io/Beneath: add OpenDirectoryBeneath()

src/io/Beneath.cxx

@@ -35,3 +35,28 @@ OpenReadOnlyBeneath(FileAt file)
 
 	return fd;
 }
+
+static constexpr struct open_how directory_beneath{
+	.flags = O_DIRECTORY|O_RDONLY|O_NOCTTY|O_CLOEXEC,
+	.resolve = RESOLVE_BENEATH|RESOLVE_NO_MAGICLINKS,
+};
+
+UniqueFileDescriptor
+TryOpenDirectoryBeneath(FileAt file) noexcept
+{
+	assert(file.directory.IsDefined());
+
+	int fd = openat2(file.directory.Get(), file.name,
+			 &directory_beneath, sizeof(directory_beneath));
+	return UniqueFileDescriptor{fd};
+}
+
+UniqueFileDescriptor
+OpenDirectoryBeneath(FileAt file)
+{
+	auto fd = TryOpenDirectoryBeneath(file);
+	if (!fd.IsDefined())
+		throw FmtErrno("Failed to open '{}'", file.name);
+
+	return fd;
+}

src/io/Beneath.hxx

@@ -11,3 +11,9 @@ TryOpenReadOnlyBeneath(FileAt file) noexcept;
 
 UniqueFileDescriptor
 OpenReadOnlyBeneath(FileAt file);
+
+UniqueFileDescriptor
+TryOpenDirectoryBeneath(FileAt file) noexcept;
+
+UniqueFileDescriptor
+OpenDirectoryBeneath(FileAt file);