Patch to allow C extension modules to initialize/finish

PostgreSQL provides a way to load C extension modules with its internal
FMGR. Unfortunately there is no portable way for an extension module to
initialize (directly after the pg_dlopen() of the DSO) and to finish
(directly before the pg_dlclose() of the DSO). This way it is mostly
impossible to write a more complex extension module in a portable way.

The only to me known workarounds are either to call an own
initialization function at the start of _EVERY_ exported function
manually (works, but is ugly and especially doesn't work for the
finishing function!) or to leverage some platform specific hacks like
the implicitly called _init and _fini functions (is what the ODBC
extension module currently does, but is horribly platform specific and
not portable).

Hence I propose the patch below (applies to PostgreSQL 8.1.4) which
mimics the dlopen(3) and dlclose(3) behaviour of some Unix platforms
and resolves and calls _PG_init and _PG_fini functions of an extension
module right after/before the pg_dlopen/pg_dlclose calls in the FMGR.
This is both a fully portable solution and fully backward compatible to
existing and forthcoming extension modules (except they really would
have _PG_init and _PG_fini functions already defined).

Ralf S. Engelschall
rse(at)engelschall(dot)com
www.engelschall.com

Index: src/backend/utils/fmgr/dfmgr.c
--- src/backend/utils/fmgr/dfmgr.c.orig 2005-10-15 04:49:32 +0200
+++ src/backend/utils/fmgr/dfmgr.c 2006-08-02 20:48:48 +0200
@@ -60,6 +60,10 @@
static char *expand_dynamic_library_name(const char *name);
static char *substitute_libpath_macro(const char *name);

+/* types for PostgreSQL-specific DSO init/fini functions */
+typedef void (*PG_init_t)(void);
+typedef void (*PG_fini_t)(void);
+
/*
* Load the specified dynamic-link library file, and look for a function
* named funcname in it. (funcname can be NULL to just load the file.)
@@ -82,6 +86,7 @@
char *load_error;
struct stat stat_buf;
char *fullname;
+ PG_init_t *PG_init;

fullname = expand_dynamic_library_name(filename);
if (!fullname)
@@ -146,6 +151,13 @@
fullname, load_error)));
}

+ /* optionally give the DSO a chance to initialize by calling a
+ PostgreSQL-specific (and this way portable) "_PG_init" function
+ similar to what dlopen(3) implicitly does with "_init" on some
+ Unix platforms. */
+ if ((PG_init = (PG_init_t *)pg_dlsym(file_scanner->handle, "_PG_init")) != NULL)
+ (*PG_init)();
+
/* OK to link it into list */
if (file_list == NULL)
file_list = file_scanner;
@@ -192,6 +204,7 @@
*nxt;
struct stat stat_buf;
char *fullname;
+ PG_fini_t *PG_fini;

fullname = expand_dynamic_library_name(filename);
if (!fullname)
@@ -224,6 +237,14 @@
else
file_list = nxt;
clear_external_function_hash(file_scanner->handle);
+
+ /* optionally give the DSO a chance to finish by calling
+ a PostgreSQL-specific (and this way portable) "_PG_fini"
+ function similar to what dlopen(3) implicitly does with
+ "_fini" on some Unix platforms. */
+ if ((PG_fini = (PG_init_t *)pg_dlsym(file_scanner->handle, "_PG_fini")) != NULL)
+ (*PG_fini)();
+
pg_dlclose(file_scanner->handle);
free((char *) file_scanner);
/* prv does not change */