Horton

Re: pg_preadv() and pg_pwritev()

Sergey Shinderuk <s.shinderuk@postgrespro.ru>

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From: Sergey Shinderuk <s.shinderuk@postgrespro.ru>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Thomas Munro <thomas.munro@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2021-01-15T01:12:01Z
Lists: pgsql-hackers 
On 14.01.2021 21:05, Tom Lane wrote:
> After considerable playing around, I'm guessing that the reason
> -no_weak_imports doesn't help is that it rejects calls that are
> marked as weak references on the *calling* side.  Since AC_CHECK_FUNCS
> doesn't bother to #include the relevant header file, the compiler
> doesn't know that preadv() ought to be marked as a weak reference.
> Then, when the test program gets linked against the stub libc that's
> provided by the SDK, there is a version of preadv() there so no link
> failure occurs.  (There are way more moving parts in this weak-reference
> thing than I'd realized.)
> 

Oh, that's interesting. I've just played with it a bit and it looks 
exactly as you say.

> Another thing I've been realizing while poking at this is that we
> might not need to set -isysroot explicitly at all, which would then
> lead to the compiler using its default sysroot automatically.
> In some experimentation, it seems like what we need PG_SYSROOT for
> is just for configure to be able to find tclConfig.sh and the Perl
> header files.  So at this point I'm tempted to try ripping that
> out altogether.  If you remove the lines in src/template/darwin
> that inject PG_SYSROOT into CPPFLAGS and LDFLAGS, do things
> work for you?

Yes, it works fine.

Commits

  1. Improve our heuristic for selecting PG_SYSROOT on macOS.

  2. Move our p{read,write}v replacements into their own files.

  3. Don't use elog() in src/port/pwrite.c.

  4. Use vectored I/O to fill new WAL segments.

  5. Provide pg_preadv() and pg_pwritev().