allow clearing an uncommited batch

index.js

@@ -813,8 +813,8 @@ module.exports = class Hypercore extends EventEmitter {
     return true
   }
 
-  batch ({ checkout = -1, autoClose = true, session = true, restore = false } = {}) {
-    return new Batch(session ? this.session() : this, checkout, autoClose, restore)
+  batch ({ checkout = -1, autoClose = true, session = true, restore = false, clear = false } = {}) {
+    return new Batch(session ? this.session() : this, checkout, autoClose, restore, clear)
   }
 
   async seek (bytes, opts) {

lib/batch.js

@@ -4,7 +4,7 @@ const c = require('compact-encoding')
 const b4a = require('b4a')
 
 module.exports = class HypercoreBatch extends EventEmitter {
-  constructor (session, checkoutLength, autoClose, restore) {
+  constructor (session, checkoutLength, autoClose, restore, clear) {
     super()
 
     this.session = session
@@ -25,8 +25,10 @@ module.exports = class HypercoreBatch extends EventEmitter {
     this._sessionByteLength = 0
     this._sessionBatch = null
     this._flushing = null
+    this._clear = clear
 
-    this.opening = this.ready().catch(noop)
+    this.opening = this._open()
+    this.opening.catch(noop)
   }
 
   get id () {
@@ -69,23 +71,27 @@ module.exports = class HypercoreBatch extends EventEmitter {
     return this.session.manifest
   }
 
-  async ready () {
+  ready () {
+    return this.opening
+  }
+
+  async _open () {
     await this.session.ready()
-    if (this.opened) return
+
+    if (this._clear) this._checkoutLength = this.core.tree.length
 
     if (this._checkoutLength !== -1) {
       const batch = await this.session.core.tree.restoreBatch(this._checkoutLength)
       batch.treeLength = this._checkoutLength
-      if (this.opened) return
       this._sessionLength = batch.length
       this._sessionByteLength = batch.byteLength
       this._sessionBatch = batch
+      if (this._clear) await this.core.clearBatch()
     } else {
       const last = this.restore ? this.session.core.bitfield.findFirst(false, this.session.length) : 0
 
       if (last > this.session.length) {
         const batch = await this.session.core.tree.restoreBatch(last)
-        if (this.opened) return
         this._sessionLength = batch.length
         this._sessionByteLength = batch.byteLength - this.session.padding * batch.length
         this._sessionBatch = batch

lib/core.js

@@ -440,6 +440,40 @@ module.exports = class Core {
     }
   }
 
+  async clearBatch () {
+    await this._mutex.lock()
+
+    try {
+      const len = this.bitfield.findFirst(false, this.tree.length)
+      if (len <= this.tree.length) return
+
+      const batch = await this.tree.truncate(this.tree.length, this.tree.fork)
+
+      batch.signature = this.tree.signature // same sig
+
+      const entry = {
+        userData: null,
+        treeNodes: batch.nodes,
+        treeUpgrade: batch,
+        bitfield: {
+          drop: true,
+          start: batch.ancestors,
+          length: len - batch.ancestors
+        }
+      }
+
+      await this.oplog.append([entry], false)
+
+      this.bitfield.setRange(batch.ancestors, len - batch.ancestors, false)
+      batch.commit()
+
+      // TODO: (see below todo)
+      await this._flushOplog()
+    } finally {
+      this._mutex.unlock()
+    }
+  }
+
   async clear (start, end, cleared) {
     await this._mutex.lock()
 

test/batch.js

@@ -608,3 +608,38 @@ test('persistent batch', async function (t) {
     sub.alike(downloaded.sort(), [6, 7], 'got non pending blocks')
   })
 })
+
+test('clear', async function (t) {
+  const core = await create()
+
+  await core.append('hello')
+
+  const clone = await create(core.key)
+
+  const b = clone.batch()
+
+  await b.append('hello')
+  await b.flush()
+  await b.close()
+
+  const [s1, s2] = replicate(core, clone, t)
+
+  await eventFlush()
+  t.ok(!!(await clone.get(0)), 'got block 0 proof')
+
+  s1.destroy()
+  s2.destroy()
+
+  const b1 = clone.batch()
+  await b1.ready()
+  await b1.append('foo')
+  await b1.flush()
+  await b1.close()
+
+  t.is(clone.length, 1, 'clone length is still 1')
+
+  const b2 = clone.batch({ clear: true })
+  await b2.ready()
+
+  t.is(b2.length, 1, 'reset the batch')
+})