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We add an extra condition that makes the inequality testing in SEQUENCE::increment_value() mathematically watertight, and we cast to and from unsigned in potential underflow and overflow addition and subtractions to avoid undefined behaviour. Let's start by distinguishing between c++ expressions and mathematical expressions. by c++ expression I mean an expression with the outcome determined by the compiler/runtime. by mathematical expression I mean an expression whose value is mathematically determined. So a c++ expression -9223372036854775806 - 1000 at worst can evaluate to any value due to underflow. A mathematical expression -9223372036854775806 - 1000 evaluates to -9223372036854776806. The problem boils down to how to write a c++ expression equivalent to an mathematical expression x + y < z where x and z can take any values of long long int, and y < 0 is also a long long int. Ideally we want to avoid underflow, but I'm not sure how this can be done. The correct c++ form should be (x + y < z || x < z - y || x < z). Let M=9223372036854775808 i.e. LONGLONG_MAX + 1. We have -M < x < M - 1 -M < y < 0 -M < z < M - 1 Let's consider the case where x + y < z is true as a mathematical expression. If the first disjunct underflows, i.e. the mathematical expression x + y < -M. If the arbitrary value resulting from the underflow causes the c++ expression to hold too, then we are done. Otherwise we move onto the next expression x < z - y. If there's no overflow in z - y then we are done. If there's overflow i.e. z - y > M - 1, and the c++ expression evals to false, then we are onto x < z. There's no over or underflow here, and it will eval to true. To see this, note that x + y < -M means x < -M - y < -M - (-M) = 0 z - y > M - 1 means z > y + M - 1 > - M + M - 1 = -1 so x < z. Now let's consider the case where x + y < z is false as a mathematical expression. The first disjunct will not underflow in this case, so we move to (x < z - y). This will not overflow. To see this, note that x + y >= z means z - y <= x < M - 1 So it evals to false too. And the third disjunct x < z also evals to false because x >= z - y > z. I suspect that in either case the expression x < z does not determine the final value of the disjunction in the vast majority cases, which is why we leave it as the final one in case of the rare cases of both an underflow and an overflow happening. Here's an example of both underflow and overflow happening and the added inequality x < z saves the day: x = - M / 2 y = - M / 2 - 1 z = M / 2 x + y evals to M - 1 which is > z z - y evals to - M + 1 which is < x We can do the same to test x + y > z where the increment y is positive: (x > z - y || x + y > z || x > z) And the same analysis applies to unsigned cases. |
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Code status:
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MariaDB: The open source relational database
MariaDB was designed as a drop-in replacement of MySQL(R) with more features, new storage engines, fewer bugs, and better performance.
MariaDB is brought to you by the MariaDB Foundation and the MariaDB Corporation. Please read the CREDITS file for details about the MariaDB Foundation, and who is developing MariaDB.
MariaDB is developed by many of the original developers of MySQL who now work for the MariaDB Corporation, the MariaDB Foundation and by many people in the community.
MySQL, which is the base of MariaDB, is a product and trademark of Oracle Corporation, Inc. For a list of developers and other contributors, see the Credits appendix. You can also run 'SHOW authors' to get a list of active contributors.
A description of the MariaDB project and a manual can be found at:
https://mariadb.com/kb/en/mariadb-vs-mysql-features/
https://mariadb.com/kb/en/mariadb-versus-mysql-compatibility/
https://mariadb.com/kb/en/new-and-old-releases/
Help
More help is available from the Maria Discuss mailing list https://launchpad.net/~maria-discuss, MariaDB's Zulip instance, https://mariadb.zulipchat.com/
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MariaDB has a dedicated time each week when we answer new contributor questions live on Zulip. From 8:00 to 10:00 UTC on Mondays, and 10:00 to 12:00 UTC on Thursdays, anyone can ask any questions they’d like, and a live developer will be available to assist.
New contributors can ask questions any time, but we will provide immediate feedback during that interval.
Licensing
NOTE:
MariaDB is specifically available only under version 2 of the GNU General Public License (GPLv2). (I.e. Without the "any later version" clause.) This is inherited from MySQL. Please see the README file in the MySQL distribution for more information.
License information can be found in the COPYING file. Third party license information can be found in the THIRDPARTY file.
Bug Reports
Bug and/or error reports regarding MariaDB should be submitted at: https://jira.mariadb.org
For reporting security vulnerabilities, see our security-policy.
The code for MariaDB, including all revision history, can be found at: https://github.com/MariaDB/server