Fix pasting as LaTeX in documents with no modules

When there is no module, the old code would invoke tex2lyx with option
 -m
instead of
 -m ""
and an error would ensue.

This is because the QProcess command line parser used in Systemcall is
broken and does not recognize empty parameters as such. The solution
is to rewrite our parsecmd() parser to generate a list of parameters.
This is post-2.4.0 work.

The workaround used here is:

- set the list of modules to "," when it should be empty. In effect,
  this is a list of two empty modules.

- change tex2lyx to accept empty module names and ignore them; this is
  good in terms of robustness anyway.

Additionally, when there is no receiving buffer, set the defaults as
the BufferParams defaults instead of empty (this is cleaner, but
should not make a difference in practice).

In the long term, we should switch to use the QStringList-based API of
QProcess in Systemcall (see QTBUG-80640).

src/BufferParams.cpp

@@ -3840,5 +3840,11 @@ string const BufferParams::bibFileEncoding(string const & file) const
 }
 
 
+BufferParams const & defaultBufferParams()
+{
+	static BufferParams default_params;
+	return default_params;
+}
+
 
 } // namespace lyx

src/BufferParams.h

@@ -691,6 +691,10 @@ private:
 	support::copied_ptr<Impl, MemoryTraits> pimpl_;
 };
 
+
+///
+BufferParams const & defaultBufferParams();
+
 } // namespace lyx
 
 #endif

src/Converter.cpp

@@ -646,21 +646,24 @@ Converters::RetVal Converters::convert(Buffer const * buffer,
 				to_utf8(makeRelPath(from_utf8(outfile.absFileName()), from_utf8(path)));
 
 			string command = conv.command();
+			BufferParams const & bparams = buffer ? buffer->params() : defaultBufferParams();
 			command = subst(command, token_from, quoteName(infile2));
 			command = subst(command, token_base, quoteName(from_base));
 			command = subst(command, token_to, quoteName(outfile2));
 			command = subst(command, token_path, quoteName(onlyPath(infile.absFileName())));
 			command = subst(command, token_orig_path, quoteName(onlyPath(orig_from.absFileName())));
 			command = subst(command, token_orig_from, quoteName(onlyFileName(orig_from.absFileName())));
-			command = subst(command, token_textclass,
-			                buffer ? quoteName(buffer->params().documentClass().name())
-			                       : string());
-			command = subst(command, token_modules,
-			                buffer ? quoteName(buffer->params().getModules().asString())
-			                       : string());
-			command = subst(command, token_encoding,
-			                buffer ? quoteName(buffer->params().encoding().iconvName())
-			                       : string());
+			command = subst(command, token_textclass, quoteName(bparams.documentClass().name()));
+			string modules = bparams.getModules().asString();
+			// FIXME: remove when SystemCall uses QProcess with the list API.
+			// Currently the QProcess parser is not able to encode an
+			// empty argument as ""; work around this by passing a
+			// single comma, that will be interpreted as a list of two
+			// empty module names.
+			if (modules.empty())
+				modules = ",";
+			command = subst(command, token_modules, quoteName(modules));
+			command = subst(command, token_encoding, quoteName(bparams.encoding().iconvName()));
 			command = subst(command, token_python, os::python());
 
 			if (!conv.parselog().empty())

src/support/Systemcall.cpp

@@ -392,6 +392,11 @@ void SystemcallPrivate::startProcess(QString const & cmd, string const & path,
 	 * The cleanest solution would be to have parsecmd() produce a
 	 * QStringList for arguments, instead of transforming the string
 	 * into something that the QProcess splitter accepts.
+	 *
+	 * Another reason for doing that is that the Qt parser ignores
+	 * empty "" arguments, which are needed in some instances (see
+	 * e.g. the work around for modules in Converter:convert. See
+	 * QTBUG-80640 for a discussion.
 	*/
 	QStringList arguments = QProcess::splitCommand(toqstr(latexEnvCmdPrefix(path, lpath)) + cmd_);
 	QString command = (arguments.empty()) ? QString() : arguments.first();

src/tex2lyx/tex2lyx.cpp

@@ -699,8 +699,6 @@ int parse_class(string const & arg, string const &)
 
 int parse_module(string const & arg, string const &)
 {
-	if (arg.empty())
-		error_message("Missing modules string after -m switch");
 	split(arg, preloaded_modules, ',');
 	return 1;
 }
@@ -924,11 +922,12 @@ bool tex2lyx(idocstream & is, ostream & os, string const & encoding,
 
 	// Load preloaded modules.
 	// This needs to be done after the preamble is parsed, since the text
-	// class may not be known before. It neds to be done before parsing
+	// class may not be known before. It needs to be done before parsing
 	// body, since otherwise the commands/environments provided by the
 	// modules would be parsed as ERT.
+	// Empty module names are silently skipped.
 	for (auto const & module : preloaded_modules) {
-		if (!addModule(module)) {
+		if (!module.empty() && !addModule(module)) {
 			cerr << "Error: Could not load module \""
 			     << module << "\"." << endl;
 			return false;