use sizeDiff=1 to represent holes filled

Author: staltz

compaction.js

@@ -134,6 +134,7 @@ function Compaction(log, onDone) {
   const progress = Obv() // for the unshifted offset
   let startOffset = 0
   let version = 0
+  let holesFound = true // assume true
 
   let compactedBlockIndex = -1
   let compactedBlockBuf = null
@@ -170,13 +171,11 @@ function Compaction(log, onDone) {
       if (state.initial) {
         findStateFromLog(function foundStateFromLog(err, state) {
           if (err) return cb(err)
-          startOffset = state.compactedBlockIndex * log.blockSize
           compactedBlockIndex = state.compactedBlockIndex
+          startOffset = compactedBlockIndex * log.blockSize
           unshiftedOffset = state.unshiftedOffset
           unshiftedBlockBuf = state.unshiftedBlockBuf
-          unshiftedBlockIndex = Math.floor(
-            state.unshiftedOffset / log.blockSize
-          )
+          unshiftedBlockIndex = Math.floor(unshiftedOffset / log.blockSize)
           savePersistentState(cb)
         })
       } else {
@@ -212,6 +211,7 @@ function Compaction(log, onDone) {
       if (err) return cb(err)
       if (holeOffset === -1) {
         compactedBlockIndex = Math.floor(log.since.value / log.blockSize)
+        holesFound = false
         stop()
         return
       }
@@ -414,7 +414,13 @@ function Compaction(log, onDone) {
       if (err) return onDone(err)
       persistentState.destroy(function onStateDestroyed(err) {
         if (err) return onDone(err)
-        onDone(null, sizeDiff)
+        if (sizeDiff === 0 && holesFound) {
+          // Truncation did not make the log smaller but it did rewrite the log.
+          // So report 1 byte as a way of saying that compaction filled holes.
+          onDone(null, { sizeDiff: 1 })
+        } else {
+          onDone(null, { sizeDiff })
+        }
       })
     })
   }

index.js

@@ -485,10 +485,10 @@ module.exports = function AsyncAppendOnlyLog(filename, opts) {
     }
     onDrain(function startCompactAfterDrain() {
       onDeletesFlushed(function startCompactAfterDeletes() {
-        compaction = new Compaction(self, (err, sizeDiff) => {
+        compaction = new Compaction(self, (err, stats) => {
           compaction = null
           if (err) return cb(err)
-          compactionProgress.set({ sizeDiff, percent: 1, done: true })
+          compactionProgress.set({ ...stats, percent: 1, done: true })
           for (const callback of waitingCompaction) callback()
           waitingCompaction.length = 0
           cb()

test/compaction.js

@@ -26,10 +26,32 @@ tape('compact a log that does not have holes', async (t) => {
   await run(log.onDrain)()
   t.pass('append two records')
 
+  const progressArr = []
+  log.compactionProgress((stats) => {
+    progressArr.push(stats)
+  })
+
   const [err] = await run(log.compact)()
   await run(log.onDrain)()
   t.error(err, 'no error when compacting')
 
+  t.deepEquals(
+    progressArr,
+    [
+      {
+        sizeDiff: 0,
+        percent: 1,
+        done: true,
+      },
+      {
+        sizeDiff: 0,
+        percent: 1,
+        done: true,
+      },
+    ],
+    'progress events'
+  )
+
   await new Promise((resolve) => {
     log.stream({ offsets: false }).pipe(
       push.collect((err, ary) => {
@@ -519,7 +541,7 @@ tape('startOffset is correct', async (t) => {
         done: false,
       },
       {
-        sizeDiff: 0,
+        sizeDiff: 1,
         percent: 1,
         done: true,
       },
@@ -616,7 +638,7 @@ tape('recovers from crash just after persisting state', async (t) => {
         done: false,
       },
       {
-        sizeDiff: 0,
+        sizeDiff: 1,
         percent: 1,
         done: true,
       },
@@ -668,8 +690,8 @@ tape('recovers from crash just after persisting block', async (t) => {
   t.pass('suppose compaction was in progress: [0x22, 0x33] and [0x33, 0x44]')
 
   const version = [1, 0, 0, 0] // uint32LE
-  const startOffset = [0,0,0,0] // uint32LE
-  const truncateBlockIndex = [255, 255, 255, 255] //uint32LE
+  const startOffset = [0, 0, 0, 0] // uint32LE
+  const truncateBlockIndex = [255, 255, 255, 255] // uint32LE
   const compactingBlockIndex = [0, 0, 0, 0] // uint32LE
   const unshiftedOffset = [0, 0, 0, 0] // uint32LE
   const unshiftedBlock = [
@@ -739,7 +761,7 @@ tape('restarts from crash just before truncating log', async (t) => {
   t.pass('suppose compaction ready: [0x22, 0x44], [0x55, 0x66], [0x55, 0x66]')
 
   const version = [1, 0, 0, 0] // uint32LE
-  const startOffset = [0,0,0,0] // uint32LE
+  const startOffset = [0, 0, 0, 0] // uint32LE
   const truncateBlockIndex = [1, 0, 0, 0] //uint32LE
   const compactingBlockIndex = [0, 0, 0, 0] // uint32LE
   const unshiftedOffset = [0, 0, 0, 0] // uint32LE