stream: check done before backpressure in stream reader

Signed-off-by: Daijiro Wachi <daijiro.wachi@gmail.com>
PR-URL: #63699
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: watilde

lib/internal/streams/compose.js

@@ -204,12 +204,12 @@ module.exports = function compose(...streams) {
 try{
 const{ value, done }=awaitreader.read();
 
-if(!d.push(value)){
+if(done){
+d.push(null);
 return;
 }
 
-if(done){
-d.push(null);
+if(!d.push(value)){
 return;
 }
 }catch{

test/parallel/test-stream-compose.js

@@ -537,3 +537,24 @@ const assert = require('assert');
 assert.strictEqual(duplex.destroyed,true);
 
 }
+
+// Regression test: compose with a web TransformStream tail must always emit
+// null (EOF) when the source finishes. The done check must precede the
+// backpressure check in the reader.read() loop; otherwise push(null) can be
+// skipped if canPushMore() returns false on the final done:true read.
+{
+const{ TransformStream }=globalThis;
+const{ Readable }=require('stream');
+
+// A web TransformStream as the tail exercises the isWebStream code path
+// in compose that loops over reader.read() results.
+constts=newTransformStream();
+constsrc=Readable.from(['hello',' ','world']);
+constcomposed=compose(src,ts);
+
+letresult='';
+composed.on('data',(chunk)=>{result+=Buffer.from(chunk).toString();});
+composed.on('end',common.mustCall(()=>{
+assert.strictEqual(result,'hello world');
+}));
+}