go/nix: split Nix encoding stuff into nixwire package

go/nix/default.nix

@@ -6,6 +6,7 @@ args:
 {
   nar = import ./nar args;
   nixstore = import ./nixstore args;
+  nixwire = import ./nixwire args;
   bcachegc = import ./bcachegc args;
   bcacheup = import ./bcacheup args;
 }

go/nix/nar/default.nix

@@ -10,6 +10,9 @@
     ./nar.go
     ./dirfs.go
   ];
+  deps = with depot; [
+    go.nix.nixwire
+  ];
 }) // {
   narinfo = import ./narinfo args;
 }

go/nix/nar/nar.go

@@ -1,48 +1,15 @@
 package nar
 
 import (
-	"encoding/binary"
 	"fmt"
 	"io"
 	"io/fs"
 	"path"
 	"sort"
+
+	"hg.lukegb.com/lukegb/depot/go/nix/nixwire"
 )
 
-type serializeWriter struct {
-	io.Writer
-}
-
-func (w serializeWriter) WritePadding(n int64) (int64, error) {
-	if n%8 > 0 {
-		n, err := w.Write(make([]byte, 8-(n%8)))
-		return int64(n), err
-	}
-	return 0, nil
-}
-
-func (w serializeWriter) WriteUint64(n uint64) (int64, error) {
-	buf := make([]byte, 8)
-	binary.LittleEndian.PutUint64(buf, n)
-	wrote, err := w.Write(buf)
-	return int64(wrote), err
-}
-
-func (w serializeWriter) WriteString(s string) (int64, error) {
-	nSize, err := w.WriteUint64(uint64(len(s)))
-	if err != nil {
-		return int64(nSize), err
-	}
-
-	nData, err := w.Write([]byte(s))
-	if err != nil {
-		return int64(nSize) + int64(nData), err
-	}
-
-	nPad, err := w.WritePadding(int64(len(s)))
-	return int64(nSize) + int64(nData) + int64(nPad), err
-}
-
 type FS interface {
 	Open(string) (fs.File, error)
 	Stat(string) (fs.FileInfo, error)
@@ -50,7 +17,7 @@ type FS interface {
 	Readlink(string) (string, error)
 }
 
-func packFile(sw serializeWriter, root FS, fn string, stat fs.FileInfo) (int64, error) {
+func packFile(sw nixwire.Serializer, root FS, fn string, stat fs.FileInfo) (int64, error) {
 
 	var nSoFar int64
 	write := func(data ...any) (int64, error) {
@@ -179,7 +146,7 @@ func packFile(sw serializeWriter, root FS, fn string, stat fs.FileInfo) (int64,
 }
 
 func Pack(w io.Writer, fs FS, fn string) (int64, error) {
-	sw := serializeWriter{w}
+	sw := nixwire.Serializer{w}
 
 	n, err := sw.WriteString("nix-archive-1")
 	if err != nil {

go/nix/nixwire/default.nix

@@ -0,0 +1,12 @@
+# SPDX-FileCopyrightText: 2022 Luke Granger-Brown <depot@lukegb.com>
+#
+# SPDX-License-Identifier: Apache-2.0
+
+{ depot, ... }@args:
+depot.third_party.buildGo.package {
+  name = "nixwire";
+  path = "hg.lukegb.com/lukegb/depot/go/nix/nixwire";
+  srcs = [
+    ./nixwire.go
+  ];
+}

go/nix/nixwire/nixwire.go

@@ -0,0 +1,40 @@
+package nixwire
+
+import (
+	"encoding/binary"
+	"io"
+)
+
+type Serializer struct {
+	io.Writer
+}
+
+func (w Serializer) WritePadding(n int64) (int64, error) {
+	if n%8 > 0 {
+		n, err := w.Write(make([]byte, 8-(n%8)))
+		return int64(n), err
+	}
+	return 0, nil
+}
+
+func (w Serializer) WriteUint64(n uint64) (int64, error) {
+	buf := make([]byte, 8)
+	binary.LittleEndian.PutUint64(buf, n)
+	wrote, err := w.Write(buf)
+	return int64(wrote), err
+}
+
+func (w Serializer) WriteString(s string) (int64, error) {
+	nSize, err := w.WriteUint64(uint64(len(s)))
+	if err != nil {
+		return int64(nSize), err
+	}
+
+	nData, err := w.Write([]byte(s))
+	if err != nil {
+		return int64(nSize) + int64(nData), err
+	}
+
+	nPad, err := w.WritePadding(int64(len(s)))
+	return int64(nSize) + int64(nData) + int64(nPad), err
+}