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Tom Lane be7b2848c6 Make the different Unix-y semaphore implementations ABI-compatible. 
Previously, the "sem" field of PGPROC varied in size depending on which
kernel semaphore API we were using.  That was okay as long as there was
only one likely choice per platform, but in the wake of commit ecb0d20a9,
that assumption seems rather shaky.  It doesn't seem out of the question
anymore that an extension compiled against one API choice might be loaded
into a postmaster built with another choice.  Moreover, this prevents any
possibility of selecting the semaphore API at postmaster startup, which
might be something we want to do in future.

Hence, change PGPROC.sem to be PGSemaphore (i.e. a pointer) for all Unix
semaphore APIs, and turn the pointed-to data into an opaque struct whose
contents are only known within the responsible modules.

For the SysV and unnamed-POSIX APIs, the pointed-to data has to be
allocated elsewhere in shared memory, which takes a little bit of
rejiggering of the InitShmemAllocation code sequence.  (I invented a
ShmemAllocUnlocked() function to make that a little cleaner than it used
to be.  That function is not meant for any uses other than the ones it
has now, but it beats having InitShmemAllocation() know explicitly about
allocation of space for semaphores and spinlocks.)  This change means an
extra indirection to access the semaphore data, but since we only touch
that when blocking or awakening a process, there shouldn't be any
meaningful performance penalty.  Moreover, at least for the unnamed-POSIX
case on Linux, the sem_t type is quite a bit wider than a pointer, so this
reduces sizeof(PGPROC) which seems like a good thing.

For the named-POSIX API, there's effectively no change: the PGPROC.sem
field was and still is a pointer to something returned by sem_open() in
the postmaster's memory space.  Document and check the pre-existing
limitation that this case can't work in EXEC_BACKEND mode.

It did not seem worth unifying the Windows semaphore ABI with the Unix
cases, since there's no likelihood of needing ABI compatibility much less
runtime switching across those cases.  However, we can simplify the Windows
code a bit if we define PGSemaphore as being directly a HANDLE, rather than
pointer to HANDLE, so let's do that while we're here.  (This also ends up
being no change in what's physically stored in PGPROC.sem.  We're just
moving the HANDLE fetch from callees to callers.)

It would take a bunch of additional code shuffling to get to the point of
actually choosing a semaphore API at postmaster start, but the effects
of that would now be localized in the port/XXX_sema.c files, so it seems
like fit material for a separate patch.  The need for it is unproven as
yet, anyhow, whereas the ABI risk to extensions seems real enough.

Discussion: https://postgr.es/m/4029.1481413370@sss.pgh.pa.us
2016-12-12 13:32:10 -05:00
config Fix python shlib probe for Cygwin. 2016-10-07 11:27:34 -04:00
contrib Add support for temporary replication slots 2016-12-12 08:38:17 -05:00
doc Add support for temporary replication slots 2016-12-12 08:38:17 -05:00
src Make the different Unix-y semaphore implementations ABI-compatible. 2016-12-12 13:32:10 -05:00
.dir-locals.el emacs: Set indent-tabs-mode in perl-mode 2015-04-12 23:53:23 -04:00
.gitattributes Fix whitespace and remove obsolete gitattributes entry 2016-03-13 16:03:13 -04:00
.gitignore Allow .so minor version numbers above 9 in .gitignore. 2016-08-15 17:35:35 -04:00
aclocal.m4 Replace our hacked version of ax_pthread.m4 with latest upstream version. 2015-07-08 20:36:06 +03:00
configure Fix broken autoconf test for random number source. 2016-12-12 09:26:42 +02:00
configure.in Fix broken autoconf test for random number source. 2016-12-12 09:26:42 +02:00
COPYRIGHT Update copyright for 2016 2016-01-02 13:33:40 -05:00
GNUmakefile.in Have "make coverage" recurse into contrib as well 2016-09-05 18:44:36 -03:00
HISTORY Improve text of stub HISTORY file. 2014-02-12 18:16:17 -05:00
Makefile Allow make check in PL directories 2011-02-15 06:52:12 +02:00
README Don't generate plain-text HISTORY and src/test/regress/README anymore. 2014-02-10 20:48:04 -05:00
README.git Don't generate plain-text HISTORY and src/test/regress/README anymore. 2014-02-10 20:48:04 -05:00

README 

PostgreSQL Database Management System
=====================================

This directory contains the source code distribution of the PostgreSQL
database management system.

PostgreSQL is an advanced object-relational database management system
that supports an extended subset of the SQL standard, including
transactions, foreign keys, subqueries, triggers, user-defined types
and functions.  This distribution also contains C language bindings.

PostgreSQL has many language interfaces, many of which are listed here:

	http://www.postgresql.org/download

See the file INSTALL for instructions on how to build and install
PostgreSQL.  That file also lists supported operating systems and
hardware platforms and contains information regarding any other
software packages that are required to build or run the PostgreSQL
system.  Copyright and license information can be found in the
file COPYRIGHT.  A comprehensive documentation set is included in this
distribution; it can be read as described in the installation
instructions.

The latest version of this software may be obtained at
http://www.postgresql.org/download/.  For more information look at our
web site located at http://www.postgresql.org/.