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Limit returned lsn for timestamp by the planned gc cutoff (#10678)

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Often the output of the timestamp->lsn API is used as input for branch
creation, and branch creation takes the planned lsn into account, i.e.
rejects lsn's as branch lsns that are before the planned lsn.

This patch doesn't fix all race conditions, it's still racy. But at
least it is a step into the right direction.

For #10639
This commit is contained in:
Arpad Müller
2025-02-06 12:17:08 +01:00
committed by GitHub
parent f4cfa725b8
commit 67b71538d0
1 changed files with 8 additions and 1 deletions
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9
pageserver/src/pgdatadir_mapping.rs
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@@ -612,11 +612,18 @@ impl Timeline {
pausable_failpoint!("find-lsn-for-timestamp-pausable");
let gc_cutoff_lsn_guard = self.get_latest_gc_cutoff_lsn();
let gc_cutoff_planned = {
let gc_info = self.gc_info.read().unwrap();
gc_info.min_cutoff()
};
// Usually the planned cutoff is newer than the cutoff of the last gc run,
// but let's be defensive.
let gc_cutoff = gc_cutoff_planned.max(*gc_cutoff_lsn_guard);
// We use this method to figure out the branching LSN for the new branch, but the
// GC cutoff could be before the branching point and we cannot create a new branch
// with LSN < `ancestor_lsn`. Thus, pick the maximum of these two to be
// on the safe side.
let min_lsn = std::cmp::max(*gc_cutoff_lsn_guard, self.get_ancestor_lsn());
let min_lsn = std::cmp::max(gc_cutoff, self.get_ancestor_lsn());
let max_lsn = self.get_last_record_lsn();
// LSNs are always 8-byte aligned. low/mid/high represent the