[PATCH 1/4] reftable/stack: don't perform auto-compaction with less than two tables

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.

This is mostly defense in depth: `stack_table_sizes_for_compaction()`
may try to allocate a zero-byte object when there aren't any readers,
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