Rationalizing declarations of src/common/ variables

We've been burnt by this issue repeatedly (cf c2d1eea9e, d025cf88b,
11b500072) so I think it's time to try to formalize and document
what to do to export a variable from src/common/ or src/port/.

Here's a draft patch.  I'm not in love with the name "PGDLLIMPORT_FE"
and would welcome better ideas.

            regards, tom lane

diff --git a/src/include/c.h b/src/include/c.h
index c8ede08273..e124e02d62 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -1312,10 +1312,34 @@ extern long long strtoll(const char *str, char **endptr, int base);
 extern unsigned long long strtoull(const char *str, char **endptr, int base);
 #endif

-/* no special DLL markers on most ports */
+/*
+ * Use "extern PGDLLIMPORT ..." to declare variables that are defined in
+ * the core backend and need to be accessible by loadable modules.
+ * No special marking is required on most ports.
+ */
 #ifndef PGDLLIMPORT
 #define PGDLLIMPORT
 #endif
+
+/*
+ * Use "extern PGDLLIMPORT_FE ..." to declare variables that are defined in
+ * common frontend/backend libraries (src/common/ or src/port/).  In the
+ * server, these are defined in the core backend and need to be accessible by
+ * loadable modules.  In frontend programs, these are defined locally and need
+ * no marking.  In any case, no special marking is required on most ports.
+ */
+#ifndef FRONTEND
+#define PGDLLIMPORT_FE PGDLLIMPORT
+#else
+#define PGDLLIMPORT_FE
+#endif
+
+/*
+ * 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.
+ */
 #ifndef PGDLLEXPORT
 #define PGDLLEXPORT
 #endif
diff --git a/src/include/common/keywords.h b/src/include/common/keywords.h
index 19e4eda8f9..54e40ac66c 100644
--- a/src/include/common/keywords.h
+++ b/src/include/common/keywords.h
@@ -22,14 +22,8 @@
 #define TYPE_FUNC_NAME_KEYWORD    2
 #define RESERVED_KEYWORD        3

-#ifndef FRONTEND
-extern PGDLLIMPORT const ScanKeywordList ScanKeywords;
-extern PGDLLIMPORT const uint8 ScanKeywordCategories[];
-extern PGDLLIMPORT const bool ScanKeywordBareLabel[];
-#else
-extern const ScanKeywordList ScanKeywords;
-extern const uint8 ScanKeywordCategories[];
-extern const bool ScanKeywordBareLabel[];
-#endif
+extern PGDLLIMPORT_FE const ScanKeywordList ScanKeywords;
+extern PGDLLIMPORT_FE const uint8 ScanKeywordCategories[];
+extern PGDLLIMPORT_FE const bool ScanKeywordBareLabel[];

 #endif                            /* KEYWORDS_H */
diff --git a/src/include/common/pg_prng.h b/src/include/common/pg_prng.h
index e4df5165d7..a07214fd11 100644
--- a/src/include/common/pg_prng.h
+++ b/src/include/common/pg_prng.h
@@ -26,11 +26,7 @@ typedef struct pg_prng_state
  * Callers not needing local PRNG series may use this global state vector,
  * after initializing it with one of the pg_prng_...seed functions.
  */
-#ifndef FRONTEND
-extern PGDLLIMPORT pg_prng_state pg_global_prng_state;
-#else
-extern pg_prng_state pg_global_prng_state;
-#endif
+extern PGDLLIMPORT_FE pg_prng_state pg_global_prng_state;

 extern void pg_prng_seed(pg_prng_state *state, uint64 seed);
 extern void pg_prng_fseed(pg_prng_state *state, double fseed);
diff --git a/src/include/port/pg_bitutils.h b/src/include/port/pg_bitutils.h
index 7dd7fef4f7..0cdbfe7a14 100644
--- a/src/include/port/pg_bitutils.h
+++ b/src/include/port/pg_bitutils.h
@@ -13,15 +13,9 @@
 #ifndef PG_BITUTILS_H
 #define PG_BITUTILS_H

-#ifndef FRONTEND
-extern PGDLLIMPORT const uint8 pg_leftmost_one_pos[256];
-extern PGDLLIMPORT const uint8 pg_rightmost_one_pos[256];
-extern PGDLLIMPORT const uint8 pg_number_of_ones[256];
-#else
-extern const uint8 pg_leftmost_one_pos[256];
-extern const uint8 pg_rightmost_one_pos[256];
-extern const uint8 pg_number_of_ones[256];
-#endif
+extern PGDLLIMPORT_FE const uint8 pg_leftmost_one_pos[256];
+extern PGDLLIMPORT_FE const uint8 pg_rightmost_one_pos[256];
+extern PGDLLIMPORT_FE const uint8 pg_number_of_ones[256];

 /*
  * pg_leftmost_one_pos32