io/FileDescriptor: add OpenReadOnly() overload with directory fd

src/io/FileDescriptor.cxx

@@ -122,6 +122,16 @@ FileDescriptor::OpenReadOnly(const char *pathname) noexcept
 	return Open(pathname, O_RDONLY);
 }
 
+#ifdef __linux__
+
+bool
+FileDescriptor::OpenReadOnly(FileDescriptor dir, const char *pathname) noexcept
+{
+	return Open(dir, pathname, O_RDONLY);
+}
+
+#endif // __linux__
+
 #ifndef _WIN32
 
 bool

src/io/FileDescriptor.hxx

@@ -27,8 +27,7 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef FILE_DESCRIPTOR_HXX
-#define FILE_DESCRIPTOR_HXX
+#pragma once
 
 #include <cstddef>
 #include <utility>
@@ -135,6 +134,11 @@ public:
 
 	bool OpenReadOnly(const char *pathname) noexcept;
 
+#ifdef __linux__
+	bool OpenReadOnly(FileDescriptor dir,
+			  const char *pathname) noexcept;
+#endif
+
 #ifndef _WIN32
 	bool OpenNonBlocking(const char *pathname) noexcept;
 #endif
@@ -271,5 +275,3 @@ public:
 	bool IsReadyForWriting() const noexcept;
 #endif
 };
-
-#endif