From 73d5a1c55b9c3f0166623d7a89b7fb1a73160c42 Mon Sep 17 00:00:00 2001 From: Robert Haas Date: Thu, 16 Apr 2020 12:15:57 -0400 Subject: [PATCH v2 3/6] Fix bugs in MaintainOldSnapshotTimeMapping. --- src/backend/utils/time/snapmgr.c | 30 +++++++++++++++++++++++++++--- 1 file changed, 27 insertions(+), 3 deletions(-) diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index abaaea569a..72b2c61a07 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -1926,10 +1926,32 @@ MaintainOldSnapshotTimeMapping(TimestampTz whenTaken, TransactionId xmin) else { /* We need a new bucket, but it might not be the very next one. */ - int advance = ((ts - oldSnapshotControl->head_timestamp) - / USECS_PER_MINUTE); + int distance_to_new_tail; + int distance_to_current_tail; + int advance; - oldSnapshotControl->head_timestamp = ts; + /* + * Our goal is for the new "tail" of the mapping, that is, the entry + * which is newest and thus furthest from the "head" entry, to + * correspond to "ts". Since there's one entry per minute, the + * distance between the current head and the new tail is just the + * number of minutes of difference between ts and the current + * head_timestamp. + * + * The distance from the current head to the current tail is one + * less than the number of entries in the mapping, because the + * entry at the head_offset is for 0 minutes after head_timestamp. + * + * The difference between these two values is the number of minutes + * by which we need to advance the mapping, either adding new entries + * or rotating old ones out. + */ + distance_to_new_tail = + (ts - oldSnapshotControl->head_timestamp) / USECS_PER_MINUTE; + distance_to_current_tail = + oldSnapshotControl->count_used - 1; + advance = distance_to_new_tail - distance_to_current_tail; + Assert(advance > 0); if (advance >= OLD_SNAPSHOT_TIME_MAP_ENTRIES) { @@ -1937,6 +1959,7 @@ MaintainOldSnapshotTimeMapping(TimestampTz whenTaken, TransactionId xmin) oldSnapshotControl->head_offset = 0; oldSnapshotControl->count_used = 1; oldSnapshotControl->xid_by_minute[0] = xmin; + oldSnapshotControl->head_timestamp = ts; } else { @@ -1955,6 +1978,7 @@ MaintainOldSnapshotTimeMapping(TimestampTz whenTaken, TransactionId xmin) else oldSnapshotControl->head_offset = old_head + 1; oldSnapshotControl->xid_by_minute[old_head] = xmin; + oldSnapshotControl->head_timestamp += USECS_PER_MINUTE; } else { -- 2.20.1