thodg/libgit2

diff --git a/include/git2/errors.h b/include/git2/errors.h
index 5c85c4d..8887b32 100644
--- a/include/git2/errors.h
+++ b/include/git2/errors.h
@@ -107,7 +107,8 @@ typedef enum {
 	GIT_ERROR_PATCH,
 	GIT_ERROR_WORKTREE,
 	GIT_ERROR_SHA1,
-	GIT_ERROR_HTTP
+	GIT_ERROR_HTTP,
+	GIT_ERROR_INTERNAL
 } git_error_t;
 
 /**
diff --git a/src/assert_safe.h b/src/assert_safe.h
new file mode 100644
index 0000000..8c26110
--- /dev/null
+++ b/src/assert_safe.h
@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#ifndef INCLUDE_assert_safe_h__
+#define INCLUDE_assert_safe_h__
+
+/*
+ * In a debug build, we'll assert(3) for aide in debugging.  In release
+ * builds, we will provide macros that will set an error message that
+ * indicate a failure and return.  Note that memory leaks can occur in
+ * a release-mode assertion failure -- it is impractical to provide
+ * safe clean up routines in these very extreme failures, but care
+ * should be taken to not leak very large objects.
+ */
+
+#if (defined(_DEBUG) || defined(GIT_ASSERT_HARD)) && GIT_ASSERT_HARD != 0
+# include <assert.h>
+
+# define GIT_ASSERT(expr) assert(expr)
+# define GIT_ASSERT_ARG(expr) assert(expr)
+
+# define GIT_ASSERT_WITH_RETVAL(expr, fail) assert(expr)
+# define GIT_ASSERT_ARG_WITH_RETVAL(expr, fail) assert(expr)
+#else
+
+/** Internal consistency check to stop the function. */
+# define GIT_ASSERT(expr) GIT_ASSERT_WITH_RETVAL(expr, -1)
+
+/**
+ * Assert that a consumer-provided argument is valid, setting an
+ * actionable error message and returning -1 if it is not.
+ */
+# define GIT_ASSERT_ARG(expr) GIT_ASSERT_ARG_WITH_RETVAL(expr, -1)
+
+/** Internal consistency check to return the `fail` param on failure. */
+# define GIT_ASSERT_WITH_RETVAL(expr, fail) \
+	GIT_ASSERT__WITH_RETVAL(expr, GIT_ERROR_INTERNAL, "unrecoverable internal error", fail)
+
+/**
+ * Assert that a consumer-provided argument is valid, setting an
+ * actionable error message and returning the `fail` param if not.
+ */
+# define GIT_ASSERT_ARG_WITH_RETVAL(expr, fail) \
+	GIT_ASSERT__WITH_RETVAL(expr, GIT_ERROR_INVALID, "invalid argument", fail)
+
+# define GIT_ASSERT__WITH_RETVAL(expr, code, msg, fail) do { \
+		if (!(expr)) { \
+			git_error_set(code, "%s: '%s'", msg, #expr); \
+			return fail; \
+		} \
+	} while(0)
+
+#endif /* GIT_ASSERT_HARD */
+
+#endif
diff --git a/src/common.h b/src/common.h
index a4152ca..2b1a4a4 100644
--- a/src/common.h
+++ b/src/common.h
@@ -80,6 +80,7 @@
 #include "errors.h"
 #include "thread-utils.h"
 #include "integer.h"
+#include "assert_safe.h"
 
 /*
  * Include the declarations for deprecated functions; this ensures
diff --git a/tests/core/assert.c b/tests/core/assert.c
new file mode 100644
index 0000000..ef75624
--- /dev/null
+++ b/tests/core/assert.c
@@ -0,0 +1,94 @@
+#ifdef GIT_ASSERT_HARD
+# undef GIT_ASSERT_HARD
+#endif
+
+#define GIT_ASSERT_HARD 0
+
+#include "clar_libgit2.h"
+
+static const char *hello_world = "hello, world";
+static const char *fail = "FAIL";
+
+static int dummy_fn(const char *myarg)
+{
+	GIT_ASSERT_ARG(myarg);
+	GIT_ASSERT_ARG(myarg != hello_world);
+	return 0;
+}
+
+static const char *fn_returns_string(const char *myarg)
+{
+	GIT_ASSERT_ARG_WITH_RETVAL(myarg, fail);
+	GIT_ASSERT_ARG_WITH_RETVAL(myarg != hello_world, fail);
+
+	return myarg;
+}
+
+static int bad_math(void)
+{
+	GIT_ASSERT(1 + 1 == 3);
+	return 42;
+}
+
+static const char *bad_returns_string(void)
+{
+	GIT_ASSERT_WITH_RETVAL(1 + 1 == 3, NULL);
+	return hello_world;
+}
+
+void test_core_assert__argument(void)
+{
+	cl_git_fail(dummy_fn(NULL));
+	cl_assert(git_error_last());
+	cl_assert_equal_i(GIT_ERROR_INVALID, git_error_last()->klass);
+	cl_assert_equal_s("invalid argument: 'myarg'", git_error_last()->message);
+
+	cl_git_fail(dummy_fn(hello_world));
+	cl_assert(git_error_last());
+	cl_assert_equal_i(GIT_ERROR_INVALID, git_error_last()->klass);
+	cl_assert_equal_s("invalid argument: 'myarg != hello_world'", git_error_last()->message);
+
+	cl_git_pass(dummy_fn("foo"));
+}
+
+void test_core_assert__argument_with_non_int_return_type(void)
+{
+	const char *foo = "foo";
+
+	cl_assert_equal_p(fail, fn_returns_string(NULL));
+	cl_assert_equal_i(GIT_ERROR_INVALID, git_error_last()->klass);
+	cl_assert_equal_s("invalid argument: 'myarg'", git_error_last()->message);
+
+	cl_assert_equal_p(fail, fn_returns_string(hello_world));
+	cl_assert_equal_i(GIT_ERROR_INVALID, git_error_last()->klass);
+	cl_assert_equal_s("invalid argument: 'myarg != hello_world'", git_error_last()->message);
+
+	cl_assert_equal_p(foo, fn_returns_string(foo));
+}
+
+void test_core_assert__argument_with_void_return_type(void)
+{
+	const char *foo = "foo";
+
+	git_error_clear();
+	fn_returns_string(hello_world);
+	cl_assert_equal_i(GIT_ERROR_INVALID, git_error_last()->klass);
+	cl_assert_equal_s("invalid argument: 'myarg != hello_world'", git_error_last()->message);
+
+	git_error_clear();
+	cl_assert_equal_p(foo, fn_returns_string(foo));
+	cl_assert_equal_p(NULL, git_error_last());
+}
+
+void test_core_assert__internal(void)
+{
+	cl_git_fail(bad_math());
+	cl_assert(git_error_last());
+	cl_assert_equal_i(GIT_ERROR_INTERNAL, git_error_last()->klass);
+	cl_assert_equal_s("unrecoverable internal error: '1 + 1 == 3'", git_error_last()->message);
+
+	cl_assert_equal_p(NULL, bad_returns_string());
+	cl_assert(git_error_last());
+	cl_assert_equal_i(GIT_ERROR_INTERNAL, git_error_last()->klass);
+	cl_assert_equal_s("unrecoverable internal error: '1 + 1 == 3'", git_error_last()->message);
+}