replace backpressure mechanism

replace standard backpressure mechanism that involved queueing write
requests with pausing/resuming source stream

that seems to solve incomplete/unfinished part stream issues that
started appearing in node 4.4.5 and later

probably related to nodejs/node#7278
although not fixed in 4.4.7 as this issue is supposed to be

lib/Dicer.js

@@ -38,19 +38,23 @@ function Dicer(cfg) {
   this._firstWrite = true;
   this._inHeader = true;
   this._part = undefined;
-  this._cb = undefined;
   this._ignoreData = false;
   this._partOpts = (typeof cfg.partHwm === 'number'
                     ? { highWaterMark: cfg.partHwm }
                     : {});
-  this._pause = false;
-
   this._hparser = new HeaderParser(cfg);
   this._hparser.on('header', function(header) {
     self._inHeader = false;
     self._part.emit('header', header);
   });
 
+  this.on('pipe', function(source) {
+    this._source = source;
+  });
+
+  this.on('unpipe', function() {
+    this._source = undefined;
+  });
 }
 inherits(Dicer, WritableStream);
 
@@ -108,10 +112,7 @@ Dicer.prototype._write = function(data, encoding, cb) {
 
   this._bparser.push(data);
 
-  if (this._pause)
-    this._cb = cb;
-  else
-    cb();
+  cb();
 };
 
 Dicer.prototype.reset = function() {
@@ -174,7 +175,7 @@ Dicer.prototype._oninfo = function(isMatch, data, start, end) {
   if (!this._part) {
     this._part = new PartStream(this._partOpts);
     this._part._read = function(n) {
-      self._unpause();
+      self._sourceResume();
     };
     ev = this._isPreamble ? 'preamble' : 'part';
     if (this._events[ev])
@@ -190,7 +191,7 @@ Dicer.prototype._oninfo = function(isMatch, data, start, end) {
         shouldWriteMore = this._part.push(buf);
       shouldWriteMore = this._part.push(data.slice(start, end));
       if (!shouldWriteMore)
-        this._pause = true;
+        this._sourcePause();
     } else if (!this._isPreamble && this._inHeader) {
       if (buf)
         this._hparser.push(buf);
@@ -212,7 +213,9 @@ Dicer.prototype._oninfo = function(isMatch, data, start, end) {
             self.emit('finish');
             self._realFinish = false;
           } else {
-            self._unpause();
+            if (self._source) {
+              self._sourceResume();
+            }
           }
         }
       });
@@ -225,15 +228,15 @@ Dicer.prototype._oninfo = function(isMatch, data, start, end) {
   }
 };
 
-Dicer.prototype._unpause = function() {
-  if (!this._pause)
-    return;
+Dicer.prototype._sourcePause = function() {
+  if (this._source) {
+    this._source.pause();
+  }
+};
 
-  this._pause = false;
-  if (this._cb) {
-    var cb = this._cb;
-    this._cb = undefined;
-    cb();
+Dicer.prototype._sourceResume = function() {
+  if (this._source) {
+    this._source.resume();
   }
 };