[#52] vdfs: throw a custom exception if the VDFs signature is not known

This fixes a crash with VDFs generate by Union and potentially other 3rd-party tools.

include/phoenix/vdfs.hh

@@ -36,6 +36,16 @@ namespace phoenix {
 	/// \note This will convert to a ``DOS`` timestamp in the ``GMT`` timezone.
 	std::uint32_t unix_time_to_dos(std::time_t tm) noexcept;
 
+	/// \brief An exception thrown if the signature of a VDF file is not recognized.
+	///
+	/// If the signature is not recognized, this means that the VDF file was created using a 3rd-party tool
+	/// like Union and is not necessarily compatible with the VDFS spec. Accepted signatures are
+	/// phoenix::VDF_SIGNATURE_G1 and phoenix::VDF_SIGNATURE_G2.
+	class vdfs_signature_error : public error {
+	public:
+		explicit vdfs_signature_error(const std::string& signature);
+	};
+
 	/// \brief Represents the header of a VDF.
 	class vdf_header {
 	public:

source/vdfs.cc

@@ -30,6 +30,9 @@ namespace phoenix {
 		return dos;
 	}
 
+	vdfs_signature_error::vdfs_signature_error(const std::string& signature)
+	    : error("VDF signature not recognized: \"" + signature + "\"") {}
+
 	bool vdf_entry_comparator::operator()(const vdf_entry& a, const vdf_entry& b) const {
 		return icompare(a.name, b.name);
 	}
@@ -236,6 +239,12 @@ namespace phoenix {
 		vdf_file vdf {};
 
 		vdf.header = vdf_header::read(buf);
+
+		// TODO: Reverse-engineer Union VDF format
+		if (vdf.header.signature != VDF_SIGNATURE_G1 && vdf.header.signature != VDF_SIGNATURE_G2) {
+			throw vdfs_signature_error {vdf.header.signature};
+		}
+
 		buf.position(vdf.header.catalog_offset);
 
 		const vdf_entry* entry = nullptr;