In order to compact tables we need at least two tables. Bail out early
from `reftable_stack_auto_compact()` in case we have less than two
tables.

In the original, `stack_table_sizes_for_compaction()` yields an array
that has the same length as the number of tables. This array is then
passed on to `suggest_compaction_segment()`, which returns an empty
segment in case we have less than two tables. The segment is then passed
to `segment_size()`, which will return `0` because both start and end of
the segment are `0`. And because we only call `stack_compact_range()` in
case we have a positive segment size we don't perform auto-compaction at
all. Consequently, this change does not result in a user-visible change
in behaviour when called with a single table.

But when called with no tables this protects us against a potential
out-of-memory error: `stack_table_sizes_for_compaction()` would try to
allocate a zero-byte object when there aren't any tables, and that may
lead to a `NULL` pointer on some platforms like NonStop which causes us
to bail out with an out-of-memory error.

Signed-off-by: Patrick Steinhardt <redacted>
---
 reftable/stack.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/reftable/stack.c b/reftable/stack.c
index 59fd695a12c2033ed589a21ef1c9155eeecc4641..6ca21965d8e1135d986043113d465abd14cd532c 100644
--- a/reftable/stack.c
+++ b/reftable/stack.c

@@ -1627,6 +1627,9 @@ int reftable_stack_auto_compact(struct reftable_stack *st)
 	struct segment seg;
 	uint64_t *sizes;
 
+	if (st->merged->readers_len < 2)
+		return 0;
+
 	sizes = stack_table_sizes_for_compaction(st);
 	if (!sizes)
 		return REFTABLE_OUT_OF_MEMORY_ERROR;

-- 
2.48.0.rc0.184.g0fc57dec57.dirty