Fix relptr's encoding of the base address.

Previously, we encoded both NULL and the first byte at the base address
as 0.  That confusion led to the assertion in commit e07d4ddc, which
failed when min_dynamic_shared_memory was used.  Give them distinct
encodings, by switching to 1-based offsets for non-NULL pointers.  Also
improve macro hygiene in passing (missing/misplaced parentheses), and
remove open-coded access to the raw offset value from freepage.c/h.

Although e07d4ddc was back-patched to 10, the only code that actually
makes use of relptr at the base address arrived in 84b1c63a, so no need
to back-patch further than 14 for now.
Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/20220519193839.GT19626%40telsasoft.com

src/backend/utils/mmgr/freepage.c

@@ -434,7 +434,7 @@ FreePageManagerDump(FreePageManager *fpm)
 
 	/* Dump general stuff. */
 	appendStringInfo(&buf, "metadata: self %zu max contiguous pages = %zu\n",
-					 fpm->self.relptr_off, fpm->contiguous_pages);
+					 relptr_offset(fpm->self), fpm->contiguous_pages);
 
 	/* Dump btree. */
 	if (fpm->btree_depth > 0)
@@ -1269,7 +1269,7 @@ FreePageManagerDumpBtree(FreePageManager *fpm, FreePageBtree *btp,
 		if (btp->hdr.magic == FREE_PAGE_INTERNAL_MAGIC)
 			appendStringInfo(buf, " %zu->%zu",
 							 btp->u.internal_key[index].first_page,
-							 btp->u.internal_key[index].child.relptr_off / FPM_PAGE_SIZE);
+							 relptr_offset(btp->u.internal_key[index].child) / FPM_PAGE_SIZE);
 		else
 			appendStringInfo(buf, " %zu(%zu)",
 							 btp->u.leaf_key[index].first_page,
@@ -1859,7 +1859,7 @@ FreePagePopSpanLeader(FreePageManager *fpm, Size pageno)
 	{
 		Size		f = Min(span->npages, FPM_NUM_FREELISTS) - 1;
 
-		Assert(fpm->freelist[f].relptr_off == pageno * FPM_PAGE_SIZE);
+		Assert(relptr_offset(fpm->freelist[f]) == pageno * FPM_PAGE_SIZE);
 		relptr_copy(fpm->freelist[f], span->next);
 	}
 }

src/include/utils/freepage.h

@@ -78,11 +78,11 @@ struct FreePageManager
 #define fpm_pointer_is_page_aligned(base, ptr)		\
 	(((Size) (((char *) (ptr)) - (base))) % FPM_PAGE_SIZE == 0)
 #define fpm_relptr_is_page_aligned(base, relptr)		\
-	((relptr).relptr_off % FPM_PAGE_SIZE == 0)
+	(relptr_offset(relptr) % FPM_PAGE_SIZE == 0)
 
 /* Macro to find base address of the segment containing a FreePageManager. */
 #define fpm_segment_base(fpm)	\
-	(((char *) fpm) - fpm->self.relptr_off)
+	(((char *) fpm) - relptr_offset(fpm->self))
 
 /* Macro to access a FreePageManager's largest consecutive run of pages. */
 #define fpm_largest(fpm) \

src/include/utils/relptr.h

@@ -42,7 +42,7 @@
 #define relptr_access(base, rp) \
 	(AssertVariableIsOfTypeMacro(base, char *), \
 	 (__typeof__((rp).relptr_type)) ((rp).relptr_off == 0 ? NULL : \
-		(base + (rp).relptr_off)))
+		(base) + (rp).relptr_off - 1))
 #else
 /*
  * If we don't have __builtin_types_compatible_p, assume we might not have
@@ -50,12 +50,15 @@
  */
 #define relptr_access(base, rp) \
 	(AssertVariableIsOfTypeMacro(base, char *), \
-	 (void *) ((rp).relptr_off == 0 ? NULL : (base + (rp).relptr_off)))
+	 (void *) ((rp).relptr_off == 0 ? NULL : (base) + (rp).relptr_off - 1))
 #endif
 
 #define relptr_is_null(rp) \
 	((rp).relptr_off == 0)
 
+#define relptr_offset(rp) \
+	((rp).relptr_off - 1)
+
 /* We use this inline to avoid double eval of "val" in relptr_store */
 static inline Size
 relptr_store_eval(char *base, char *val)
@@ -64,8 +67,8 @@ relptr_store_eval(char *base, char *val)
 		return 0;
 	else
 	{
-		Assert(val > base);
-		return val - base;
+		Assert(val >= base);
+		return val - base + 1;
 	}
 }
 
@@ -73,7 +76,7 @@ relptr_store_eval(char *base, char *val)
 #define relptr_store(base, rp, val) \
 	(AssertVariableIsOfTypeMacro(base, char *), \
 	 AssertVariableIsOfTypeMacro(val, __typeof__((rp).relptr_type)), \
-	 (rp).relptr_off = relptr_store_eval(base, (char *) (val)))
+	 (rp).relptr_off = relptr_store_eval((base), (char *) (val)))
 #else
 /*
  * If we don't have __builtin_types_compatible_p, assume we might not have
@@ -81,7 +84,7 @@ relptr_store_eval(char *base, char *val)
  */
 #define relptr_store(base, rp, val) \
 	(AssertVariableIsOfTypeMacro(base, char *), \
-	 (rp).relptr_off = relptr_store_eval(base, (char *) (val)))
+	 (rp).relptr_off = relptr_store_eval((base), (char *) (val)))
 #endif
 
 #define relptr_copy(rp1, rp2) \