The `ErrFileNotFound` was used for for three cases:
1. There is no any WAL files (probably due to no read permission);
2. There is no WAL files matching the snapshot index;
3. The WAL file seqs do not increase continuously.
It's not good for debug when users see the `ErrFileNotFound` error,
so in this PR, a different error is returned for each case above.
Signed-off-by: Benjamin Wang <wachao@vmware.com>
Currently the max size of each WAL entry is hard coded as 10MB. If users
set a value > 10MB for the flag --max-request-bytes, then etcd may run
into a situation that it successfully processes a big request, but fails
to decode it when replaying the WAL file on startup.
On the other hand, we can't just remove the limitation, because if a
WAL entry is somehow corrupted, and its recByte is a huge value, then
etcd may run out of memory. So the solution is to restrict the max size
of each WAL entry as a dynamic value, which is the remaining size of
the WAL file.
Signed-off-by: Benjamin Wang <wachao@vmware.com>
