osv: add error class

Different source bases have different error conventions; libc has 0/-1+errno,
while the rest os the source base uses 0/error.

Wrap errors in a class to prevent confusion between the two.

include/osv/error.h

@@ -0,0 +1,69 @@
+#ifndef ERROR_H_
+#define ERROR_H_
+
+#include <errno.h>
+
+struct error {
+#ifdef __cplusplus
+public:
+    explicit error() = default;
+    explicit error(int _errno) : _errno(_errno) {}
+    error(const error& e) = default;
+    bool bad() const { return _errno != 0; }
+    int get() const { return _errno; }
+    int to_libc() const;
+private:
+#endif
+    int _errno = 0;
+};
+
+typedef struct error error;
+
+static inline error no_error()
+{
+    return (error) { 0 };
+}
+
+static inline struct error make_error(int _errno)
+{
+    return (error) { _errno };
+}
+
+static inline bool error_bad(error e)
+{
+#ifdef __cplusplus
+    return e.bad();
+#else
+    return e._errno != 0;
+#endif
+}
+
+static inline int error_get(error e)
+{
+#ifdef __cplusplus
+    return e.get();
+#else
+    return e._errno;
+#endif
+}
+
+inline static int error_to_libc(error e)
+{
+    if (error_bad(e)) {
+        return 0;
+    } else {
+        errno = error_get(e);
+        return -1;
+    }
+}
+
+#ifdef __cplusplus
+
+inline int error::to_libc() const
+{
+    return error_to_libc(*this);
+}
+
+#endif
+
+#endif /* ERROR_H_ */