QProcess::startDetached: set the error condition on failure to start

And set *pid to -1.

[ChangeLog][QtCore][QProcess] If a startDetached() fails to start the
target application, the QProcess object should now have a proper error
string in errorString().

Change-Id: Ic90d8429a0eb4837971dfffd1664e825ffcb923e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
(cherry picked from commit 73a04edce151b21ea5d07494603c0317716e99d9)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qprocess.cpp

@@ -2132,11 +2132,12 @@ void QProcess::startCommand(const QString &command, OpenMode mode)
     temporarily freeze.
 
     If the function is successful then *\a pid is set to the process identifier
-    of the started process. Note that the child process may exit and the PID
+    of the started process; otherwise, it's set to -1. Note that the child
-    may become invalid without notice. Furthermore, after the child process
+    process may exit and the PID may become invalid without notice.
-    exits, the same PID may be recycled and used by a completely different
+    Furthermore, after the child process exits, the same PID may be recycled
-    process. User code should be careful when using this variable, especially
+    and used by a completely different process. User code should be careful
-    if one intends to forcibly terminate the process by operating system means.
+    when using this variable, especially if one intends to forcibly terminate
+    the process by operating system means.
 
     Only the following property setters are supported by startDetached():
     \list

src/corelib/io/qprocess_unix.cpp

@@ -1,7 +1,8 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2020 The Qt Company Ltd.
-** Copyright (C) 2016 Intel Corporation.
+** Copyright (C) 2021 Intel Corporation.
+** Copyright (C) 2021 Alex Trotsenko.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -883,17 +884,21 @@ bool QProcessPrivate::startDetached(qint64 *pid)
 
     // To catch the startup of the child
     int startedPipe[2];
-    if (qt_safe_pipe(startedPipe) != 0)
+    if (qt_safe_pipe(startedPipe) != 0) {
+        setErrorAndEmit(QProcess::FailedToStart, QLatin1String("pipe: ") + qt_error_string(errno));
         return false;
+    }
     // To communicate the pid of the child
     int pidPipe[2];
     if (qt_safe_pipe(pidPipe) != 0) {
+        setErrorAndEmit(QProcess::FailedToStart, QLatin1String("pipe: ") + qt_error_string(errno));
         qt_safe_close(startedPipe[0]);
         qt_safe_close(startedPipe[1]);
         return false;
     }
 
     if (!openChannelsForDetached()) {
+        // openChannel sets the error string
         closeChannel(&stdinChannel);
         closeChannel(&stdoutChannel);
         closeChannel(&stderrChannel);
@@ -981,6 +986,7 @@ bool QProcessPrivate::startDetached(qint64 *pid)
         ::_exit(1);
     }
 
+    int savedErrno = errno;
     closeChannel(&stdinChannel);
     closeChannel(&stdoutChannel);
     closeChannel(&stderrChannel);
@@ -990,6 +996,7 @@ bool QProcessPrivate::startDetached(qint64 *pid)
     if (childPid == -1) {
         qt_safe_close(startedPipe[0]);
         qt_safe_close(pidPipe[0]);
+        setErrorAndEmit(QProcess::FailedToStart, QLatin1String("fork: ") + qt_error_string(savedErrno));
         return false;
     }
 
@@ -998,14 +1005,17 @@ bool QProcessPrivate::startDetached(qint64 *pid)
     int result;
     qt_safe_close(startedPipe[0]);
     qt_safe_waitpid(childPid, &result, 0);
+
     bool success = (startResult != -1 && reply == '\0');
     if (success && pid) {
-        pid_t actualPid = 0;
+        pid_t actualPid;
-        if (qt_safe_read(pidPipe[0], (char *)&actualPid, sizeof(pid_t)) == sizeof(pid_t)) {
+        if (qt_safe_read(pidPipe[0], &actualPid, sizeof(pid_t)) != sizeof(pid_t))
-            *pid = actualPid;
+            actualPid = 0;
-        } else {
+        *pid = actualPid;
-            *pid = 0;
+    } else if (!success) {
-        }
+        if (pid)
+            *pid = -1;
+        setErrorAndEmit(QProcess::FailedToStart);
     }
     qt_safe_close(pidPipe[0]);
     return success;

src/corelib/io/qprocess_win.cpp

@@ -869,6 +869,7 @@ bool QProcessPrivate::startDetached(qint64 *pid)
     static const DWORD errorElevationRequired = 740;
 
     if (!openChannelsForDetached()) {
+        // openChannel sets the error string
         closeChannel(&stdinChannel);
         closeChannel(&stdoutChannel);
         closeChannel(&stderrChannel);
@@ -913,6 +914,11 @@ bool QProcessPrivate::startDetached(qint64 *pid)
         success = startDetachedUacPrompt(program, arguments, nativeArguments,
                                          workingDirectory, pid);
     }
+    if (!success) {
+        if (pid)
+            *pid = -1;
+        setErrorAndEmit(QProcess::FailedToStart);
+    }
 
     closeChannel(&stdinChannel);
     closeChannel(&stdoutChannel);

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

@@ -2279,6 +2279,8 @@ void tst_QProcess::detachedSetNonExistentWorkingDirectory()
     qint64 pid = -1;
     QVERIFY(!process.startDetached(&pid));
     QCOMPARE(pid, -1);
+    QCOMPARE(process.error(), QProcess::FailedToStart);
+    QVERIFY(process.errorString() != "Unknown error");
 }
 
 void tst_QProcess::startFinishStartFinish()