QLockFile: Move the check for Linux /proc to a central place

I'll need it soon in QTemporaryFile. This also reduces the overhead,
since QFile::exists -> QFileInfo::exists needs to check if there's a
file engine that handles "/proc/version" (there isn't), convert the
UTF-16 filename to UTF-8 and QFileSystemEngine::fillMetaData will issue
a more expensive stat(2) syscall.

access(2) is cheaper.

This commit also introduces QT_LINUX_ALWAYS_HAVE_PROCFS that isn't
defined anywhere, but could be passed during configure with -D if a
build wants it.

Change-Id: I1eba2b016de74620bfc8fffd14ccfd1593d59ade
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/io/qlockfile_unix.cpp

@@ -199,7 +199,7 @@ QLockFile::LockError QLockFilePrivate::tryLock_sys()
     }
 
     if (qt_write_loop(fd, fileData.constData(), fileData.size()) < fileData.size()) {
-        close(fd);
+        qt_safe_close(fd);
         if (!QFile::remove(fileName))
             qWarning("QLockFile: Could not remove our own lock file %s.", qPrintable(fileName));
         return QLockFile::UnknownError; // partition full
@@ -258,7 +258,7 @@ QString QLockFilePrivate::processNameByPid(qint64 pid)
     proc_name(pid, name, sizeof(name) / sizeof(char));
     return QFile::decodeName(name);
 #elif defined(Q_OS_LINUX)
-    if (!QFile::exists(QStringLiteral("/proc/version")))
+    if (!qt_haveLinuxProcfs())
         return QString();
     char exePath[64];
     char buf[PATH_MAX + 1];

src/corelib/kernel/qcore_unix_p.h

@@ -340,6 +340,21 @@ static inline pid_t qt_safe_waitpid(pid_t pid, int *status, int options)
 timespec qt_gettime() Q_DECL_NOTHROW;
 void qt_nanosleep(timespec amount);
 
+/* non-static */
+inline bool qt_haveLinuxProcfs()
+{
+#ifdef Q_OS_LINUX
+#  ifdef QT_LINUX_ALWAYS_HAVE_PROCFS
+    return true;
+#  else
+    static const bool present = (access("/proc/version", F_OK) == 0);
+    return present;
+#  endif
+#else
+    return false;
+#endif
+}
+
 Q_CORE_EXPORT int qt_safe_poll(struct pollfd *fds, nfds_t nfds, const struct timespec *timeout_ts);
 
 static inline int qt_poll_msecs(struct pollfd *fds, nfds_t nfds, int timeout)