vinyl compaction may delete files needed for backup

[locker]

Branch 1.9, commit 8ed45ce.

The following script demonstrates the problem:

log = require('log')
fio = require('fio')
fiber = require('fiber')

box.cfg{}

memtx = box.schema.space.create('memtx', {engine = 'memtx'})
memtx:create_index('pk')
vinyl = box.schema.space.create('vinyl', {engine = 'vinyl'})
vinyl:create_index('pk')
pad = string.rep('x', 1024)
for i = 1, 1000 do
    vinyl:delete{i}
    vinyl:replace{i + 1}
    memtx:delete{i}
    memtx:replace{i + 1}
    box.snapshot()
    files = box.backup.start()
    for _, f in pairs(files) do
        if not fio.path.exists(f) then
            log.error('File %s not found!', f)
            os.exit(1)
        end
    end
    box.backup.stop()
end
os.exit(0)

(If you don't get a failure, try running it on tmpfs)

Looks like the bug was introduced by commit 35db70f.