feat: allow for truncating files

License: MIT
Signed-off-by: achingbrain <alex@achingbrain.net>

src/core/write/index.js

@@ -119,7 +119,7 @@ module.exports = function mfsWrite (ipfs) {
       return callback(new Error('cannot have negative byte count'))
     }
 
-    if (options.length === 0) {
+    if (options.length === 0 && !options.truncate) {
       return callback()
     }
 

src/core/write/truncate-node.js

@@ -0,0 +1,43 @@
+'use strict'
+
+const exporter = require('ipfs-unixfs-engine').exporter
+const importer = require('ipfs-unixfs-engine').importer
+const pull = require('pull-stream/pull')
+const values = require('pull-stream/sources/values')
+const asyncMap = require('pull-stream/throughs/async-map')
+const collect = require('pull-stream/sinks/collect')
+const log = require('debug')('mfs:write:truncate-node')
+const {
+  loadNode
+} = require('../utils')
+
+const truncateNode = (ipfs, dagNode, newLength, options, callback) => {
+  log(`Truncating ${dagNode.multihash} to ${newLength} bytes`)
+
+  pull(
+    exporter(dagNode.multihash, ipfs._ipld, {
+      offset: 0,
+      length: newLength
+    }),
+    asyncMap((file, cb) => {
+      pull(
+        values([{
+          content: file.content
+        }]),
+        importer(ipfs._ipld, {
+          progress: options.progress,
+          hashAlg: options.hash,
+          cidVersion: options.cidVersion,
+          strategy: options.strategy
+        }),
+        collect(cb)
+      )
+    }),
+    asyncMap((imported, cb) => loadNode(ipfs, imported[0], cb)),
+    collect((error, results) => {
+      callback(error, results.pop())
+    })
+  )
+}
+
+module.exports = truncateNode

src/core/write/update-node.js

@@ -25,6 +25,7 @@ const {
 } = require('../utils')
 const importNode = require('./import-node')
 const updateNodeBytes = require('./update-tree')
+const truncateNode = require('./truncate-node')
 
 const updateNode = (ipfs, cidToUpdate, source, options, callback) => {
   let offset = options.offset || 0
@@ -159,6 +160,13 @@ const updateNode = (ipfs, cidToUpdate, source, options, callback) => {
           }
 
           next(null, updatedNode)
+        },
+        (updatedNode, cb) => {
+          if (options.truncate) {
+            return truncateNode(ipfs, updatedNode, streamEnd, options, cb)
+          }
+
+          cb(null, updatedNode)
         }
       ], done)
     }

test/write.spec.js

@@ -369,7 +369,7 @@ describe('write', function () {
   })
 
   runTest(({type, path, content}) => {
-    it.skip(`truncates a file when requested (${type})`, () => {
+    it(`truncates a file after writing (${type})`, () => {
       const newContent = Buffer.from('Oh hai!')
 
       return mfs.write(path, content, {