cli: fix the STDIN pipe on Windows (#5045)

* cli: pipe stdin

* uggh, here is the actual change

* update cli unit tests

* add unit test

cli/lib/exec/spawn.js

@@ -135,12 +135,25 @@ module.exports = {
         child.on('close', resolve)
         child.on('error', reject)
 
-        child.stdin && child.stdin.pipe(process.stdin)
-        child.stdout && child.stdout.pipe(process.stdout)
+        // if stdio options is set to 'pipe', then
+        //   we should set up pipes:
+        //  process STDIN (read stream) => child STDIN (writeable)
+        //  child STDOUT => process STDOUT
+        //  child STDERR => process STDERR with additional filtering
+        if (child.stdin) {
+          debug('piping process STDIN into child STDIN')
+          process.stdin.pipe(child.stdin)
+        }
+
+        if (child.stdout) {
+          debug('piping child STDOUT to process STDOUT')
+          child.stdout.pipe(process.stdout)
+        }
 
         // if this is defined then we are manually piping for linux
         // to filter out the garbage
-        child.stderr &&
+        if (child.stderr) {
+          debug('piping child STDERR to process STDERR')
           child.stderr.on('data', (data) => {
             const str = data.toString()
 
@@ -158,6 +171,7 @@ module.exports = {
             // else pass it along!
             process.stderr.write(data)
           })
+        }
 
         // https://github.com/cypress-io/cypress/issues/1841
         // In some versions of node, it will throw on windows

cli/test/lib/exec/spawn_spec.js

@@ -39,7 +39,10 @@ describe('lib/exec/spawn', function () {
       },
     }
 
-    sinon.stub(process, 'stdin').value(new EE)
+    // process.stdin is both an event emitter and a readable stream
+    this.processStdin = new EE()
+    this.processStdin.pipe = sinon.stub().returns(undefined)
+    sinon.stub(process, 'stdin').value(this.processStdin)
     sinon.stub(cp, 'spawn').returns(this.spawnedProcess)
     sinon.stub(xvfb, 'start').resolves()
     sinon.stub(xvfb, 'stop').resolves()
@@ -292,6 +295,9 @@ describe('lib/exec/spawn', function () {
       return spawn.start()
       .then(() => {
         expect(cp.spawn.firstCall.args[2].stdio).to.deep.eq('pipe')
+        // parent process STDIN was piped to child process STDIN
+        expect(this.processStdin.pipe, 'process.stdin').to.have.been.calledOnce
+        .and.to.have.been.calledWith(this.spawnedProcess.stdin)
       })
     })