions in a virtual machine program used to implement an SQL statement. If sqlite3_prepare_v2() or the equivalent tries to allocate space for more than this many opcodes in a single prepared statement, an SQLITE_NOMEM error is returned.

SQLITE_LIMIT_FUNCTION_ARG
The maximum number of arguments on a function.

SQLITE_LIMIT_ATTACHED
The maximum number of attached databases.

SQLITE_LIMIT_LIKE_PATTERN_LENGTH
The maximum length of the pattern argument to the LIKE or GLOB operators.

SQLITE_LIMIT_VARIABLE_NUMBER
The maximum index number of any parameter in an SQL statement.

SQLITE_LIMIT_TRIGGER_DEPTH
The maximum depth of recursion for triggers.

SQLITE_LIMIT_WORKER_THREADS
The maximum number of auxiliary worker threads that a single prepared statement may start.

[erlend-aasland]

Christian, how about adding an audit event for something like sqlite3.Connection.setlimit()? My initial thought is: yes.

[pablogsal]

New changeset b6b38a8 by Erlend Egeberg Aasland in branch 'main':
bpo-45243: Add support for setting/getting sqlite3 connection limits (GH-28463)
b6b38a8

[erlend-aasland]

Steve, do you think it is worth it adding an audit hook for setting connection limits?

Most of the limits are harmless, but limits that control recursion are more interesting.

SQLITE_LIMIT_EXPR_DEPTH:

Maximum Depth Of An Expression Tree

SQLite parses expressions into a tree for processing. During code
generation, SQLite walks this tree recursively. The depth of expression
trees is therefore limited in order to avoid using too much stack space.
[...] If the value is 0, then no limit is enforced.

SQLITE_LIMIT_TRIGGER_DEPTH:

Maximum Depth Of Trigger Recursion

SQLite limits the depth of recursion of triggers in order to prevent a
statement involving recursive triggers from using an unbounded amount of
memory.

Note also, how the SQLite docs talk about SQLITE_LIMIT_LENGTH:

Maximum length of a string or BLOB

[...] In security-sensitive applications it is best not to try to increase
the maximum string and blob length. In fact, you might do well to lower
the maximum string and blob length to something more in the range of a few
million if that is possible.

[serhiy-storchaka]

New changeset 3d42cd9 by Erlend Egeberg Aasland in branch 'main':
bpo-45243: Use connection limits to simplify sqlite3 tests (GH-29356)
3d42cd9