[PATCH v2 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.

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 <ps@pks.im>
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 reftable/stack.c | 3 +++
 1 file changed, 3 insertions(+)