Add option to disable prewrite hook

[vweevers]

Context

I'm working on a rewrite of level-ttl, not because I need it, but to make it use hooks and find out what gaps we in that API.

Problem

If a plugin like level-ttl uses the prewrite hook but also has a background job that writes to the same db, it will trigger its own hook function (as well as other hook functions). E.g.:

module.exports = function ttl (db) {
  // Imagine a sublevel to which we write expiry metadata
  const sublevel = ...

  db.hooks.prewrite.add(function (op, batch) {
    if (op.type === 'put') {
      const exp = Date.now() + op.ttl
      batch.add({ type: 'put', sublevel, key: op.key, value: exp })
    } else {
      batch.add({ type: 'del', sublevel, key: op.key })
    }
  })

  // Background job
  setInterval(function sweep () {
    // Imagine we're deleting an expired key
    db.del('foo').catch(..)
  }, 60e3)
}

Solution

db.del('foo', { prewrite: false })

As well as:

db.put('foo', 'bar', { prewrite: false })
db.batch([], { prewrite: false })
db.batch().put('foo', 'bar', { prewrite: false })
db.batch().del('foo', { prewrite: false })

[vweevers]

Only question is, if db is a sublevel, should it forward { prewrite: false } to its parent database? I think no, to keep it isolated.

[vweevers]

The prewrite hook is fast, so technically this could also be solved in userland. Like so:

const bypass = Symbol('bypass')

db.hooks.prewrite.add(function (op, batch) {
  if (op[bypass]) return
})

await db.del('foo', { [bypass]: true })

And adding a prewrite option wouldn't be a breaking change, so I'm removing this from the v2 milestone. Not a blocker.