WriteEntry backpressure

lib/write-entry.js

@@ -21,6 +21,8 @@ const OPENFILE = Symbol('openfile')
 const ONOPENFILE = Symbol('onopenfile')
 const CLOSE = Symbol('close')
 const MODE = Symbol('mode')
+const AWAITDRAIN = Symbol('awaitDrain')
+const ONDRAIN = Symbol('ondrain')
 const warner = require('./warn-mixin.js')
 const winchars = require('./winchars.js')
 const stripAbsolutePath = require('./strip-absolute-path.js')
@@ -232,7 +234,7 @@ const WriteEntry = warner(class WriteEntry extends MiniPass {
     this.pos = 0
     this.remain = this.stat.size
     this.length = this.buf.length
-    this[READ](this.stat.size)
+    this[READ]()
   }
 
   [READ] () {
@@ -284,13 +286,23 @@ const WriteEntry = warner(class WriteEntry extends MiniPass {
 
     const writeBuf = this.offset === 0 && bytesRead === this.buf.length ?
       this.buf : this.buf.slice(this.offset, this.offset + bytesRead)
-    this.remain -= bytesRead
-    this.blockRemain -= bytesRead
-    this.pos += bytesRead
-    this.offset += bytesRead
+    this.remain -= writeBuf.length
+    this.blockRemain -= writeBuf.length
+    this.pos += writeBuf.length
+    this.offset += writeBuf.length
+
+    const flushed = this.write(writeBuf)
+    if (!flushed)
+      this[AWAITDRAIN](() => this[ONDRAIN]())
+    else
+      this[ONDRAIN]()
+  }
 
-    this.write(writeBuf)
+  [AWAITDRAIN] (cb) {
+    this.once('drain', cb)
+  }
 
+  [ONDRAIN] () {
     if (!this.remain) {
       if (this.blockRemain)
         this.write(Buffer.alloc(this.blockRemain))
@@ -339,6 +351,10 @@ class WriteEntrySync extends WriteEntry {
     }
   }
 
+  [AWAITDRAIN] (cb) {
+    cb()
+  }
+
   [CLOSE] (cb) {
     fs.closeSync(this.fd)
     cb()

test/write-entry.js

@@ -254,6 +254,7 @@ t.test('zero-byte file, but close fails', t => {
     t.match(er, { message: 'poop' })
     t.end()
   })
+  ws.resume()
 })
 
 t.test('hardlinks', t => {
@@ -597,7 +598,7 @@ t.test('read overflow expectation', t => {
   t.throws(_ => new WriteEntry.Sync(f, { cwd: files, maxReadSize: 2 }), expect)
   new WriteEntry(f, { cwd: files, maxReadSize: 2 }).on('error', er => {
     t.match(er, expect)
-  })
+  }).resume()
 })
 
 t.test('short reads', t => {