Re: Annoying build warnings from latest Apple toolchain

I wrote:
> Assuming that this problem is restricted to initdb, which I think
> is true, probably the best fix is to cause the initdb link *only*
> to link libpgcommon before libpq.  Every other non-backend program
> is interested in libpq's encoding IDs if it cares at all.

The more I thought about that the less I liked it.  We're trying to
get away from link order dependencies, not add more.  And the fact
that we've had a latent bug for awhile from random-ish changes in
link order should reinforce our desire to get out of that business.

So I experimented with fixing things so that the versions of these
functions exported by libpq have physically different names from those
that you'd get from linking to libpgcommon.a or libpgcommon_srv.a.
Then, there's certainty about which one a given usage will link to,
based on what the #define environment is when the call is compiled.

This leads to a pleasingly small patch, at least in the Makefile
universe (I've not attempted to sync the meson or MSVC infrastructure
with this yet).  As a bonus, it should silence those new warnings
on AIX.  A disadvantage is that this causes an ABI break for
backend extensions, so we couldn't consider back-patching it.
But I think that's fine given that the problem is only latent
in released branches.

Thoughts?

            regards, tom lane


diff --git a/src/bin/initdb/Makefile b/src/bin/initdb/Makefile
index d69bd89572..e80e57e457 100644
--- a/src/bin/initdb/Makefile
+++ b/src/bin/initdb/Makefile
@@ -16,13 +16,12 @@ subdir = src/bin/initdb
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global

-override CPPFLAGS := -I$(libpq_srcdir) -I$(top_srcdir)/src/timezone $(ICU_CFLAGS) $(CPPFLAGS)
-
 # Note: it's important that we link to encnames.o from libpgcommon, not
 # from libpq, else we have risks of version skew if we run with a libpq
-# shared library from a different PG version.  The libpq_pgport macro
-# should ensure that that happens.
-#
+# shared library from a different PG version.  Define
+# USE_PRIVATE_ENCODING_FUNCS to ensure that that happens.
+override CPPFLAGS := -DUSE_PRIVATE_ENCODING_FUNCS -I$(libpq_srcdir) -I$(top_srcdir)/src/timezone $(ICU_CFLAGS)
$(CPPFLAGS)
+
 # We need libpq only because fe_utils does.
 LDFLAGS_INTERNAL += -L$(top_builddir)/src/fe_utils -lpgfeutils $(libpq_pgport) $(ICU_LIBS)

diff --git a/src/common/Makefile b/src/common/Makefile
index cc5c54dcee..70884be00c 100644
--- a/src/common/Makefile
+++ b/src/common/Makefile
@@ -140,6 +140,13 @@ libpgcommon.a: $(OBJS_FRONTEND)
     rm -f $@
     $(AR) $(AROPT) $@ $^

+#
+# Files in libpgcommon.a should use/export the "xxx_private" versions
+# of pg_char_to_encoding() and friends.
+#
+$(OBJS_FRONTEND): CPPFLAGS += -DUSE_PRIVATE_ENCODING_FUNCS
+
+
 #
 # Shared library versions of object files
 #
diff --git a/src/include/mb/pg_wchar.h b/src/include/mb/pg_wchar.h
index 25276b199f..7d2fad91e6 100644
--- a/src/include/mb/pg_wchar.h
+++ b/src/include/mb/pg_wchar.h
@@ -13,6 +13,8 @@
  *        included by libpq client programs.  In particular, a libpq client
  *        should not assume that the encoding IDs used by the version of libpq
  *        it's linked to match up with the IDs declared here.
+ *        To help prevent mistakes, relevant functions that are exported by
+ *        libpq have a physically different name when being referenced directly.
  *
  *-------------------------------------------------------------------------
  */
@@ -562,6 +564,23 @@ surrogate_pair_to_codepoint(pg_wchar first, pg_wchar second)
 }


+/*
+ * The functions in this list are exported by libpq, and we need to be sure
+ * that we know which calls are satisfied by libpq and which are satisfied
+ * by static linkage to libpgcommon.  (This is because we might be using a
+ * libpq.so that's of a different major version and has different encoding
+ * IDs from what libpgcommon knows.)  The official function names are what
+ * is actually used in and exported by libpq, while the names exported by
+ * libpgcommon.a and libpgcommon_srv.a end in "_private".
+ */
+#if defined(USE_PRIVATE_ENCODING_FUNCS) || !defined(FRONTEND)
+#define pg_char_to_encoding            pg_char_to_encoding_private
+#define pg_encoding_to_char            pg_encoding_to_char_private
+#define pg_valid_server_encoding    pg_valid_server_encoding_private
+#define pg_valid_server_encoding_id    pg_valid_server_encoding_id_private
+#define pg_utf_mblen                pg_utf_mblen_private
+#endif
+
 /*
  * These functions are considered part of libpq's exported API and
  * are also declared in libpq-fe.h.