Fix integer underflow in shared memory debugging

dsa_dump would print a large negative number instead of zero for
segment bin 0.  Fix by explicitly checking for underflow and add
special case for bin 0. Backpatch to all supported versions.

Author: Ian Ilyasov <ianilyasov@outlook.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/GV1P251MB1004E0D09D117D3CECF9256ECD502@GV1P251MB1004.EURP251.PROD.OUTLOOK.COM
Backpatch-through: v12

diff --git a/src/backend/utils/mmgr/dsa.c b/src/backend/utils/mmgr/dsa.c
index a6b728ba9ffb21e83a03dedab9dcdc6bf9143893..b9e7f224d517ee9a2f25f5bf58c39e05ee60a2c7 100644 (file)
--- a/src/backend/utils/mmgr/dsa.c
+++ b/src/backend/utils/mmgr/dsa.c
@@ -1113,9 +1113,13 @@ dsa_dump(dsa_area *area)
        {
            dsa_segment_index segment_index;
 
-           fprintf(stderr,
-                   "    segment bin %zu (at least %d contiguous pages free):\n",
-                   i, 1 << (i - 1));
+           if (i == 0)
+               fprintf(stderr,
+                       "    segment bin %zu (no contiguous free pages):\n", i);
+           else
+               fprintf(stderr,
+                       "    segment bin %zu (at least %d contiguous pages free):\n",
+                       i, 1 << (i - 1));
            segment_index = area->control->segment_bins[i];
            while (segment_index != DSA_SEGMENT_INDEX_NONE)
            {