Patch to allow C extension modules to initialize/finish - Mailing list pgsql-hackers

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@engelschall.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(constchar *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
NULLto 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*/