Prevent misuse of stale errno value in QSerialPort read notifier

Unix syscalls don't modify the value of errno unless they are returning
an error code. Thus it is important to check that value only after
receiving an erroenus syscall result. The readNotification() which
handles incoming serial port data should not treat read() returning
0 as an error - it is not an error and it doesn't update the errno.
Consequently, zero-length reads (ie. no data currently available)
would use the errno value set by any previous failed syscall. In some
cases it led to disabling the read notifier and complete stall of the
incoming data stream. Now the zero-length read is handled separately
which avoids that problem.

Pick-to: 6.7 6.6 6.5 6.2 5.15
Change-Id: I851aff0be7220f8a8f6418ddd40abbdf70d1382b
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/serialport/qserialport_unix.cpp b/src/serialport/qserialport_unix.cpp
index 1223f824..7e642e57 100644
--- a/src/serialport/qserialport_unix.cpp
+++ b/src/serialport/qserialport_unix.cpp
@@ -751,7 +751,7 @@ bool QSerialPortPrivate::readNotification()
 
     buffer.chop(bytesToRead - qMax(readBytes, qint64(0)));
 
-    if (readBytes <= 0) {
+    if (readBytes < 0) {
         QSerialPortErrorInfo error = getSystemError();
         if (error.errorCode != QSerialPort::ResourceError)
             error.errorCode = QSerialPort::ReadError;
@@ -759,6 +759,8 @@ bool QSerialPortPrivate::readNotification()
             setReadNotificationEnabled(false);
         setError(error);
         return false;
+    } else if (readBytes == 0) {
+        return false;
     }
 
     newBytes = buffer.size() - newBytes;