#12523 check for existence of usable Python interpreter

Backport of change bbc2270

- present appropriate alert message in case of missing Python
- add the option to quit LyX immediately
- recheck for Python interpreter on reconfigure if it was missing

src/LyX.cpp

@@ -585,10 +585,11 @@ void LyX::execCommands()
 {
 	// The advantage of doing this here is that the event loop
 	// is already started. So any need for interaction will be
-	// aknowledged.
+	// acknowledged.
 
 	// if reconfiguration is needed.
-	if (LayoutFileList::get().empty()) {
+	const bool noLayouts = LayoutFileList::get().empty();
+	if (noLayouts && os::hasPython()) {
 		switch (Alert::prompt(
 			_("No textclass is found"),
 			_("LyX will only have minimal functionality because no textclasses "
@@ -597,7 +598,8 @@ void LyX::execCommands()
 			0, 2,
 			_("&Reconfigure"),
 			_("&Without LaTeX"),
-			_("&Continue")))
+			_("&Continue"),
+			_("&Exit LyX")))
 		{
 		case 0:
 			// regular reconfigure
@@ -608,6 +610,24 @@ void LyX::execCommands()
 			lyx::dispatch(FuncRequest(LFUN_RECONFIGURE,
 				" --without-latex-config"));
 			break;
+		case 3:
+			lyx::dispatch(FuncRequest(LFUN_LYX_QUIT, ""));
+			return;
+		default:
+			break;
+		}
+	} else if (noLayouts) {
+		switch (Alert::prompt(
+			_("No python is found"),
+			_("LyX will only have minimal functionality because no python interpreter "
+				"has been found. Consider download and install of an python interpreter."),
+			0, 1,
+			_("&Continue"),
+			_("&Exit LyX")))
+		{
+		case 1:
+			lyx::dispatch(FuncRequest(LFUN_LYX_QUIT, ""));
+			return;
 		default:
 			break;
 		}

src/support/Package.cpp

@@ -158,9 +158,9 @@ Package::Package(string const & command_line_arg0,
 
 int Package::reconfigureUserLyXDir(string const & option) const
 {
-	if (configure_command_.empty()) {
+	if (configure_command_.empty() || !os::hasPython()) {
 		FileName const configure_script(addName(system_support().absFileName(), "configure.py"));
-		configure_command_ = os::python() + ' ' +
+		configure_command_ = os::python(true) + ' ' +
 			quoteName(configure_script.toFilesystemEncoding()) +
 			with_version_suffix() + " --binary-dir=" +
 			quoteName(FileName(binary_dir().absFileName()).toFilesystemEncoding());

src/support/os.cpp

@@ -104,6 +104,12 @@ string const python(bool reset)
 	return command;
 }
 
+
+bool hasPython()
+{
+	return !(python23(python()).empty());
+}
+
 } // namespace os
 } // namespace support
 } // namespace lyx

src/support/os.h

@@ -62,6 +62,9 @@ int timeout_min();
 /// @param reset True if the python path should be recomputed
 std::string const python(bool reset = false);
 
+/// Check for availability of the python interpreter
+bool hasPython();
+
 ///
 bool isFilesystemCaseSensitive();