QProcess/Unix: add a few, basic session & terminal management flags

Doing setsid() and disconnecting from the controlling terminal are, in
addition to resetting the standard file descriptors to /dev/null, a
common task that daemons do. These options allow a QProcess to force a
child to be a daemon.

QProcess ensures that the operations are done in the correct order.

Change-Id: I3e3bfef633af4130a03afffd175e9451d2716d7a
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/io/qprocess.cpp

@@ -849,6 +849,22 @@ void QProcessPrivate::Channel::clear()
            child. The \c stdin, \c stdout, and \c stderr file descriptors are
            never closed.
 
+    \value [since 6.7] CreateNewSession  Starts a new process session, by calling
+           \c{setsid(2)}. This allows the child process to outlive the session
+           the current process is in. This is one of the steps that
+           startDetached() takes to allow the process to detach, and is also one
+           of the steps to daemonize a process.
+
+    \value [since 6.7] DisconnectControllingTerminal   Requests that the process
+           disconnect from its controlling terminal, if it has one. If it has
+           none, nothing happens. Processes still connected to a controlling
+           terminal may get a Hang Up (\c SIGHUP) signal if the terminal
+           closes, or one of the other terminal-control signals (\c SIGTSTP, \c
+           SIGTTIN, \c SIGTTOU). Note that on some operating systems, a process
+           may only disconnect from the controlling terminal if it is the
+           session leader, meaning the \c CreateNewSession flag may be
+           required. Like it, this is one of the steps to daemonize a process.
+
     \value IgnoreSigPipe    Always sets the \c SIGPIPE signal to ignored
            (\c SIG_IGN), even if the \c ResetSignalHandlers flag was set. By
            default, if the child attempts to write to its standard output or

src/corelib/io/qprocess.h

@@ -182,6 +182,8 @@ public:
         // some room if we want to add IgnoreSigHup or so
         CloseFileDescriptors                = 0x0010,
         UseVFork                            = 0x0020, // like POSIX_SPAWN_USEVFORK
+        CreateNewSession                    = 0x0040, // like POSIX_SPAWN_SETSID
+        DisconnectControllingTerminal       = 0x0080,
     };
     Q_DECLARE_FLAGS(UnixProcessFlags, UnixProcessFlag)
     struct UnixProcessParameters

src/corelib/io/qprocess_unix.cpp

@@ -39,6 +39,9 @@
 #include <sys/resource.h>
 #include <unistd.h>
 
+#if __has_include(<paths.h>)
+#  include <paths.h>
+#endif
 #if __has_include(<linux/close_range.h>)
 // FreeBSD's is in <unistd.h>
 #  include <linux/close_range.h>
@@ -51,6 +54,12 @@
 #ifndef O_PATH
 #  define O_PATH        0
 #endif
+#ifndef _PATH_DEV
+#  define _PATH_DEV     "/dev/"
+#endif
+#ifndef _PATH_TTY
+#  define _PATH_TTY     _PATH_DEV "tty"
+#endif
 
 #ifdef Q_OS_FREEBSD
 __attribute__((weak))
@@ -774,7 +783,7 @@ void QProcess::failChildProcessModifier(const char *description, int error) noex
 }
 
 // See IMPORTANT notice below
-static void applyProcessParameters(const QProcess::UnixProcessParameters &params)
+static const char *applyProcessParameters(const QProcess::UnixProcessParameters &params)
 {
     // Apply Unix signal handler parameters.
     // We don't expect signal() to fail, so we ignore its return value
@@ -817,6 +826,29 @@ static void applyProcessParameters(const QProcess::UnixProcessParameters &params
                 close(fd);
         }
     }
+
+    // Apply session and process group settings. This may fail.
+    if (params.flags.testFlag(QProcess::UnixProcessFlag::CreateNewSession)) {
+        if (setsid() < 0)
+            return "setsid";
+    }
+
+    // Disconnect from the controlling TTY. This probably won't fail. Must be
+    // done after the session settings from above.
+    if (params.flags.testFlag(QProcess::UnixProcessFlag::DisconnectControllingTerminal)) {
+        if (int fd = open(_PATH_TTY, O_RDONLY | O_NOCTTY); fd >= 0) {
+            // we still have a controlling TTY; give it up
+            int r = ioctl(fd, TIOCNOTTY);
+            int savedErrno = errno;
+            close(fd);
+            if (r != 0) {
+                errno = savedErrno;
+                return "ioctl";
+            }
+        }
+    }
+
+    return nullptr;
 }
 
 // the noexcept here adds an extra layer of protection
@@ -857,7 +889,8 @@ void QChildProcess::startProcess() const noexcept
         callChildProcessModifier(d);
 
         // then we apply our other user-provided parameters
-        applyProcessParameters(d->unixExtras->processParameters);
+        if (const char *what = applyProcessParameters(d->unixExtras->processParameters))
+            failChildProcess(d, what, errno);
 
         auto flags = d->unixExtras->processParameters.flags;
         using P = QProcess::UnixProcessFlag;

tests/auto/corelib/io/qprocess/testUnixProcessParameters/main.cpp

@@ -80,6 +80,22 @@ int main(int argc, char **argv)
         return EXIT_SUCCESS;
     }
 
+    if (cmd == "noctty") {
+        int fd = open("/dev/tty", O_RDONLY);
+        if (fd == -1)
+            return EXIT_SUCCESS;
+        fprintf(stderr, "Could open /dev/tty\n");
+        return EXIT_FAILURE;
+    }
+
+    if (cmd == "setsid") {
+        pid_t pgid = getpgrp();
+        if (pgid == getpid())
+            return EXIT_SUCCESS;
+        fprintf(stderr, "Process group was %d\n", pgid);
+        return EXIT_FAILURE;
+    }
+
     fprintf(stderr, "Unknown command \"%s\"", cmd.data());
     return EXIT_FAILURE;
 }

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

@@ -126,6 +126,8 @@ private slots:
     void raiseInChildProcessModifier();
     void unixProcessParameters_data();
     void unixProcessParameters();
+    void impossibleUnixProcessParameters_data();
+    void impossibleUnixProcessParameters();
     void unixProcessParametersAndChildModifier();
     void unixProcessParametersOtherFileDescriptors();
 #endif
@@ -1730,6 +1732,10 @@ void tst_QProcess::unixProcessParameters_data()
     addRow("reset-sighand", P::ResetSignalHandlers);
     addRow("ignore-sigpipe", P::IgnoreSigPipe);
     addRow("file-descriptors", P::CloseFileDescriptors);
+    addRow("setsid", P::CreateNewSession);
+
+    // On FreeBSD, we need to be session leader to disconnect from the CTTY
+    addRow("noctty", P::DisconnectControllingTerminal | P::CreateNewSession);
 }
 
 void tst_QProcess::unixProcessParameters()
@@ -1778,6 +1784,13 @@ void tst_QProcess::unixProcessParameters()
         }
     } scope;
 
+    if (params.flags & QProcess::UnixProcessFlag::DisconnectControllingTerminal) {
+        if (int fd = open("/dev/tty", O_RDONLY); fd < 0) {
+            qInfo("Process has no controlling terminal; this test will do nothing");
+            close(fd);
+        }
+    }
+
     QProcess process;
     process.setUnixProcessParameters(params);
     process.setStandardInputFile(QProcess::nullDevice());   // so we can't mess with SIGPIPE
@@ -1798,6 +1811,31 @@ void tst_QProcess::unixProcessParameters()
     QCOMPARE(process.exitStatus(), QProcess::NormalExit);
 }
 
+void tst_QProcess::impossibleUnixProcessParameters_data()
+{
+    using P = QProcess::UnixProcessParameters;
+    QTest::addColumn<P>("params");
+    QTest::newRow("setsid") << P{ QProcess::UnixProcessFlag::CreateNewSession };
+}
+
+void tst_QProcess::impossibleUnixProcessParameters()
+{
+    QFETCH(QProcess::UnixProcessParameters, params);
+
+    QProcess process;
+    if (params.flags & QProcess::UnixProcessFlag::CreateNewSession) {
+        process.setChildProcessModifier([]() {
+            // double setsid() should cause the second to fail
+            setsid();
+        });
+    }
+    process.setUnixProcessParameters(params);
+    process.start("testProcessNormal/testProcessNormal");
+
+    QVERIFY(!process.waitForStarted(5000));
+    qDebug() << process.errorString();
+}
+
 void tst_QProcess::unixProcessParametersAndChildModifier()
 {
     static constexpr char message[] = "Message from the handler function\n";
@@ -1806,6 +1844,8 @@ void tst_QProcess::unixProcessParametersAndChildModifier()
     QAtomicInt vforkControl;
     int pipes[2];
 
+    pid_t oldpgid = getpgrp();
+
     QVERIFY2(pipe(pipes) == 0, qPrintable(qt_error_string()));
     auto pipeGuard0 = qScopeGuard([=] { close(pipes[0]); });
     {
@@ -1813,10 +1853,14 @@ void tst_QProcess::unixProcessParametersAndChildModifier()
 
         // verify that our modifier runs before the parameters are applied
         process.setChildProcessModifier([=, &vforkControl] {
+            const char *pgidmsg = "PGID mismatch. ";
+            if (getpgrp() != oldpgid)
+                write(pipes[1], pgidmsg, strlen(pgidmsg));
             write(pipes[1], message, strlen(message));
             vforkControl.storeRelaxed(1);
         });
         auto flags = QProcess::UnixProcessFlag::CloseFileDescriptors |
+                QProcess::UnixProcessFlag::CreateNewSession |
                 QProcess::UnixProcessFlag::UseVFork;
         process.setUnixProcessParameters({ flags });
         process.setProgram("testUnixProcessParameters/testUnixProcessParameters");