tst_QProcess::startStopStartStopBuffers: depend less on OS configuration

The comment in this function made it clear that it really depended on
the size of the pipe buffer in the OS. I don't see a way to make a pipe
default to a different size on Linux -- it always defaults to
PIPE_DEF_BUFFERS (16) and that value is only increased as a result of
fcntl(F_SETPIPE_SZ), which we don't do. But we can be defensive and
simply write until the OS can't take any more data.

Drive-by update the comment on Windows to be clear that bytesToWrite()
does work, but only while the child process is still running.

Pick-to: 6.6
Task-number: QTBUG-80953
Change-Id: I9d43e5b91eb142d6945cfffd17866d22a4127e5e
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

tests/auto/corelib/io/qprocess/tst_qprocess.cpp

@@ -2889,14 +2889,18 @@ void tst_QProcess::startStopStartStopBuffers()
     }
 
     // We want to test that the write buffer still has bytes after the child
-    // exiting. We do that by writing to a child process that never reads. We
+    // exits. We can do that by writing data until the OS stops consuming data,
-    // just have to write more data than a pipe can hold, so that even if
+    // indicating that the pipe buffers are full. The initial value of 128 kB
-    // QProcess finds the pipe writable (during waitForFinished() or in the
+    // should make this loop typicall run only once; the worst case I know of
-    // QWindowsPipeWriter thread), some data will remain. The worst case I know
+    // is Linux, which defaults to 64 kB of buffer.
-    // of is Linux, which defaults to 64 kB of buffer.
 
-    process.write(QByteArray(128 * 1024, 'a'));
+    QByteArray chunk(128 * 1024, 'a');
+    do {
+        process.write(chunk);
         QVERIFY(process.bytesToWrite() > 0);
+        process.waitForBytesWritten(1);
+    } while (process.bytesToWrite() == 0);
+    chunk = {};
     process.kill();
 
     QVERIFY(process.waitForFinished());
@@ -2904,7 +2908,8 @@ void tst_QProcess::startStopStartStopBuffers()
 #ifndef Q_OS_WIN
     // confirm that our buffers are still full
     // Note: this doesn't work on Windows because our buffers are drained into
-    // QWindowsPipeWriter before being sent to the child process.
+    // QWindowsPipeWriter before being sent to the child process and are lost
+    // in waitForFinished() -> processFinished() -> cleanup().
     QVERIFY(process.bytesToWrite() > 0);
     QVERIFY(process.bytesAvailable() > 0); // channelMode1 is not ForwardedChannels
     if (channelMode1 == QProcess::SeparateChannels || channelMode1 == QProcess::ForwardedOutputChannel) {