fix(index): ignore index removal races when inserting

lib/entry-index.js

@@ -30,16 +30,16 @@ module.exports.insert = insert
 function insert (cache, key, digest, opts) {
   opts = opts || {}
   const bucket = bucketPath(cache, key)
+  const entry = {
+    key: key,
+    digest: digest,
+    hashAlgorithm: opts.hashAlgorithm || 'sha512',
+    time: +(new Date()),
+    metadata: opts.metadata
+  }
   return fixOwner.mkdirfix(
     path.dirname(bucket), opts.uid, opts.gid
   ).then(() => {
-    const entry = {
-      key: key,
-      digest: digest,
-      hashAlgorithm: opts.hashAlgorithm || 'sha512',
-      time: +(new Date()),
-      metadata: opts.metadata
-    }
     const stringified = JSON.stringify(entry)
     // NOTE - Cleverness ahoy!
     //
@@ -50,12 +50,18 @@ function insert (cache, key, digest, opts) {
     // Thanks to @isaacs for the whiteboarding session that ended up with this.
     return appendFileAsync(
       bucket, `\n${hashEntry(stringified)}\t${stringified}`
-    ).then(() => entry)
-  }).then(entry => (
-    fixOwner.chownr(bucket, opts.uid, opts.gid).then(() => (
-      formatEntry(cache, entry)
-    ))
-  ))
+    )
+  }).then(
+    () => fixOwner.chownr(bucket, opts.uid, opts.gid)
+  ).catch({code: 'ENOENT'}, () => {
+    // There's a class of race conditions that happen when things get deleted
+    // during fixOwner, or between the two mkdirfix/chownr calls.
+    //
+    // It's perfectly fine to just not bother in those cases and lie
+    // that the index entry was written. Because it's a cache.
+  }).then(() => {
+    return formatEntry(cache, entry)
+  })
 }
 
 module.exports.find = find