Breaking: parent db must support deferredOpen

DeferredOpen means that the db opens itself and defers operations
until it's open. Currently that's only supported by level(up) and
friends. Before, subleveldown would also accept abstract-leveldown
db's that were not wrapped in levelup.

Opening and closing a sublevel no longer opens or closes the parent
db. The sublevel does wait for the parent to open (which in the
case of levelup already happens automatically) but never initiates
a state change.

Drops support of old modules:

- memdb (use level-mem instead)
- deferred-leveldown < 2.0.0 (and thus levelup < 2.0.0)
- abstract-leveldown < 2.4.0

Closes #84, #83 and #60.

leveldown.js

@@ -64,12 +64,36 @@ function SubDown (db, prefix, opts) {
   })
 
   this.db = db
-  this.leveldown = null
-  this.ownPrefix = separator + prefix + separator
-  this.prefix = this.ownPrefix
+  this.prefix = separator + prefix + separator
   this._beforeOpen = opts.open
 
   var self = this
+  var manifest = db.supports || {}
+
+  // The parent db must open itself or be (re)opened by the user because a
+  // sublevel can't (shouldn't) initiate state changes on the rest of the db.
+  if (!manifest.deferredOpen && !reachdown.is(db, 'levelup')) {
+    throw new Error('The parent db must support deferredOpen')
+  }
+
+  var subdb = reachdown(db, 'subleveldown')
+
+  if (subdb) {
+    // Old subleveldown doesn't know its prefix and leveldown until opened
+    if (!subdb.prefix || !subdb.leveldown) {
+      throw new Error('Incompatible with subleveldown < 5.0.0')
+    }
+
+    this.prefix = subdb.prefix + this.prefix
+    this.leveldown = subdb.leveldown
+  } else {
+    this.leveldown = reachdown(db, matchdown, false)
+  }
+
+  // Old deferred-leveldown doesn't expose its underlying db until opened
+  if (reachdown.is(this.leveldown, 'deferred-leveldown')) {
+    throw new Error('Incompatible with deferred-leveldown < 2.0.0')
+  }
 
   this._wrap = {
     gt: function (x) {
@@ -91,28 +115,29 @@ inherits(SubDown, abstract.AbstractLevelDOWN)
 
 SubDown.prototype.type = 'subleveldown'
 
+// TODO: remove _open() once abstract-leveldown supports deferredOpen,
+// because that means we can always do operations on this.leveldown.
 SubDown.prototype._open = function (opts, cb) {
   var self = this
 
-  this.db.open(function (err) {
-    if (err) return cb(err)
+  // TODO: make _isOpening public in levelup or add a method like
+  // ready(cb) which waits for - but does not initiate - a state change.
+  var m = typeof this.db.isOpening === 'function' ? 'isOpening' : '_isOpening'
 
-    var subdb = reachdown(self.db, 'subleveldown')
+  if (this.db[m]()) {
+    this.db.once('open', onopen)
+  } else {
+    this._nextTick(onopen)
+  }
 
-    if (subdb && subdb.prefix) {
-      self.prefix = subdb.prefix + self.ownPrefix
-      self.leveldown = subdb.leveldown
-    } else {
-      self.leveldown = reachdown(self.db, matchdown, false)
-    }
+  function onopen () {
+    if (!self.db.isOpen()) return cb(new Error('Parent db is not open'))
 
-    if (self._beforeOpen) self._beforeOpen(cb)
-    else cb()
-  })
-}
+    // TODO: add hooks to abstract-leveldown
+    if (self._beforeOpen) return self._beforeOpen(cb)
 
-SubDown.prototype._close = function (cb) {
-  this.leveldown.close(cb)
+    cb()
+  }
 }
 
 SubDown.prototype._serializeKey = function (key) {

matchdown.js

@@ -3,6 +3,7 @@ module.exports = function matchdown (db, type) {
   if (type === 'levelup') return false
   if (type === 'encoding-down') return false
   if (type === 'deferred-leveldown') return false
+  if (type === 'subleveldown') return false
 
   return true
 }

package.json

@@ -16,12 +16,12 @@
     "test": "test"
   },
   "dependencies": {
-    "abstract-leveldown": "^6.1.1",
+    "abstract-leveldown": "^6.2.3",
     "encoding-down": "^6.2.0",
     "inherits": "^2.0.3",
     "level-option-wrap": "^1.1.0",
     "levelup": "^4.3.1",
-    "reachdown": "^1.0.0"
+    "reachdown": "github:vweevers/reachdown"
   },
   "devDependencies": {
     "after": "^0.8.2",
@@ -31,7 +31,6 @@
     "hallmark": "^2.0.0",
     "level-community": "^3.0.0",
     "level-concat-iterator": "^2.0.1",
-    "memdb": "^1.3.1",
     "memdown": "^5.0.0",
     "nyc": "^14.0.0",
     "standard": "^14.0.0",