feat(cache): add versioning to content and index

Fixes: #50

This will allow us to do automated migrations in the future
as the cache format changes.

BREAKING CHANGE: index/content directories are now versioned. Previous caches are no longer compatible and cannot be migrated.

lib/content/path.js

@@ -1,11 +1,21 @@
 'use strict'
 
+var contentVer = require('../../package.json')['cache-version'].content
 var path = require('path')
 
+// Current format of content file path:
+//
+// ~/.my-cache/content-v1/sha512/ba/bada55deadbeefc0ffee
+//
 module.exports = contentPath
 function contentPath (cache, address, hashAlgorithm) {
   address = address && address.toLowerCase()
   hashAlgorithm = hashAlgorithm ? hashAlgorithm.toLowerCase() : 'sha512'
   return path.join(
-    cache, 'content', hashAlgorithm, address.slice(0, 2), address)
+    cache,
+    `content-v${contentVer}`,
+    hashAlgorithm,
+    address.slice(0, 2),
+    address
+  )
 }

lib/entry-index.js

@@ -12,6 +12,8 @@ const Promise = require('bluebird')
 const split = require('split')
 const through = require('mississippi').through
 
+const indexV = require('../package.json')['cache-version'].index
+
 module.exports.insert = insert
 function insert (cache, key, digest, opts) {
   opts = opts || {}
@@ -105,7 +107,7 @@ function del (cache, key) {
 
 module.exports.lsStream = lsStream
 function lsStream (cache) {
-  const indexDir = path.join(cache, 'index')
+  const indexDir = bucketDir(cache)
   const stream = through.obj()
   fs.readdir(indexDir, function (err, buckets) {
     if (err && err.code === 'ENOENT') {
@@ -177,10 +179,14 @@ function notFoundError (cache, key) {
   return err
 }
 
+function bucketDir (cache) {
+  return path.join(cache, `index-v${indexV}`)
+}
+
 module.exports._bucketPath = bucketPath
 function bucketPath (cache, key) {
   const hashed = hashKey(key)
-  return path.join(cache, 'index', hashed.slice(0, 2), hashed)
+  return path.join(bucketDir(cache), hashed.slice(0, 2), hashed)
 }
 
 module.exports._hashKey = hashKey

package.json

@@ -1,6 +1,10 @@
 {
   "name": "cacache",
   "version": "5.0.3",
+  "cache-version": {
+    "content": "1",
+    "index": "1"
+  },
   "description": "General content-addressable cache system that maintains a filesystem registry of file data.",
   "main": "index.js",
   "files": [