cmd/vendorhash: track vendor SRI in flakehashes.json

Move the headscale vendorHash out of flake.nix into a content-
addressed flakehashes.json maintained by a small Go tool. The
schema and goModFingerprint algorithm mirror upstream tailscale's
tool/updateflakes so a future shared library extraction is trivial.

vendorhash check verifies flakehashes.json against the current
go.mod/go.sum. Hot path is a sha256 over those two files, so
re-runs without input change are essentially free; only an actual
fingerprint drift triggers go mod vendor + nardump.SRI.

vendorhash update recomputes both fields and rewrites the JSON.
The nix-vendor-sri devShell shim now wraps it.

cmd/vendorhash/main.go

@@ -0,0 +1,221 @@
+// vendorhash maintains the Nix SRI hash for the Go module vendor tree
+// and stores it in flakehashes.json alongside a content fingerprint of
+// go.mod and go.sum.
+//
+// Each block records its input fingerprint (goModSum) so that re-runs
+// with no input change are essentially free: the fast path is just a
+// sha256 over two small files. The vendor tree is only re-walked when
+// the fingerprint actually drifts.
+//
+// Subcommands:
+//
+//	vendorhash check   exit non-zero if flakehashes.json is stale
+//	vendorhash update  recompute and rewrite flakehashes.json
+//
+// The JSON schema and goModFingerprint algorithm mirror upstream
+// tailscale's tool/updateflakes so a future shared library extraction
+// is straightforward.
+package main
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+
+	"tailscale.com/cmd/nardump/nardump"
+)
+
+const (
+	hashesFile = "flakehashes.json"
+	goModFile  = "go.mod"
+	goSumFile  = "go.sum"
+)
+
+type FlakeHashes struct {
+	Vendor VendorBlock `json:"vendor"`
+}
+
+type VendorBlock struct {
+	GoModSum string `json:"goModSum"`
+	SRI      string `json:"sri"`
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		usage()
+		os.Exit(2)
+	}
+
+	ctx := context.Background()
+
+	var err error
+
+	switch os.Args[1] {
+	case "check":
+		err = cmdCheck(ctx)
+	case "update":
+		err = cmdUpdate(ctx)
+	case "-h", "--help", "help":
+		usage()
+		return
+	default:
+		usage()
+		os.Exit(2)
+	}
+
+	if err != nil {
+		if errors.Is(err, errStale) {
+			os.Exit(1)
+		}
+
+		fmt.Fprintln(os.Stderr, "vendorhash:", err)
+		os.Exit(1)
+	}
+}
+
+func usage() {
+	fmt.Fprintln(os.Stderr, "usage: vendorhash <check|update>")
+}
+
+// errStale signals to main that the check found a mismatch; it has
+// already printed a remediation message, so main should exit 1
+// silently.
+var errStale = errors.New("vendor hash stale")
+
+// cmdCheck verifies that flakehashes.json matches the current
+// go.mod/go.sum. The fast path (fingerprint unchanged) costs only
+// a sha256 over the two files. On mismatch, it computes the actual
+// SRI so the failure message gives the developer the value to paste
+// (or to run `vendorhash update`).
+func cmdCheck(ctx context.Context) error {
+	hashes, err := loadHashes()
+	if err != nil {
+		return err
+	}
+
+	curFP, err := goModFingerprint()
+	if err != nil {
+		return err
+	}
+
+	if curFP == hashes.Vendor.GoModSum {
+		return nil
+	}
+
+	curSRI, err := hashVendor(ctx)
+	if err != nil {
+		return err
+	}
+
+	fmt.Fprintln(os.Stderr, "vendor hash is stale.")
+	fmt.Fprintf(os.Stderr, "  expected goModSum: %s\n", hashes.Vendor.GoModSum)
+	fmt.Fprintf(os.Stderr, "    actual goModSum: %s\n", curFP)
+	fmt.Fprintf(os.Stderr, "       expected sri: %s\n", hashes.Vendor.SRI)
+	fmt.Fprintf(os.Stderr, "         actual sri: %s\n", curSRI)
+	fmt.Fprintln(os.Stderr, "run: go run ./cmd/vendorhash update")
+	// Also emit machine-parseable lines so CI can pick them up.
+	fmt.Printf("expected_sri=%s\n", hashes.Vendor.SRI)
+	fmt.Printf("actual_sri=%s\n", curSRI)
+
+	return errStale
+}
+
+func cmdUpdate(ctx context.Context) error {
+	fp, err := goModFingerprint()
+	if err != nil {
+		return err
+	}
+
+	sri, err := hashVendor(ctx)
+	if err != nil {
+		return err
+	}
+
+	return writeHashes(FlakeHashes{
+		Vendor: VendorBlock{
+			GoModSum: fp,
+			SRI:      sri,
+		},
+	})
+}
+
+// goModFingerprint returns a content fingerprint of go.mod and go.sum
+// that changes whenever either file changes. The byte layout matches
+// upstream tailscale's tool/updateflakes.
+func goModFingerprint() (string, error) {
+	h := sha256.New()
+
+	for _, f := range []string{goModFile, goSumFile} {
+		b, err := os.ReadFile(f)
+		if err != nil {
+			return "", err
+		}
+
+		fmt.Fprintf(h, "%s %d\n", f, len(b))
+		h.Write(b)
+	}
+
+	return "sha256-" + base64.StdEncoding.EncodeToString(h.Sum(nil)), nil
+}
+
+// hashVendor runs `go mod vendor` into a temporary directory and
+// returns the Nix SRI hash of the resulting tree.
+func hashVendor(ctx context.Context) (string, error) {
+	out, err := os.MkdirTemp("", "nar-vendor-")
+	if err != nil {
+		return "", err
+	}
+	// `go mod vendor -o` requires the destination to not already exist.
+	err = os.Remove(out)
+	if err != nil {
+		return "", err
+	}
+
+	defer os.RemoveAll(out)
+
+	cmd := exec.CommandContext(ctx, "go", "mod", "vendor", "-o", out)
+
+	cmd.Env = append(os.Environ(), "GOWORK=off")
+	cmd.Stderr = os.Stderr
+
+	err = cmd.Run()
+	if err != nil {
+		return "", fmt.Errorf("go mod vendor: %w", err)
+	}
+
+	return nardump.SRI(os.DirFS(out))
+}
+
+func loadHashes() (FlakeHashes, error) {
+	var h FlakeHashes
+
+	b, err := os.ReadFile(hashesFile)
+	if err != nil {
+		return h, err
+	}
+
+	err = json.Unmarshal(b, &h)
+	if err != nil {
+		return h, fmt.Errorf("%s: %w", hashesFile, err)
+	}
+
+	return h, nil
+}
+
+func writeHashes(h FlakeHashes) error {
+	b, err := json.MarshalIndent(h, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	b = append(b, '\n')
+
+	// flakehashes.json is committed source read by Nix during evaluation;
+	// world-readable matches every other tracked file in the repo.
+	return os.WriteFile(hashesFile, b, 0o644) //nolint:gosec
+}

flake.nix

@@ -27,7 +27,7 @@
         let
           pkgs = nixpkgs.legacyPackages.${prev.stdenv.hostPlatform.system};
           buildGo = pkgs.buildGo126Module;
-          vendorHash = "sha256-Jquzx8xIkV28S8DnZVH8apQ4q+Q92e5yVX13fxodw8Y=";
+          vendorHash = (builtins.fromJSON (builtins.readFile ./flakehashes.json)).vendor.sri;
         in
         {
           headscale = buildGo {
@@ -38,8 +38,8 @@
             # Only run unit tests when testing a build
             checkFlags = [ "-short" ];
 
-            # When updating go.mod or go.sum, a new sha will need to be calculated,
+            # vendorHash is read from flakehashes.json; refresh via:
-            # update this if you have a mismatch after doing a change to those files.
+            #   go run ./cmd/vendorhash update
             inherit vendorHash;
 
             subPackages = [ "cmd/headscale" ];
@@ -223,13 +223,7 @@
                 "nix-vendor-sri"
                 ''
                   set -eu
-
+                  exec go run ./cmd/vendorhash update "$@"
-                  OUT=$(mktemp -d -t nar-hash-XXXXXX)
-                  rm -rf "$OUT"
-
-                  go mod vendor -o "$OUT"
-                  go run tailscale.com/cmd/nardump --sri "$OUT"
-                  rm -rf "$OUT"
                 '')
 
               (pkgs.writeShellScriptBin

flakehashes.json

@@ -0,0 +1,6 @@
+{
+  "vendor": {
+    "goModSum": "sha256-IE0n9cSqO4XNX4RN+CGBk9VC46iACiZKDFf/215iivk=",
+    "sri": "sha256-ijEIP9NSomhlWOgsVN7tPvSuvkTiLtnvXvhZmatIDLM="
+  }
+}