Default to hidden visibility for extension libraries where possible

Until now postgres built extension libraries with global visibility, i.e.
exporting all symbols.  On the one platform where that behavior is not
natively available, namely windows, we emulate it by analyzing the input files
to the shared library and exporting all the symbols therein.

Not exporting all symbols is actually desirable, as it can improve loading
speed, reduces the likelihood of symbol conflicts and can improve intra
extension library function call performance. It also makes the non-windows
builds more similar to windows builds.

Additionally, with meson implementing the export-all-symbols behavior for
windows, turns out to be more verbose than desirable.

This patch adds support for hiding symbols by default and, to counteract that,
explicit symbol visibility annotation for compilers that support
__attribute__((visibility("default"))) and -fvisibility=hidden. That is
expected to be most, if not all, compilers except msvc (for which we already
support explicit symbol export annotations).

Now that extension library symbols are explicitly exported, we don't need to
export all symbols on windows anymore, hence remove that behavior from
src/tools/msvc. The supporting code can't be removed, as we still need to
export all symbols from the main postgres binary.

Author: Andres Freund <andres@anarazel.de>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20211101020311.av6hphdl6xbjbuif@alap3.anarazel.de

1 files changed, 9 insertions, 4 deletions

diff --git a/src/include/c.h b/src/include/c.h
index 863a16c6a6..2cc2784750 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -1347,14 +1347,19 @@ extern unsigned long long strtoull(const char *str, char **endptr, int base);
 
 /*
  * Use "extern PGDLLEXPORT ..." to declare functions that are defined in
- * loadable modules and need to be callable by the core backend.  (Usually,
- * this is not necessary because our build process automatically exports
- * such symbols, but sometimes manual marking is required.)
- * No special marking is required on most ports.
+ * loadable modules and need to be callable by the core backend or other
+ * loadable modules.
+ * If the compiler knows __attribute__((visibility("*"))), we use that,
+ * unless we already have a platform-specific definition.  Otherwise,
+ * no special marking is required.
  */
 #ifndef PGDLLEXPORT
+#ifdef HAVE_VISIBILITY_ATTRIBUTE
+#define PGDLLEXPORT __attribute__((visibility("default")))
+#else
 #define PGDLLEXPORT
 #endif
+#endif
 
 /*
  * The following is used as the arg list for signal handlers.  Any ports