Horton

Re: Large writable variables

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, Thomas Munro <thomas.munro@enterprisedb.com>
Date: 2018-10-16T16:26:21Z
Lists: pgsql-hackers 
On 2018-10-16 12:19:55 -0400, Robert Haas wrote:
> On Tue, Oct 16, 2018 at 12:06 PM Andres Freund <andres@anarazel.de> wrote:
> > But yea, it definitely should have a big red warning
> > label.
> 
> That's all I'm asking.  And the warning label shouldn't just say "use
> with caution!" but should rather explain how to know whether you're
> doing the right thing.

I'm thinking of something like:

/*
 * Macro that allows to cast constness away from a variable, but doesn't
 * allow changing the underlying type.  Enforcement of the latter
 * currently only works for gcc like compilers.
 *
 * Please note IT IS NOT SAFE to cast constness away if the variable will ever
 * be modified (it would be undefined behaviour). Doing so anyway can cause
 * compiler misoptimizations or runtime crashes (modifying readonly memory).
 * It is only safe to use when the the variabble will not be modified, but API
 * design or language restrictions prevent you from declaring that
 * (e.g. because a function returns both const and non-const variables).
 */

Greetings,

Andres Freund

Commits

  1. Apply unconstify() in more places

  2. Improve unconstify() documentation

  3. Drop const cast from dlsym() calls

  4. Const-ify a few more large static tables.

  5. Improve tzparse's handling of TZDEFRULES ("posixrules") zone data.

  6. Avoid statically allocating statement cache in ecpglib/prepare.c.

  7. Reorder FmgrBuiltin members, saving 25% in size.

  8. Add macro to cast away const without allowing changes to underlying type.

  9. Mark constantly allocated dest receiver as const.

  10. Avoid statically allocating formatting.c's format string caches.

  11. Correct constness of system attributes in heap.c & prerequisites.

  12. Avoid statically allocating gmtsub()'s timezone workspace.

  13. Correct constness of a few variables.

  14. Move the replication lag tracker into heap memory.