Preserve libgit2 error codes

Throw a system_error with a libgit2_category error category for errors
that propagate from libgit2.

docs/api/reference.rst

@@ -56,3 +56,5 @@ LOOT uses error category objects to identify errors with codes that originate in
 lower-level libraries.
 
 .. doxygenfunction:: loot::libloadorder_category
+
+.. doxygenfunction:: loot::libgit2_category

include/loot/error_categories.h

@@ -36,6 +36,13 @@ namespace loot {
              derived from std::error_category.
  */
 LOOT_API const std::error_category& libloadorder_category();
+
+/** @brief Get the error category that can be used to identify system_error
+ *         exceptions that are due to libgit2 errors.
+ *  @returns A reference to the static object of unspecified runtime type,
+             derived from std::error_category.
+ */
+LOOT_API const std::error_category& libgit2_category();
 }
 
 #endif

src/backend/error_categories.cpp

@@ -39,10 +39,29 @@ class libloadorder_category : public std::error_category {
     return code.category().name() == name();
   }
 };
+
+class libgit2_category : public std::error_category {
+  virtual const char* name() const noexcept {
+    return "libgit2";
+  }
+
+  virtual std::string message(int ev) const {
+    return "libgit2 error";
+  }
+
+  virtual bool equivalent(const std::error_code& code, int condition) const noexcept {
+    return code.category().name() == name();
+  }
+};
 }
 
 LOOT_API const std::error_category& libloadorder_category() {
   static detail::libloadorder_category instance;
   return instance;
 }
+
+LOOT_API const std::error_category& libgit2_category() {
+  static detail::libgit2_category instance;
+  return instance;
+}
 }

src/backend/helpers/git_helper.cpp

@@ -29,6 +29,7 @@
 #include <boost/log/trivial.hpp>
 
 #include "loot/error.h"
+#include "loot/error_categories.h"
 
 using boost::locale::translate;
 using std::string;
@@ -104,7 +105,7 @@ void GitHelper::Call(int error_code) {
     errorMessage_ = (boost::format(translate("Git operation failed. Error: %1%")) % gitError).str();
 
   BOOST_LOG_TRIVIAL(error) << "Git operation failed. Error: " << gitError;
-  throw Error(Error::Code::git_error, errorMessage_);
+  throw std::system_error(error_code, libgit2_category(), errorMessage_);
 }
 
 void GitHelper::SetErrorMessage(const std::string& message) {

src/tests/backend/helpers/git_helper_test.h

@@ -94,8 +94,8 @@ TEST_F(GitHelperTest, callShouldNotThrowIfPassedAZeroValue) {
 }
 
 TEST_F(GitHelperTest, callShouldThrowIfPassedANonZeroValue) {
-  EXPECT_THROW(git_.Call(1), Error);
-  EXPECT_THROW(git_.Call(-1), Error);
+  EXPECT_THROW(git_.Call(1), std::system_error);
+  EXPECT_THROW(git_.Call(-1), std::system_error);
 }
 
 TEST_F(GitHelperTest, setErrorMessageShouldSetTheMessageForThrownExceptions) {
@@ -105,7 +105,7 @@ TEST_F(GitHelperTest, setErrorMessageShouldSetTheMessageForThrownExceptions) {
   try {
     git_.Call(1);
     ADD_FAILURE() << "An exception should have been thrown.";
-  } catch (Error& e) {
+  } catch (std::system_error& e) {
     EXPECT_NE(nullptr, strstr(e.what(), errorMessage));
   }
 }