lib: use Writable#_final for 'finish' tracking

Instead overriding `emit` method and using `this._realFinish` to detect
unexpected end of data - implement `_final` method and track the logic
there.

The only behavior change from the use of `_final` is that we would no
longer emit `finish` when the input data is terminated early. Instead we
would emit `error` as before and stop the processing. Note that this is
the behavior provided by `stream.Writable`, and it is thus conformant
with the specification.

Additionally, replace uses of private `_events` property with either
straight `emit()` with return value check, or `listenerCount()` in the
situations where there might be an overhead from the constructed event
arguments.

Replace `emit('error', error)` with `destroy(error)` calls as well, as
otherwise the behavior is undefined.

Fix: mscdex#26

lib/Dicer.js

@@ -48,44 +48,8 @@ class Dicer extends Writable {
       this._part.emit('header', header);
     });
     this._hparser.on('error', (err) => {
-      if (this._part && !this._ignoreData) {
-        this._part.emit('error', err);
-        this._part.push(null);
-      }
-    });
-  }
-
-  emit(ev) {
-    if (ev !== 'finish' || this._realFinish) {
-      Writable.prototype.emit.apply(this, arguments);
-      return;
-    }
-
-    if (this._finished)
-      return;
-
-    process.nextTick(() => {
-      this.emit('error', new Error('Unexpected end of multipart data'));
-
-      if (this._part && !this._ignoreData) {
-        const type = (this._isPreamble ? 'Preamble' : 'Part');
-        this._part.emit(
-          'error',
-          new Error(`${type} terminated early due to `
-                    + 'unexpected end of multipart data')
-        );
-        this._part.push(null);
-        process.nextTick(() => {
-          this._realFinish = true;
-          this.emit('finish');
-          this._realFinish = false;
-        });
-        return;
-      }
-
-      this._realFinish = true;
-      this.emit('finish');
-      this._realFinish = false;
+      if (this._part && !this._ignoreData)
+        this._part.destroy(err);
     });
   }
 
@@ -97,9 +61,7 @@ class Dicer extends Writable {
     if (this._headerFirst && this._isPreamble) {
       if (!this._part) {
         this._part = new PartStream(this._partOpts);
-        if (this._events.preamble)
-          this.emit('preamble', this._part);
-        else
+        if (!this.emit('preamble', this._part))
           ignore(this);
       }
       const r = this._hparser.push(data);
@@ -123,6 +85,34 @@ class Dicer extends Writable {
       cb();
   }
 
+  _final(cb) {
+    if (this._finished) {
+      if (this._parts !== 0) {
+        this._pause = true;
+        this._cb = cb;
+        return;
+      }
+
+      cb();
+      return;
+    }
+
+    if (this._part && !this._ignoreData) {
+      const type = (this._isPreamble ? 'Preamble' : 'Part');
+      this._part.destroy(
+        new Error(`${type} terminated early due to `
+                  + 'unexpected end of multipart data')
+      );
+      ignore(this);
+    }
+
+    // Node <= 12 compatibility, otherwise `part`'s 'error'/'end' have no chance
+    // to emit.
+    process.nextTick(() => {
+      cb(new Error('Unexpected end of multipart data'));
+    });
+  }
+
   reset() {
     this._part = undefined;
     this._bparser = undefined;
@@ -154,16 +144,14 @@ function onInfo(isMatch, data, start, end) {
       }
     }
     if (this._dashes === 2) {
-      if ((start + i) < end && this._events.trailer)
+      if ((start + i) < end && this.listenerCount('trailer'))
         this.emit('trailer', data.slice(start + i, end));
       this.reset();
       this._finished = true;
       // No more parts will be added
-      if (this._parts === 0) {
-        this._realFinish = true;
-        this.emit('finish');
-        this._realFinish = false;
-      }
+      if (this._parts === 0)
+        unpause(this);
+
     }
     if (this._dashes)
       return;
@@ -176,9 +164,7 @@ function onInfo(isMatch, data, start, end) {
       unpause(this);
     };
     ev = this._isPreamble ? 'preamble' : 'part';
-    if (this._events[ev])
-      this.emit(ev, this._part);
-    else
+    if (!this.emit(ev, this._part))
       ignore(this);
     if (!this._isPreamble)
       this._inHeader = true;
@@ -205,15 +191,8 @@ function onInfo(isMatch, data, start, end) {
     } else {
       ++this._parts;
       this._part.on('end', () => {
-        if (--this._parts === 0) {
-          if (this._finished) {
-            this._realFinish = true;
-            this.emit('finish');
-            this._realFinish = false;
-          } else {
-            unpause(this);
-          }
-        }
+        if (--this._parts === 0)
+          unpause(this);
       });
     }
     this._part.push(null);

test/test-multipart.js

@@ -84,6 +84,13 @@ function next() {
       header: undefined
     };
 
+    const onPreambleEnd = () => {
+      if (preamble.body)
+        preamble.body = Buffer.concat(preamble.body, preamble.bodylen);
+      if (preamble.body || preamble.header)
+        state.preamble = preamble;
+    };
+
     p.on('header', (h) => {
       preamble.header = h;
       if (v.setBoundary)
@@ -100,12 +107,8 @@ function next() {
       preamble.bodylen += data.length;
     }).on('error', (err) => {
       preamble.error = err;
-    }).on('end', () => {
-      if (preamble.body)
-        preamble.body = Buffer.concat(preamble.body, preamble.bodylen);
-      if (preamble.body || preamble.header)
-        state.preamble = preamble;
-    });
+      onPreambleEnd();
+    }).on('end', onPreambleEnd);
   });
   dicer.on('part', (p) => {
     const part = {
@@ -115,6 +118,12 @@ function next() {
       header: undefined
     };
 
+    const onPartEnd = () => {
+      if (part.body)
+        part.body = Buffer.concat(part.body, part.bodylen);
+      state.parts.push(part);
+    };
+
     p.on('header', (h) => {
       part.header = h;
     }).on('data', (data) => {
@@ -126,15 +135,23 @@ function next() {
     }).on('error', (err) => {
       part.error = err;
       ++partErrors;
-    }).on('end', () => {
-      if (part.body)
-        part.body = Buffer.concat(part.body, part.bodylen);
-      state.parts.push(part);
-    });
-  }).on('error', (err) => {
-    error = err;
-  }).on('finish', () => {
-    assert(finishes++ === 0, makeMsg(v.what, 'finish emitted multiple times'));
+      onPartEnd();
+    }).on('end', onPartEnd);
+  }).on('error', onFinish).on('finish', onFinish);
+
+  function onFinish(err) {
+    if (err) {
+      assert(
+        error === undefined,
+        makeMsg(v.what, 'error emitted multiple times')
+      );
+      error = err;
+    }
+
+    // Node <= 12 emits both 'error' and 'end', while Node > 14 emits only
+    // 'error'.
+    if (finishes++ > 0)
+      return;
 
     if (v.dicerError) {
       assert(error !== undefined, makeMsg(v.what, 'Expected error'));
@@ -242,7 +259,7 @@ function next() {
     }
     ++t;
     next();
-  });
+  }
 
   fs.createReadStream(fixtureBase + '/original').pipe(dicer);
 }

test/test-pipeline.js

@@ -0,0 +1,30 @@
+'use strict';
+
+const assert = require('assert');
+
+const { Readable, pipeline } = require('stream');
+const Dicer = require('..');
+
+const r = new Readable({ read() {} });
+const d = new Dicer({ boundary: 'a' });
+
+let isFinished = false;
+
+d.on('part', async (part) => {
+  part.resume();
+});
+
+r.push('--a\r\nA: 1\r\nB: 1\r\n\r\n123\r\n--a\r\n\r\n456\r\n--a--\r\n');
+setImmediate(() => {
+  r.push(null);
+});
+
+pipeline(r, d, (error) => {
+  assert(isFinished === false, 'Double-invocation of pipeline callback');
+  assert(error === undefined, 'Unexpected pipeline error');
+  isFinished = true;
+});
+
+process.on('exit', () => {
+  assert(isFinished === true, 'Should finish before exiting');
+});