Detect diagsession files in "Compound File Binary" format

The diagsession files created by the Visual Studio GUI are in CFB format and not just simple ZIP files (in contrast to what is created by vsdiagnostics.exe).

CFB is not supported yet, but we will at least give a meaningful error message.

snail/analysis/diagsession_data_provider.cpp

@@ -1,9 +1,14 @@
 
 #include <snail/analysis/diagsession_data_provider.hpp>
 
+#include <cassert>
+
+#include <array>
+#include <bit>
 #include <filesystem>
 #include <format>
 #include <fstream>
+#include <span>
 #include <stdexcept>
 
 #include <libzippp.h>
@@ -17,6 +22,52 @@ using namespace snail::analysis;
 
 namespace {
 
+template<std::size_t MaxSize>
+struct file_magic
+{
+    char* data()
+    {
+        return reinterpret_cast<char*>(buffer_.data());
+    }
+
+    void set_size(std::size_t size)
+    {
+        assert(size <= MaxSize);
+        size_ = size;
+    }
+
+    bool equals(std::span<const std::byte> expected) const
+    {
+        if(size_ < expected.size()) return false;
+
+        return std::ranges::equal(std::span(buffer_).subspan(0, expected.size()), expected);
+    }
+
+private:
+    std::size_t                    size_;
+    std::array<std::byte, MaxSize> buffer_;
+};
+
+template<std::size_t MaxSize>
+file_magic<MaxSize> try_read_magic(const std::filesystem::path& file_path)
+{
+    std::ifstream file_stream(file_path, std::ios::in | std::ios::binary);
+    if(!file_stream.is_open())
+    {
+        throw std::runtime_error(std::format("Could not open file '{}'", file_path.string()));
+    }
+    assert(file_stream.tellg() == 0);
+
+    file_magic<MaxSize> magic;
+    file_stream.read(magic.data(), MaxSize);
+    if(!file_stream.good() && !file_stream.eof())
+    {
+        throw std::runtime_error(std::format("Failed to read magic from '{}'", file_path.string()));
+    }
+    magic.set_size(static_cast<std::size_t>(file_stream.tellg()));
+    return magic;
+}
+
 std::optional<std::string_view> extract_xml_attribute(std::string_view xml_node, std::string_view attribute_name)
 {
     const auto attr_name_offset = xml_node.find(attribute_name);
@@ -47,6 +98,20 @@ diagsession_data_provider::~diagsession_data_provider()
 
 void diagsession_data_provider::process(const std::filesystem::path& file_path)
 {
+    static constexpr std::array<std::uint8_t, 4> zip_magic           = {0x50, 0x4b, 0x03, 0x04};
+    static constexpr std::array<std::uint8_t, 8> compound_file_magic = {0xd0, 0xcf, 0x11, 0xe0, 0xa1, 0xb1, 0x1a, 0xe1};
+
+    const auto file_magic = try_read_magic<8>(file_path);
+    if(file_magic.equals(std::as_bytes(std::span(compound_file_magic))))
+    {
+        throw std::runtime_error(std::format("Could not open diagsession file '{}': Compound File Binary Format not yet supported", file_path.string()));
+    }
+
+    if(!file_magic.equals(std::as_bytes(std::span(zip_magic))))
+    {
+        throw std::runtime_error(std::format("Could not open diagsession file '{}': Invalid file format. It is neither a ZIP file, nor in Compound File Binary Format.", file_path.string()));
+    }
+
     libzippp::ZipArchive archive(file_path.string());
     if(!archive.open(libzippp::ZipArchive::ReadOnly))
     {