After removal, the next_sibling pointer of a node was sometimes incorrectly
left to point to another node in the heap, which meant that a node was
sometimes linked twice into the heap. Surprisingly that didn't cause any
crashes in my testing, but it was clearly wrong and could easily segfault
in other scenarios.
Also always keep the prev_or_parent pointer as NULL on the root node. That
was not a correctness issue AFAICS, but let's be tidy.
Add a debugging function, to dump the contents of a pairing heap as a
string. It's #ifdef'd out, as it's not used for anything in any normal
code, but it was highly useful in debugging this. Let's keep it handy for
further reference.
Currently, a backend will reset it's PGXACT->xmin value when it doesn't
have any registered snapshots left. That covered the common case that a
transaction in read committed mode runs several queries, one after each
other, as there would be no snapshots active between those queries.
However, if you hold cursors across each of the query, we didn't get a
chance to reset xmin.
To make that better, keep all the registered snapshots in a pairing heap,
ordered by xmin so that it's always quick to find the snapshot with the
smallest xmin. That allows us to advance PGXACT->xmin whenever the oldest
snapshot is deregistered, even if there are others still active.
Per discussion originally started by Jeff Davis back in 2009 and more
recently by Robert Haas.
This performs slightly better, uses less memory, and needs slightly less
code in GiST, than the Red-Black tree previously used.
Reviewed by Peter Geoghegan
