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 <[email protected]>
Reviewed-by: Robert Haas <[email protected]>
Discussion: https://postgr.es/m/20220519193839.GT19626%40telsasoft.com

diff --git a/src/backend/utils/mmgr/freepage.c b/src/backend/utils/mmgr/freepage.c
index b26a295a4ead5e308ea215b9c68635c39a9178fc..dcf246faf111097e3c6054ca8f8e30738e450c80 100644 (file)
--- a/src/backend/utils/mmgr/freepage.c
+++ b/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);
    }
 }

diff --git a/src/include/utils/freepage.h b/src/include/utils/freepage.h
index 08064b10f9756867f7c4dc22f97f1c69e129b6e7..e69b2804ec146569b553a7be965e9a6d7f3d99db 100644 (file)
--- a/src/include/utils/freepage.h
+++ b/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) \

diff --git a/src/include/utils/relptr.h b/src/include/utils/relptr.h
index cc80a7200de144b544009bb8467b5c1e3a7802d5..9364dd6694838f4e1af5ad74555f90f78d1e9e33 100644 (file)
--- a/src/include/utils/relptr.h
+++ b/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) \