[wip] tufclient: implement cross-repository links

cmd/quarry-client/http.go

@@ -224,7 +224,11 @@ func proxyTargetFile(rw http.ResponseWriter, req *http.Request) (Err error) {
 	// TODO(uapi16): Once we get UAPI.16 support into systemd, we can serve a
 	// manifest that lists every URL candidate as an alternative contents source
 	// (as "quarry-client list --uapi-16" does) and drop this shim entirely.
-	for url, err := range infoExt.FetchURLs(&info.Repo.DataRootURL.URL) {
+	dataRootURL, err := info.Repo.DataURL()
+	if err != nil {
+		return fmt.Errorf("bad repo %s definition: cannot compute target url: %w", info.Repo.Name, err)
+	}
+	for url, err := range infoExt.FetchURLs(dataRootURL) {
 		if err != nil {
 			return fmt.Errorf("bad target data in repo %s for target %s: cannot compute target url: %w", info.Repo.Name, targetPath, err)
 		}

cmd/quarry-client/list.go

@@ -118,7 +118,11 @@ func (o *uapi16ListFormatter) Begin(ctx context.Context, client *tufclient.Clien
 }
 
 func (o *uapi16ListFormatter) Output(_ context.Context, target *tufclient.TargetInfo) error {
-	file, err := uapi16ext.FromTargetFile(target.TargetFiles, &target.Repo.DataRootURL.URL)
+	dataRootURL, err := target.Repo.DataURL()
+	if err != nil {
+		return err
+	}
+	file, err := uapi16ext.FromTargetFile(target.TargetFiles, dataRootURL)
 	if err != nil {
 		return err
 	}

cmd/quarry-client/list_test.go

@@ -33,9 +33,8 @@ const (
 	emptyHash = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
 )
 
-// testRepo returns a [config.Repository] for a dummy repository. The config
-// has to be parsed because the URL fields are not exported types.
-func testRepo(t *testing.T) *config.Repository {
+// testRepo returns the [tufext.Repository] for a dummy repository config.
+func testRepo(t *testing.T) *tufext.Repository {
 	t.Helper()
 	cfg, err := config.Parse(strings.NewReader(`
 config_version = 1
@@ -49,7 +48,7 @@ data_root_url = "https://example.com/data"
 	require.NoError(t, err)
 	repo, ok := cfg.Repos["test-repo"]
 	require.True(t, ok, "test-repo should be in the parsed config")
-	return repo
+	return repo.AsRepository()
 }
 
 // testClient returns a client with no repositories configured. That is enough

cmd/quarry-client/utils_tuf.go

@@ -20,7 +20,6 @@ import (
 	"go.amutable.dev/quarry/internal/expand"
 	"go.amutable.dev/quarry/internal/third_party/funchelpers"
 	"go.amutable.dev/quarry/internal/tufclient"
-	"go.amutable.dev/quarry/internal/tufclient/config"
 	"go.amutable.dev/quarry/internal/tufext"
 )
 
@@ -86,7 +85,7 @@ func pprintHashes(wtr io.Writer, prefix string, hashes tufmetadata.Hashes) {
 	}
 }
 
-func pprintTargetFile(wtr io.Writer, prefix string, repo *config.Repository, target *tufmetadata.TargetFiles) {
+func pprintTargetFile(wtr io.Writer, prefix string, repo *tufext.Repository, target *tufmetadata.TargetFiles) {
 	targetExt := tufext.TargetFilesExt(target)
 
 	mustFprintf(wtr, "%s%s:\n", prefix, target.Path)
@@ -98,11 +97,15 @@ func pprintTargetFile(wtr io.Writer, prefix string, repo *config.Repository, tar
 		mustFprintf(wtr, "%sInline data: %d bytes\n", prefix, len(data))
 	}
 	mustFprintf(wtr, "%sURL(s):\n", prefix)
-	for url, err := range targetExt.FetchURLs(&repo.DataRootURL.URL) {
-		if err != nil {
-			mustFprintf(wtr, "%s - <invalid target url: %v>\n", prefix, err)
+	if dataRootURL, err := repo.DataURL(); err != nil {
+		mustFprintf(wtr, "%s - <invalid repo data url: %v>\n", prefix, err)
+	} else {
+		for url, err := range targetExt.FetchURLs(dataRootURL) {
+			if err != nil {
+				mustFprintf(wtr, "%s - <invalid target url: %v>\n", prefix, err)
+			}
+			mustFprintf(wtr, "%s - %s\n", prefix, url)
 		}
-		mustFprintf(wtr, "%s - %s\n", prefix, url)
 	}
 	mustFprintf(wtr, "%sSize: %d\n", prefix, target.Length)
 	pprintHashes(wtr, prefix, target.Hashes)
@@ -113,7 +116,7 @@ func pprintTargetFile(wtr io.Writer, prefix string, repo *config.Repository, tar
 	// TODO(ext): UnrecognisedFields
 }
 
-func expandTargetFile(wtr io.Writer, fmtStr string, repo *config.Repository, target *tufmetadata.TargetFiles) error {
+func expandTargetFile(wtr io.Writer, fmtStr string, repo *tufext.Repository, target *tufmetadata.TargetFiles) error {
 	targetExt := tufext.TargetFilesExt(target)
 
 	expander := expand.NewExpansions().
@@ -131,7 +134,11 @@ func expandTargetFile(wtr io.Writer, fmtStr string, repo *config.Repository, tar
 			} else if data != nil {
 				return "data:;base64," + base64.StdEncoding.EncodeToString(data), nil
 			}
-			for url, err := range targetExt.FetchURLs(&repo.DataRootURL.URL) {
+			dataRootURL, err := repo.DataURL()
+			if err != nil {
+				return "", err
+			}
+			for url, err := range targetExt.FetchURLs(dataRootURL) {
 				var urlStr string
 				if url != nil {
 					urlStr = url.String()

internal/generics/error.go

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (C) 2026 Amutable GmbH
+
+package generics
+
+// TakeError can be used to wrap a function that returns (T, erorr) to extract
+// just the error in one line.
+func TakeError[T any](_ T, err error) error {
+	return err
+}

internal/jsonutils/jsonext.go

@@ -13,10 +13,10 @@ import (
 // when given a top-level structure that doesn't support UnrecognizedFields.
 var ErrNotExtensible = errors.New("json structure not extensible")
 
-// reParseJSON takes an arbitrary object and then re-parses as though it were
+// ReparseJSON takes an arbitrary object and then re-parses as though it were
 // JSON for the given type parameter. This is necessary to "cast" pre-parsed
 // any interfaces into something strongly typed.
-func reParseJSON[T any](data any) (T, error) {
+func ReparseJSON[T any](data any) (T, error) {
 	var (
 		encoded []byte
 		err     error
@@ -39,7 +39,7 @@ func GetExtensionJSON[T any](extStruct any, field string) (*T, error) {
 	// Get the set of structure fields as json.RawMessage so we can re-parse
 	// them slightly more efficiently and without triggering parsing errors for
 	// other fields.
-	structFields, err := reParseJSON[map[string]json.RawMessage](extStruct)
+	structFields, err := ReparseJSON[map[string]json.RawMessage](extStruct)
 	if err != nil {
 		return nil, fmt.Errorf("%w: re-parse %T as generic struct: %w", ErrNotExtensible, extStruct, err)
 	}
@@ -67,23 +67,23 @@ func SetExtensionJSON[W any](extStruct *W, field string, value any) (json.RawMes
 	if err != nil {
 		return nil, fmt.Errorf("marshal value %T: %w", value, err)
 	}
-	structFields, err := reParseJSON[map[string]json.RawMessage](*extStruct)
+	structFields, err := ReparseJSON[map[string]json.RawMessage](*extStruct)
 	if err != nil {
 		return nil, fmt.Errorf("%w: re-parse %T as generic struct: %w", ErrNotExtensible, *new(W), err)
 	}
 
 	var oldExtBytes json.RawMessage
 	oldExtBytes, structFields[field] = structFields[field], json.RawMessage(extBytes)
 
-	newExtStruct, err := reParseJSON[W](structFields)
+	newExtStruct, err := ReparseJSON[W](structFields)
 	if err != nil {
 		return nil, fmt.Errorf("%w: re-parse generic struct to %T: %w", ErrNotExtensible, *new(W), err)
 	}
 
 	// Make sure that round-tripping the new extension structure through
 	// encoding still includes the same extension fields. If not, then the
 	// struct doesn't support extensions of this form.
-	roundTripStructFields, err := reParseJSON[map[string]json.RawMessage](newExtStruct)
+	roundTripStructFields, err := ReparseJSON[map[string]json.RawMessage](newExtStruct)
 	if err != nil {
 		return nil, fmt.Errorf("%w: re-parse modified %T as generic struct: %w", ErrNotExtensible, *new(W), err)
 	}

internal/serde/doc.go

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (C) 2026 Amutable GmbH
+
+// Package serde provides a poor-mans scheme for serialising and deserialising
+// data in different formats.
+package serde

internal/serde/errors.go

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (C) 2026 Amutable GmbH
+
+package serde
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrMissingField is returned (usually wrapped) when a required field is
+// missing entirely. [Validator] implementations should wrap it when reporting
+// missing required fields.
+var ErrMissingField = errors.New("missing required field")
+
+// UnknownTypeError is returned when the tag of a [Union] value does not match
+// any of its variants.
+type UnknownTypeError struct {
+	Union string // [UnionOptions.Name]
+	Tag   string
+}
+
+func (err *UnknownTypeError) Error() string {
+	return fmt.Sprintf("unsupported %s type %q", err.Union, err.Tag)
+}
+
+// UnknownFieldError is returned when a variant of a [Union] is given a field
+// that it does not declare. Only the first unknown field is reported.
+type UnknownFieldError struct {
+	Union string // [UnionOptions.Name]
+	Tag   string
+	// Field is the name of the unknown field (for TOML, this is the full
+	// dotted key of the field).
+	Field string
+}
+
+func (err *UnknownFieldError) Error() string {
+	return fmt.Sprintf("%s %q has unsupported field %q", err.Union, err.Tag, err.Field)
+}