Key goproxy merge cache on owners and cache module misses

README.md

@@ -33,7 +33,7 @@ Each type's paths, Terraform examples and local/virtual behaviour are in `docs/<
 - `docker`: [remote](docs/docker/remote.md), [local](docs/docker/local.md)
 - `generic`: [remote](docs/generic/remote.md)
 - `gems`: [remote](docs/gems/remote.md), [local](docs/gems/local.md), [virtual](docs/gems/virtual.md)
-- `goproxy`: [remote](docs/goproxy/remote.md)
+- `goproxy`: [remote](docs/goproxy/remote.md), [local](docs/goproxy/local.md), [virtual](docs/goproxy/virtual.md)
 - `helm`: [remote](docs/helm/remote.md), [local](docs/helm/local.md), [virtual](docs/helm/virtual.md)
 - `npm`: [remote](docs/npm/remote.md), [local](docs/npm/local.md), [virtual](docs/npm/virtual.md)
 - `nuget`: [remote](docs/nuget/remote.md)

docs/goproxy/local.md

@@ -0,0 +1,20 @@
+# Local Go module proxy
+
+A local `goproxy` repo serves private modules over the GOPROXY protocol
+(`@v/list`, `@latest`, `@v/<ver>.info|.mod|.zip`). Publish a version by
+uploading its files under the case-encoded module path (uppercase as `!lower`):
+
+```sh
+for ext in zip mod info; do
+  curl -fsSL --upload-file v0.1.0.$ext \
+    https://artifactapi.k8s.syd1.au.unkin.net/api/v2/remotes/go-internal/files/example.unkin.net/hello/@v/v0.1.0.$ext
+done
+GOPROXY=https://artifactapi.k8s.syd1.au.unkin.net/api/v1/local/go-internal,https://proxy.golang.org,direct \
+GONOSUMDB=example.unkin.net go get example.unkin.net/hello@latest
+```
+
+Build the zip with `golang.org/x/mod/zip` or reuse the files `go mod download`
+leaves in `$(go env GOMODCACHE)/cache/download`. Uploads are validated: zips must
+pass `zip.CheckZip` (every file under `module@version/`), `.mod` must declare the
+module, and `.info` must match the version. `.info` is optional; it is
+synthesized from the version when absent. `@v/list` lists versions with a `.mod`.

docs/goproxy/virtual.md

@@ -0,0 +1,30 @@
+# `goproxy` virtual
+
+A goproxy virtual serves one GOPROXY over local and remote goproxy members:
+`GOPROXY=https://artifactapi.example.com/api/v1/virtual/<name>`.
+
+- `<module>/@v/list` is the union of member versions, sorted by semver.
+- `<module>/@latest` is the highest release in that union, else the highest
+  prerelease; with no listed versions it falls back to the first member that
+  answers `@latest`.
+- `@latest` and `@v/<version>.{info,mod,zip}` 302 to the first member, in
+  member order, that lists the version (or, for unlisted pseudo-versions,
+  serves its `.info`). Redirects are root-relative and `no-store`.
+- Module paths are case-encoded (uppercase as `!lower`).
+- A member error fails the request with 502; a member answering 404 or 410 is
+  skipped. Merged lists are cached for 60s.
+
+```hcl
+resource "artifactapi_virtual" "go" {
+  name         = "go"
+  package_type = "goproxy"
+  members      = [artifactapi_local_goproxy.internal.name, artifactapi_remote_goproxy.goproxy.name]
+}
+```
+
+```sh
+export GOPROXY=https://artifactapi.example.com/api/v1/virtual/go GONOSUMDB=example.unkin.net
+go get example.unkin.net/hello@latest golang.org/x/text@latest
+```
+
+Publish to the local member directly; the virtual is read-only.

e2e-docker/README.md

@@ -38,7 +38,9 @@ already-running stack.
   client (merged versions, install through the member redirect); gems compact
   index merge over two locals via stock `ruby:3.3` bundler (member order wins,
   downloads 302 to the owning member); ansible over two locals via stock
-  `ansible-galaxy` (install through the virtual, download 302 to the owner).
+  `ansible-galaxy` (install through the virtual, download 302 to the owner);
+  goproxy over two locals via a stock `golang:1.25` client (merged versions,
+  member order wins, version files 302 to the owning member).
 - **Mirrorlist** — an rpm remote with a `mirrorlist` of extra upstream mirrors
   (pool = `base_url` + `mirrorlist`): round-robin distribution across both mirrors
   (constant-body `mockupstreama` / `mockupstreamb`), failover past a dead primary,

e2e-docker/goproxy_local_test.go

@@ -0,0 +1,57 @@
+//go:build dockere2e
+
+package e2edocker
+
+import (
+	"bytes"
+	"net/http"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/mod/module"
+	modzip "golang.org/x/mod/zip"
+)
+
+// TestLocalGoProxy publishes two module versions and reads them back over the
+// GOPROXY protocol.
+func TestLocalGoProxy(t *testing.T) {
+	createRepo(t, `{"name":"local-go","package_type":"goproxy","repo_type":"local"}`)
+	defer deleteRepo(t, "local-go")
+
+	const mod = "example.unkin.net/!hello"
+	gomod := []byte("module example.unkin.net/Hello\n\ngo 1.25\n")
+	var zip010 []byte
+	for _, ver := range []string{"v0.1.0", "v0.2.0"} {
+		dir := t.TempDir()
+		os.WriteFile(filepath.Join(dir, "go.mod"), gomod, 0o600)
+		os.WriteFile(filepath.Join(dir, "hello.go"), []byte("package hello\n"), 0o600)
+		var buf bytes.Buffer
+		if err := modzip.CreateFromDir(&buf, module.Version{Path: "example.unkin.net/Hello", Version: ver}, dir); err != nil {
+			t.Fatal(err)
+		}
+		if ver == "v0.1.0" {
+			zip010 = buf.Bytes()
+		}
+		uploadFile(t, "local-go", mod+"/@v/"+ver+".zip", buf.Bytes(), "application/zip")
+		uploadFile(t, "local-go", mod+"/@v/"+ver+".mod", gomod, "text/plain")
+	}
+
+	resp, body := doRequest(t, http.MethodPut, api("/api/v2/remotes/local-go/files/"+mod+"/@v/v0.3.0.zip"), []byte("junk"), "application/zip")
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Fatalf("invalid zip: status %d: %s", resp.StatusCode, body)
+	}
+
+	for path, want := range map[string]string{
+		"/@v/list":        "v0.1.0\nv0.2.0\n",
+		"/@latest":        `{"Version":"v0.2.0"}`,
+		"/@v/v0.1.0.info": `{"Version":"v0.1.0"}`,
+		"/@v/v0.1.0.mod":  string(gomod),
+		"/@v/v0.1.0.zip":  string(zip010),
+	} {
+		resp, body := doRequest(t, http.MethodGet, api("/api/v1/local/local-go/"+mod+path), nil, "")
+		if resp.StatusCode != http.StatusOK || string(body) != want {
+			t.Errorf("%s: status %d body %q", path, resp.StatusCode, body)
+		}
+	}
+}

e2e-docker/goproxy_virtual_test.go

@@ -0,0 +1,81 @@
+//go:build dockere2e
+
+package e2edocker
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"golang.org/x/mod/module"
+	modzip "golang.org/x/mod/zip"
+)
+
+// TestVirtualGoProxy publishes to two local goproxy repos and drives a stock go
+// client through a virtual over both: merged versions, member order winning a
+// duplicate version, and version files redirected to the owning member.
+func TestVirtualGoProxy(t *testing.T) {
+	network := os.Getenv("COMPOSE_NETWORK")
+	internal := os.Getenv("ARTIFACTAPI_INTERNAL")
+	if network == "" || internal == "" {
+		t.Skip("COMPOSE_NETWORK/ARTIFACTAPI_INTERNAL not set; run via scripts/docker-e2e.sh")
+	}
+	if _, err := exec.LookPath("docker"); err != nil {
+		t.Skip("docker not available on the test host")
+	}
+
+	id := time.Now().UnixNano()
+	a, b, virt := fmt.Sprintf("e2e-go-a-%d", id), fmt.Sprintf("e2e-go-b-%d", id), fmt.Sprintf("e2e-go-v-%d", id)
+	for _, name := range []string{a, b} {
+		createRepo(t, fmt.Sprintf(`{"name":%q,"package_type":"goproxy","repo_type":"local"}`, name))
+		defer deleteRepo(t, name)
+	}
+	createVirtual(t, fmt.Sprintf(`{"name":%q,"package_type":"goproxy","members":[%q,%q]}`, virt, a, b))
+	defer deleteVirtual(t, virt)
+
+	const mod = "example.unkin.net/hello"
+	gomod := []byte("module " + mod + "\n\ngo 1.25\n")
+	for _, p := range [][2]string{{a, "v0.1.0"}, {b, "v0.1.0"}, {b, "v0.2.0"}} {
+		dir := t.TempDir()
+		os.WriteFile(filepath.Join(dir, "go.mod"), gomod, 0o600)
+		os.WriteFile(filepath.Join(dir, "hello.go"), []byte("package hello\n\nconst Greeting = \""+p[0]+":"+p[1]+"\"\n"), 0o600)
+		var buf bytes.Buffer
+		if err := modzip.CreateFromDir(&buf, module.Version{Path: mod, Version: p[1]}, dir); err != nil {
+			t.Fatal(err)
+		}
+		uploadFile(t, p[0], mod+"/@v/"+p[1]+".zip", buf.Bytes(), "application/zip")
+		uploadFile(t, p[0], mod+"/@v/"+p[1]+".mod", gomod, "text/plain")
+	}
+
+	base := strings.TrimRight(internal, "/")
+	script := `set -euo pipefail
+export GOPROXY="$BASE/api/v1/virtual/$V" GONOSUMDB=example.unkin.net GOFLAGS=-mod=mod GOTOOLCHAIN=local
+mkdir -p /w && cd /w
+printf 'module consumer\n\ngo 1.25\n' > go.mod
+printf 'package main\n\nimport "example.unkin.net/hello"\n\nfunc main() { println("RUN " + hello.Greeting) }\n' > main.go
+echo "VERSIONS $(go list -m -versions example.unkin.net/hello)"
+go get example.unkin.net/hello@latest && go run . 2>&1
+go get example.unkin.net/hello@v0.1.0 && go run . 2>&1
+echo "REDIRECT $(curl -s -o /dev/null -w '%{redirect_url}' "$GOPROXY/example.unkin.net/hello/@v/v0.1.0.zip")"`
+
+	out, err := exec.Command("docker", "run", "--rm", "--network", network,
+		"-e", "BASE="+base, "-e", "V="+virt, "golang:1.25", "bash", "-c", script).CombinedOutput()
+	if err != nil {
+		t.Fatalf("go run failed: %v\n%s", err, out)
+	}
+	for _, want := range []string{
+		"VERSIONS example.unkin.net/hello v0.1.0 v0.2.0",
+		"RUN " + b + ":v0.2.0",
+		"RUN " + a + ":v0.1.0",
+		"REDIRECT " + base + "/api/v1/local/" + a + "/example.unkin.net/hello/@v/v0.1.0.zip",
+	} {
+		if !strings.Contains(string(out), want) {
+			t.Fatalf("missing %q in go output:\n%s", want, out)
+		}
+	}
+}

go.mod

@@ -20,6 +20,7 @@ require (
 	github.com/testcontainers/testcontainers-go/modules/redis v0.42.0
 	github.com/ulikunitz/xz v0.5.16
 	golang.org/x/crypto v0.54.0
+	golang.org/x/mod v0.37.0
 	golang.org/x/sync v0.22.0
 	golang.org/x/time v0.15.0
 	gopkg.in/yaml.v3 v3.0.1

go.sum

@@ -227,6 +227,8 @@ golang.org/x/crypto v0.54.0 h1:YLIA59K4fiNzHzjnZt2tUJQjQtUWfWbeHBqKtk3eScw=
 golang.org/x/crypto v0.54.0/go.mod h1:KWL8ny2AZdGR2cWmzeHrp2azQPGogOv+HeQaVEXC2dk=
 golang.org/x/exp v0.0.0-20231006140011-7918f672742d h1:jtJma62tbqLibJ5sFQz8bKtEM8rJBtfilJ2qTU199MI=
 golang.org/x/exp v0.0.0-20231006140011-7918f672742d/go.mod h1:ldy0pHrwJyGW56pPQzzkH36rKxoZW1tw7ZJpeKx+hdo=
+golang.org/x/mod v0.37.0 h1:vF1DjpVEshcIqoEaauuHebaLk1O1forxjxBaVn884JQ=
+golang.org/x/mod v0.37.0/go.mod h1:m8S8VeM9r4dzDwjrKO0a1sZP3YjeMamRRlD+fmR2Q/0=
 golang.org/x/net v0.56.0 h1:Rw8j/hFzGvJUZwNBXnAtf5sVDVt+65SK2C7IxCxZt5o=
 golang.org/x/net v0.56.0/go.mod h1:D3Ku6r+V6JROoZK144D2XfMHFcMq/0zSfLelVTCFKec=
 golang.org/x/oauth2 v0.36.0 h1:peZ/1z27fi9hUOFCAZaHyrpWG5lwe0RJEEEeH0ThlIs=
@@ -247,6 +249,8 @@ golang.org/x/text v0.40.0 h1:Ub2Z6/xjgF1WrYQz2nuITOEegKFtiIy+rieRJ5lHZKs=
 golang.org/x/text v0.40.0/go.mod h1:hpnzDAfGV753zIKo+wk3u1bVKCGPbrnF7+7LBF/UHVY=
 golang.org/x/time v0.15.0 h1:bbrp8t3bGUeFOx08pvsMYRTCVSMk89u4tKbNOZbp88U=
 golang.org/x/time v0.15.0/go.mod h1:Y4YMaQmXwGQZoFaVFk4YpCt4FLQMYKZe9oeV/f4MSno=
+golang.org/x/tools v0.47.0 h1:7Kn5x/d1svx/PzryTsqeoZN4TZwqeH5pGWjefhLi/1Q=
+golang.org/x/tools v0.47.0/go.mod h1:dFHnyTvFWY212G+h7ZY4Vsp/K3U4/7W9TyVaAul8uCA=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=

internal/api/v1/goproxy_virtual.go

@@ -0,0 +1,33 @@
+package v1
+
+import (
+	"errors"
+	"log/slog"
+	"net/http"
+
+	"git.unkin.net/unkin/artifactapi/internal/virtual"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+// serveVirtualGoProxy serves the merged @v/list and 302s @latest and version
+// files to the member owning that version.
+func (h *ProxyHandler) serveVirtualGoProxy(w http.ResponseWriter, r *http.Request, virt models.Virtual, path string) {
+	body, loc, err := h.virtualEngine.GoProxy(r.Context(), virt, path, r.URL.RawQuery)
+	switch {
+	case errors.Is(err, virtual.ErrNotFound):
+		http.Error(w, "not found", http.StatusNotFound)
+	case errors.Is(err, virtual.ErrBadPath):
+		http.Error(w, err.Error(), http.StatusBadRequest)
+	case err != nil:
+		slog.Error("virtual goproxy fetch failed", "virtual", virt.Name, "path", path, "error", err)
+		http.Error(w, "bad gateway", http.StatusBadGateway)
+	case loc != "":
+		w.Header().Set("Cache-Control", "no-store")
+		http.Redirect(w, r, loc, http.StatusFound)
+	default:
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		w.Header().Set("X-Artifact-Source", "virtual")
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write(body)
+	}
+}

internal/api/v1/proxy.go

@@ -215,6 +215,10 @@ func (h *ProxyHandler) handleVirtual(w http.ResponseWriter, r *http.Request) {
 		h.serveVirtualAnsible(w, r, *virt, path, proxyBaseURL)
 		return
 	}
+	if virt.PackageType == models.PackageGoProxy {
+		h.serveVirtualGoProxy(w, r, *virt, path)
+		return
+	}
 
 	loc, ok, err := h.virtualEngine.MemberRedirect(r.Context(), *virt, path, r.URL.RawQuery, proxyBaseURL)
 	if err != nil {

internal/api/v2/local.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"os"
 
 	"github.com/go-chi/chi/v5"
 
@@ -83,7 +84,22 @@ func (h *LocalHandler) uploadValidated(w http.ResponseWriter, r *http.Request, r
 		return
 	}
 
-	result, err := h.cas.Store(r.Context(), r.Body, contentType)
+	var body io.Reader = r.Body
+	if cv, ok := prov.(provider.ContentValidator); ok {
+		f, err := spoolBody(r.Body)
+		if err != nil {
+			http.Error(w, fmt.Sprintf("read upload: %v", err), http.StatusInternalServerError)
+			return
+		}
+		defer discard(f)
+		if err := cv.ValidateContent(storagePath, f.Name()); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+		body = f
+	}
+
+	result, err := h.cas.Store(r.Context(), body, contentType)
 	if err != nil {
 		http.Error(w, fmt.Sprintf("store failed: %v", err), http.StatusInternalServerError)
 		return
@@ -110,6 +126,27 @@ func (h *LocalHandler) uploadValidated(w http.ResponseWriter, r *http.Request, r
 	writeJSON(w, http.StatusCreated, uploader.UploadResponse(storagePath, result.ContentHash, result.SizeBytes))
 }
 
+func spoolBody(r io.Reader) (*os.File, error) {
+	f, err := os.CreateTemp("", "artifactapi-upload-*")
+	if err != nil {
+		return nil, err
+	}
+	if _, err := io.Copy(f, r); err != nil {
+		discard(f)
+		return nil, err
+	}
+	if _, err := f.Seek(0, io.SeekStart); err != nil {
+		discard(f)
+		return nil, err
+	}
+	return f, nil
+}
+
+func discard(f *os.File) {
+	_ = f.Close()
+	_ = os.Remove(f.Name())
+}
+
 func (h *LocalHandler) uploadGeneric(w http.ResponseWriter, r *http.Request, remote *models.Remote, filePath string) {
 	existing, err := h.db.GetLocalFile(r.Context(), remote.Name, filePath)
 	if err != nil {

internal/api/v2/local_validate_test.go

@@ -0,0 +1,55 @@
+package v2
+
+import (
+	"context"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"git.unkin.net/unkin/artifactapi/internal/database"
+	_ "git.unkin.net/unkin/artifactapi/internal/provider/goproxy"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+// TestLocalUploadContentRejected verifies a ContentValidator provider rejects
+// bad bytes with 400 before anything is stored.
+func TestLocalUploadContentRejected(t *testing.T) {
+	if testDSN == "" {
+		t.Skip("Docker unavailable")
+	}
+	ctx := context.Background()
+	db, err := database.New(testDSN)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	const repo = "go-local-validate"
+	if err := db.CreateRemote(ctx, &models.Remote{Name: repo, PackageType: models.PackageGoProxy, RepoType: models.RepoTypeLocal}); err != nil {
+		t.Fatal(err)
+	}
+
+	router := chi.NewRouter()
+	router.Route("/remotes/{name}/files", func(r chi.Router) {
+		r.Put("/*", NewLocalHandler(db, nil).Routes().ServeHTTP)
+	})
+
+	cases := map[string]string{
+		"example.unkin.net/hello/@v/v0.1.0.zip":  "not a zip",
+		"example.unkin.net/hello/@v/v0.1.0.mod":  "module example.unkin.net/other\n",
+		"example.unkin.net/hello/@v/v0.1.0.info": `{"Version":"v9.9.9"}`,
+		"example.unkin.net/Hello/@v/v0.1.0.mod":  "module example.unkin.net/Hello\n",
+	}
+	for path, body := range cases {
+		w := httptest.NewRecorder()
+		router.ServeHTTP(w, httptest.NewRequest("PUT", "/remotes/"+repo+"/files/"+path, strings.NewReader(body)))
+		if w.Code != 400 {
+			t.Errorf("%s: status %d, want 400 (%s)", path, w.Code, w.Body.String())
+		}
+		if f, _ := db.GetLocalFile(ctx, repo, path); f != nil {
+			t.Errorf("%s: rejected upload was recorded", path)
+		}
+	}
+}

internal/provider/cap_content_validator.go

@@ -0,0 +1,6 @@
+package provider
+
+// ContentValidator checks an upload's bytes (spooled to localFile) before it is stored.
+type ContentValidator interface {
+	ValidateContent(storagePath, localFile string) error
+}

internal/provider/goproxy/local.go

@@ -0,0 +1,223 @@
+package goproxy
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"os"
+	"slices"
+	"strings"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+	modzip "golang.org/x/mod/zip"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+)
+
+type versionFile struct {
+	module  string
+	version string
+	ext     string
+}
+
+// parseVersionFile splits "<escaped module>/@v/<version>.<ext>" into its parts.
+func parseVersionFile(path string) (versionFile, error) {
+	escMod, file, ok := strings.Cut(strings.TrimPrefix(path, "/"), "/@v/")
+	if !ok || escMod == "" || strings.Contains(file, "/") {
+		return versionFile{}, fmt.Errorf("path %q must be <module>/@v/<version>.{zip,mod,info}", path)
+	}
+	dot := strings.LastIndex(file, ".")
+	if dot <= 0 {
+		return versionFile{}, fmt.Errorf("path %q must end in .zip, .mod or .info", path)
+	}
+	ext := file[dot+1:]
+	if ext != "zip" && ext != "mod" && ext != "info" {
+		return versionFile{}, fmt.Errorf("path %q must end in .zip, .mod or .info", path)
+	}
+	mod, err := module.UnescapePath(escMod)
+	if err != nil {
+		return versionFile{}, fmt.Errorf("module path must be case-encoded (uppercase as !lower): %w", err)
+	}
+	ver, err := module.UnescapeVersion(file[:dot])
+	if err != nil {
+		return versionFile{}, err
+	}
+	if err := module.Check(mod, ver); err != nil {
+		return versionFile{}, err
+	}
+	if semver.Canonical(ver) != ver {
+		return versionFile{}, fmt.Errorf("version %q is not canonical semver", ver)
+	}
+	return versionFile{module: mod, version: ver, ext: ext}, nil
+}
+
+func (v versionFile) storagePath() string {
+	escMod, _ := module.EscapePath(v.module)
+	escVer, _ := module.EscapeVersion(v.version)
+	return escMod + "/@v/" + escVer + "." + v.ext
+}
+
+func (p *Provider) ValidateUpload(filePath string) (storagePath, contentType string, err error) {
+	vf, err := parseVersionFile(filePath)
+	if err != nil {
+		return "", "", err
+	}
+	sp := vf.storagePath()
+	return sp, p.ContentType(sp), nil
+}
+
+func (p *Provider) ValidateContent(storagePath, localFile string) error {
+	vf, err := parseVersionFile(storagePath)
+	if err != nil {
+		return err
+	}
+	switch vf.ext {
+	case "zip":
+		if _, err := modzip.CheckZip(module.Version{Path: vf.module, Version: vf.version}, localFile); err != nil {
+			return fmt.Errorf("invalid module zip (files must sit under %s@%s/): %w", vf.module, vf.version, err)
+		}
+	case "mod":
+		data, err := os.ReadFile(localFile)
+		if err != nil {
+			return err
+		}
+		mf, err := modfile.ParseLax("go.mod", data, nil)
+		if err != nil {
+			return fmt.Errorf("invalid go.mod: %w", err)
+		}
+		if mf.Module == nil || mf.Module.Mod.Path != vf.module {
+			return fmt.Errorf("go.mod must declare module %s", vf.module)
+		}
+	case "info":
+		data, err := os.ReadFile(localFile)
+		if err != nil {
+			return err
+		}
+		var info struct{ Version string }
+		if err := json.Unmarshal(data, &info); err != nil {
+			return fmt.Errorf("invalid .info JSON: %w", err)
+		}
+		if info.Version != vf.version {
+			return fmt.Errorf(".info Version %q does not match %s", info.Version, vf.version)
+		}
+	}
+	return nil
+}
+
+func (p *Provider) UploadResponse(storagePath, contentHash string, sizeBytes int64) map[string]any {
+	vf, _ := parseVersionFile(storagePath)
+	return map[string]any{
+		"module":       vf.module,
+		"version":      vf.version,
+		"path":         storagePath,
+		"content_hash": contentHash,
+		"size_bytes":   sizeBytes,
+	}
+}
+
+// versions returns the canonical versions of escMod that have a .mod uploaded, sorted ascending.
+func versions(ctx context.Context, files provider.FileStore, repoName, escMod string) (versions []string, infos map[string]bool, err error) {
+	entries, err := files.ListFilesByPrefix(ctx, repoName, escMod+"/@v/")
+	if err != nil {
+		return nil, nil, err
+	}
+	infos = map[string]bool{}
+	for _, e := range entries {
+		vf, err := parseVersionFile(e.FilePath)
+		if err != nil || vf.storagePath() != e.FilePath {
+			continue
+		}
+		switch vf.ext {
+		case "mod":
+			versions = append(versions, vf.version)
+		case "info":
+			infos[vf.version] = true
+		}
+	}
+	semver.Sort(versions)
+	return versions, infos, nil
+}
+
+// Latest is the highest release in sorted vs, else the highest prerelease.
+func Latest(vs []string) string {
+	for i := len(vs) - 1; i >= 0; i-- {
+		if semver.Prerelease(vs[i]) == "" {
+			return vs[i]
+		}
+	}
+	if len(vs) == 0 {
+		return ""
+	}
+	return vs[len(vs)-1]
+}
+
+func infoJSON(version string) []byte {
+	b, _ := json.Marshal(map[string]string{"Version": version})
+	return b
+}
+
+func (p *Provider) ServeLocalIndex(w http.ResponseWriter, r *http.Request, files provider.FileStore, repoName, path string) bool {
+	path = strings.TrimPrefix(path, "/")
+	switch {
+	case strings.HasSuffix(path, "/@v/list"), strings.HasSuffix(path, "/@latest"):
+	case strings.HasSuffix(path, ".info"):
+		vf, err := parseVersionFile(path)
+		if err != nil {
+			return false
+		}
+		escMod, _ := module.EscapePath(vf.module)
+		vs, infos, err := versions(r.Context(), files, repoName, escMod)
+		if err != nil || infos[vf.version] || !slices.Contains(vs, vf.version) {
+			return false
+		}
+	default:
+		return false
+	}
+
+	body, err := p.GenerateLocalIndex(r.Context(), files, repoName, path)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusNotFound)
+		return true
+	}
+	w.Header().Set("Content-Type", p.ContentType(path))
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write(body)
+	return true
+}
+
+func (p *Provider) GenerateLocalIndex(ctx context.Context, files provider.FileStore, repoName, path string) ([]byte, error) {
+	path = strings.TrimPrefix(path, "/")
+	if escMod, ok := strings.CutSuffix(path, "/@v/list"); ok {
+		vs, _, err := versions(ctx, files, repoName, escMod)
+		if err != nil {
+			return nil, err
+		}
+		var b strings.Builder
+		for _, v := range vs {
+			b.WriteString(v + "\n")
+		}
+		return []byte(b.String()), nil
+	}
+	if escMod, ok := strings.CutSuffix(path, "/@latest"); ok {
+		vs, _, err := versions(ctx, files, repoName, escMod)
+		if err != nil {
+			return nil, err
+		}
+		v := Latest(vs)
+		if v == "" {
+			return nil, fmt.Errorf("no versions of %s", escMod)
+		}
+		return infoJSON(v), nil
+	}
+	if strings.HasSuffix(path, ".info") {
+		vf, err := parseVersionFile(path)
+		if err != nil {
+			return nil, err
+		}
+		return infoJSON(vf.version), nil
+	}
+	return nil, fmt.Errorf("unsupported index path: %q", path)
+}

internal/provider/goproxy/local_test.go

@@ -0,0 +1,198 @@
+package goproxy
+
+import (
+	"archive/zip"
+	"context"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"golang.org/x/mod/module"
+	modzip "golang.org/x/mod/zip"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+)
+
+type fakeFileStore struct{ paths []string }
+
+func (f *fakeFileStore) ListPackages(context.Context, string) ([]string, error) { return nil, nil }
+
+func (f *fakeFileStore) ListFilesByPrefix(_ context.Context, _, prefix string) ([]provider.FileEntry, error) {
+	var out []provider.FileEntry
+	for _, p := range f.paths {
+		if strings.HasPrefix(p, prefix) {
+			out = append(out, provider.FileEntry{FilePath: p, ContentHash: "sha256:x"})
+		}
+	}
+	return out, nil
+}
+
+func TestValidateUpload(t *testing.T) {
+	p := &Provider{}
+	cases := []struct {
+		in, want string
+		ok       bool
+	}{
+		{"example.unkin.net/hello/@v/v0.1.0.zip", "example.unkin.net/hello/@v/v0.1.0.zip", true},
+		{"/example.unkin.net/hello/@v/v0.1.0.mod", "example.unkin.net/hello/@v/v0.1.0.mod", true},
+		{"github.com/!burnt!sushi/toml/@v/v1.2.0.info", "github.com/!burnt!sushi/toml/@v/v1.2.0.info", true},
+		{"example.unkin.net/hello/v2/@v/v2.0.0-rc.1.zip", "example.unkin.net/hello/v2/@v/v2.0.0-rc.1.zip", true},
+		{"github.com/BurntSushi/toml/@v/v1.2.0.zip", "", false},
+		{"example.unkin.net/hello/@v/v0.1.zip", "", false},
+		{"example.unkin.net/hello/@v/v2.0.0.zip", "", false},
+		{"example.unkin.net/hello/@v/v0.1.0.tar.gz", "", false},
+		{"example.unkin.net/hello/v0.1.0.zip", "", false},
+		{"example.unkin.net/hello/@v/list", "", false},
+	}
+	for _, c := range cases {
+		got, ct, err := p.ValidateUpload(c.in)
+		if (err == nil) != c.ok {
+			t.Errorf("%s: err=%v, want ok=%v", c.in, err, c.ok)
+			continue
+		}
+		if c.ok && (got != c.want || ct == "") {
+			t.Errorf("%s: got %q %q, want %q", c.in, got, ct, c.want)
+		}
+	}
+}
+
+func writeZip(t *testing.T, files map[string]string) string {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "m.zip")
+	f, err := os.Create(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	zw := zip.NewWriter(f)
+	for name, body := range files {
+		w, err := zw.Create(name)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if _, err := w.Write([]byte(body)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := zw.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
+
+func writeFile(t *testing.T, body string) string {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "f")
+	if err := os.WriteFile(path, []byte(body), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
+
+func TestValidateContentZip(t *testing.T) {
+	p := &Provider{}
+	sp := "example.unkin.net/hello/@v/v0.1.0.zip"
+
+	dir := t.TempDir()
+	os.WriteFile(filepath.Join(dir, "go.mod"), []byte("module example.unkin.net/hello\n"), 0o600)
+	os.WriteFile(filepath.Join(dir, "main.go"), []byte("package main\n"), 0o600)
+	good := filepath.Join(t.TempDir(), "good.zip")
+	f, _ := os.Create(good)
+	if err := modzip.CreateFromDir(f, module.Version{Path: "example.unkin.net/hello", Version: "v0.1.0"}, dir); err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+	if err := p.ValidateContent(sp, good); err != nil {
+		t.Errorf("valid zip rejected: %v", err)
+	}
+
+	bad := map[string]map[string]string{
+		"no prefix":     {"go.mod": "module example.unkin.net/hello\n"},
+		"wrong version": {"example.unkin.net/hello@v0.2.0/go.mod": "module example.unkin.net/hello\n"},
+		"wrong module":  {"example.unkin.net/other@v0.1.0/go.mod": "module example.unkin.net/other\n"},
+	}
+	for name, files := range bad {
+		if err := p.ValidateContent(sp, writeZip(t, files)); err == nil {
+			t.Errorf("%s: expected rejection", name)
+		}
+	}
+	if err := p.ValidateContent(sp, writeFile(t, "not a zip")); err == nil {
+		t.Error("non-zip accepted")
+	}
+}
+
+func TestValidateContentModInfo(t *testing.T) {
+	p := &Provider{}
+	mod := "example.unkin.net/hello/@v/v0.1.0.mod"
+	info := "example.unkin.net/hello/@v/v0.1.0.info"
+	cases := []struct {
+		path, body string
+		ok         bool
+	}{
+		{mod, "module example.unkin.net/hello\n\ngo 1.25\n", true},
+		{mod, "module example.unkin.net/other\n", false},
+		{mod, "go 1.25\n", false},
+		{mod, "module (\n", false},
+		{info, `{"Version":"v0.1.0","Time":"2026-01-01T00:00:00Z"}`, true},
+		{info, `{"Version":"v0.2.0"}`, false},
+		{info, `nope`, false},
+	}
+	for _, c := range cases {
+		if err := p.ValidateContent(c.path, writeFile(t, c.body)); (err == nil) != c.ok {
+			t.Errorf("%s %q: err=%v, want ok=%v", c.path, c.body, err, c.ok)
+		}
+	}
+}
+
+func serve(t *testing.T, fs provider.FileStore, path string) (bool, int, string) {
+	t.Helper()
+	rec := httptest.NewRecorder()
+	handled := (&Provider{}).ServeLocalIndex(rec, httptest.NewRequest("GET", "/"+path, nil), fs, "go-local", path)
+	return handled, rec.Code, rec.Body.String()
+}
+
+func TestServeLocalIndex(t *testing.T) {
+	fs := &fakeFileStore{paths: []string{
+		"example.unkin.net/hello/@v/v0.10.0.mod",
+		"example.unkin.net/hello/@v/v0.10.0.zip",
+		"example.unkin.net/hello/@v/v0.2.0.mod",
+		"example.unkin.net/hello/@v/v0.2.0.zip",
+		"example.unkin.net/hello/@v/v0.2.0.info",
+		"example.unkin.net/hello/@v/v0.11.0-rc.1.mod",
+		"example.unkin.net/hello/@v/v0.3.0.zip",
+		"example.unkin.net/hello/v2/@v/v2.0.0.mod",
+		"example.unkin.net/!pre/@v/v1.0.0-beta.mod",
+	}}
+
+	if h, code, body := serve(t, fs, "example.unkin.net/hello/@v/list"); !h || code != 200 || body != "v0.2.0\nv0.10.0\nv0.11.0-rc.1\n" {
+		t.Errorf("list: %v %d %q", h, code, body)
+	}
+	if h, code, body := serve(t, fs, "example.unkin.net/hello/@latest"); !h || code != 200 || body != `{"Version":"v0.10.0"}` {
+		t.Errorf("latest: %v %d %q", h, code, body)
+	}
+	if h, _, body := serve(t, fs, "example.unkin.net/!pre/@latest"); !h || body != `{"Version":"v1.0.0-beta"}` {
+		t.Errorf("prerelease-only latest: %q", body)
+	}
+	if h, code, _ := serve(t, fs, "example.unkin.net/missing/@latest"); !h || code != 404 {
+		t.Errorf("missing latest: %v %d", h, code)
+	}
+	if h, code, body := serve(t, fs, "example.unkin.net/missing/@v/list"); !h || code != 200 || body != "" {
+		t.Errorf("missing list: %v %d %q", h, code, body)
+	}
+	if h, _, body := serve(t, fs, "example.unkin.net/hello/@v/v0.10.0.info"); !h || body != `{"Version":"v0.10.0"}` {
+		t.Errorf("synthesized info: %v %q", h, body)
+	}
+	if h, _, _ := serve(t, fs, "example.unkin.net/hello/@v/v0.2.0.info"); h {
+		t.Error("uploaded .info must be served from storage")
+	}
+	if h, _, _ := serve(t, fs, "example.unkin.net/hello/@v/v0.3.0.info"); h {
+		t.Error(".info without a .mod must not be synthesized")
+	}
+	if h, _, _ := serve(t, fs, "example.unkin.net/hello/@v/v0.10.0.zip"); h {
+		t.Error(".zip must be served from storage")
+	}
+}

internal/virtual/engine.go

@@ -43,6 +43,8 @@ type Engine struct {
 	AnsibleLocal func(ctx context.Context, repo, path string) ([]byte, error)
 	ansibleDoc   func(ctx context.Context, remote models.Remote, path string) ([]byte, error)
 
+	goMember func(ctx context.Context, name, path string) (*GoMember, error)
+
 	mergeTTL time.Duration
 
 	sf   singleflight.Group
@@ -74,6 +76,7 @@ func NewEngine(db *database.DB, proxyEngine *proxy.Engine) *Engine {
 	e.npmMember = e.fetchNPMMember
 	e.gemsMember = e.fetchGemsMember
 	e.ansibleDoc = e.ansibleMemberDoc
+	e.goMember = e.fetchGoMember
 	return e
 }
 
@@ -428,6 +431,9 @@ func (e *Engine) fetchMemberPath(ctx context.Context, remote models.Remote, path
 	if serve != nil {
 		rec := httptest.NewRecorder()
 		if serve(rec, httptest.NewRequestWithContext(ctx, http.MethodGet, "/"+path, nil)) {
+			if rec.Code == http.StatusNotFound || rec.Code == http.StatusGone {
+				return nil, fmt.Errorf("%s/%s: %w", remote.Name, path, ErrNotFound)
+			}
 			if rec.Code != http.StatusOK {
 				return nil, fmt.Errorf("%s/%s: status %d", remote.Name, path, rec.Code)
 			}

internal/virtual/goproxy_merger.go

@@ -0,0 +1,228 @@
+package virtual
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"maps"
+	"net/http"
+	"slices"
+	"strings"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider/goproxy"
+	"git.unkin.net/unkin/artifactapi/internal/proxy"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+const maxGoModules = 1024
+
+// GoMember is one member's response for a module path; Body is nil when the
+// member does not have it (404/410).
+type GoMember struct {
+	RemoteName string
+	Body       []byte
+}
+
+// GoDoc is a goproxy virtual's merged @v/list for one module. Owners maps each
+// version to the first member listing it; missing caches that no member has it.
+type GoDoc struct {
+	Body    []byte
+	Owners  map[string]string
+	Latest  string
+	missing bool
+}
+
+func (d *GoDoc) index() []byte {
+	if d.missing {
+		return nil
+	}
+	b := append(slices.Clone(d.Body), 0)
+	for _, v := range slices.Sorted(maps.Keys(d.Owners)) {
+		b = append(b, v+"="+d.Owners[v]+"\n"...)
+	}
+	return b
+}
+
+func (d *GoDoc) files() map[string][]byte { return nil }
+
+// MergeGoList unions member @v/list documents, sorted by semver; the first
+// member listing a version owns it. ErrNotFound means no member has the module.
+func MergeGoList(members []GoMember) (*GoDoc, error) {
+	d := &GoDoc{Owners: map[string]string{}}
+	found := false
+	var vs []string
+	for _, m := range members {
+		if m.Body == nil {
+			continue
+		}
+		found = true
+		for line := range strings.Lines(string(m.Body)) {
+			f := strings.Fields(line)
+			if len(f) == 0 || !semver.IsValid(f[0]) {
+				continue
+			}
+			if _, dup := d.Owners[f[0]]; dup {
+				continue
+			}
+			d.Owners[f[0]] = m.RemoteName
+			vs = append(vs, f[0])
+		}
+	}
+	if !found {
+		return nil, ErrNotFound
+	}
+	semver.Sort(vs)
+	for _, v := range vs {
+		d.Body = append(d.Body, v+"\n"...)
+	}
+	if d.Body == nil {
+		d.Body = []byte{}
+	}
+	d.Latest = goproxy.Latest(vs)
+	return d, nil
+}
+
+// GoProxy serves a goproxy virtual path: the merged <module>/@v/list body, or a
+// root-relative redirect for @latest and @v/<version>.{info,mod,zip} to the
+// first member, in member order, that has that version.
+func (e *Engine) GoProxy(ctx context.Context, virt models.Virtual, path, rawQuery string) (body []byte, loc string, err error) {
+	escMod, rest, ok := strings.Cut(path, "/@")
+	if !ok {
+		return nil, "", ErrNotFound
+	}
+	if _, err := module.UnescapePath(escMod); err != nil {
+		return nil, "", ErrNotFound
+	}
+	if rest == "v/list" {
+		d, err := e.goList(ctx, virt, escMod)
+		if err != nil {
+			return nil, "", err
+		}
+		return d.Body, "", nil
+	}
+
+	var target, owner string
+	var d *GoDoc
+	switch {
+	case rest == "latest":
+		if d, err = e.goList(ctx, virt, escMod); err != nil {
+			return nil, "", err
+		}
+		if d.Latest == "" {
+			target = path
+			owner, err = e.goProbe(ctx, virt, target)
+			break
+		}
+		escVer, _ := module.EscapeVersion(d.Latest)
+		target, owner = escMod+"/@v/"+escVer+".info", d.Owners[d.Latest]
+	case strings.HasPrefix(rest, "v/"):
+		escVer, ext, ok := cutGoExt(rest[len("v/"):])
+		if !ok {
+			return nil, "", ErrNotFound
+		}
+		ver, verr := module.UnescapeVersion(escVer)
+		if verr != nil {
+			return nil, "", ErrNotFound
+		}
+		if d, err = e.goList(ctx, virt, escMod); err != nil && !errors.Is(err, ErrNotFound) {
+			return nil, "", err
+		}
+		if d != nil {
+			owner = d.Owners[ver]
+		}
+		if owner == "" {
+			owner, err = e.goProbe(ctx, virt, escMod+"/@v/"+escVer+".info")
+		}
+		target = escMod + "/@v/" + escVer + "." + ext
+	default:
+		return nil, "", ErrNotFound
+	}
+	if err != nil {
+		return nil, "", err
+	}
+	loc, ok, err = e.memberLocation(ctx, owner, target, "", "")
+	if err != nil || !ok {
+		return nil, "", errors.Join(err, ErrNotFound)
+	}
+	// module paths and versions are validated, so the only escape is the case-encoding '!'
+	loc = strings.ReplaceAll(loc, "%21", "!")
+	if rawQuery != "" {
+		loc += "?" + rawQuery
+	}
+	return nil, loc, nil
+}
+
+func cutGoExt(file string) (escVer, ext string, ok bool) {
+	dot := strings.LastIndex(file, ".")
+	if dot <= 0 || strings.Contains(file, "/") {
+		return "", "", false
+	}
+	switch ext = file[dot+1:]; ext {
+	case "info", "mod", "zip":
+		return file[:dot], ext, true
+	}
+	return "", "", false
+}
+
+func (e *Engine) goList(ctx context.Context, virt models.Virtual, escMod string) (*GoDoc, error) {
+	prefix := "goproxy\x00" + virt.Name + "\x00"
+	inserted := false
+	repo, err := e.merged(ctx, prefix+escMod, func(ctx context.Context) (mergedRepo, error) {
+		inserted = true
+		members, err := fetchMembers(virt, func(name string) (*GoMember, error) { return e.goMember(ctx, name, escMod+"/@v/list") })
+		if err != nil {
+			return nil, err
+		}
+		d, err := MergeGoList(members)
+		if errors.Is(err, ErrNotFound) {
+			return &GoDoc{missing: true}, nil
+		}
+		return d, err
+	})
+	if err != nil {
+		return nil, err
+	}
+	if inserted {
+		e.evictOldest(prefix, maxGoModules)
+	}
+	if d := repo.(*GoDoc); !d.missing {
+		return d, nil
+	}
+	return nil, ErrNotFound
+}
+
+// goProbe returns the first member, in member order, that serves path.
+func (e *Engine) goProbe(ctx context.Context, virt models.Virtual, path string) (string, error) {
+	for _, name := range virt.Members {
+		m, err := e.goMember(ctx, name, path)
+		if err != nil {
+			return "", fmt.Errorf("virtual %q: member %q: %v", virt.Name, name, err)
+		}
+		if m.Body != nil {
+			return name, nil
+		}
+	}
+	return "", ErrNotFound
+}
+
+func (e *Engine) fetchGoMember(ctx context.Context, name, path string) (*GoMember, error) {
+	remote, err := e.getRemote(ctx, name)
+	if err != nil {
+		return nil, fmt.Errorf("remote %q: %w", name, err)
+	}
+	body, err := e.fetchMemberPath(ctx, *remote, path)
+	var pe *proxy.ProxyError
+	if errors.Is(err, ErrNotFound) || errors.As(err, &pe) && (pe.Status == http.StatusNotFound || pe.Status == http.StatusGone) {
+		return &GoMember{RemoteName: name}, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	if body == nil {
+		body = []byte{}
+	}
+	return &GoMember{RemoteName: name, Body: body}, nil
+}

internal/virtual/goproxy_merger_test.go

@@ -0,0 +1,183 @@
+package virtual
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"testing"
+	"time"
+
+	"git.unkin.net/unkin/artifactapi/internal/database"
+	"git.unkin.net/unkin/artifactapi/internal/testsupport"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func TestMergeGoList(t *testing.T) {
+	d, err := MergeGoList([]GoMember{
+		{RemoteName: "internal", Body: []byte("v0.2.0\nv0.1.0\n")},
+		{RemoteName: "absent"},
+		{RemoteName: "goproxy", Body: []byte("v0.1.0 extra\nv1.0.0-rc.1\n\nnot-a-version\nv0.3.0\n")},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := "v0.1.0\nv0.2.0\nv0.3.0\nv1.0.0-rc.1\n"; string(d.Body) != want {
+		t.Fatalf("list = %q, want %q", d.Body, want)
+	}
+	if d.Latest != "v0.3.0" {
+		t.Fatalf("latest = %q, want the highest release over the prerelease", d.Latest)
+	}
+	for v, owner := range map[string]string{"v0.1.0": "internal", "v0.2.0": "internal", "v0.3.0": "goproxy", "v1.0.0-rc.1": "goproxy"} {
+		if d.Owners[v] != owner {
+			t.Errorf("owner of %s = %q, want %q", v, d.Owners[v], owner)
+		}
+	}
+
+	d, _ = MergeGoList([]GoMember{{RemoteName: "a", Body: []byte("v1.0.0-beta\nv1.0.0-alpha\n")}})
+	if d.Latest != "v1.0.0-beta" {
+		t.Fatalf("latest with only prereleases = %q", d.Latest)
+	}
+	if d, err := MergeGoList([]GoMember{{RemoteName: "a", Body: []byte{}}}); err != nil || string(d.Body) != "" || d.Latest != "" {
+		t.Fatalf("empty list must merge to an empty 200 body: %v", err)
+	}
+	if _, err := MergeGoList([]GoMember{{RemoteName: "a"}, {RemoteName: "b"}}); !errors.Is(err, ErrNotFound) {
+		t.Fatalf("want ErrNotFound, got %v", err)
+	}
+}
+
+func goEngine(docs map[string]string, fail string) (*Engine, map[string]int) {
+	var mu sync.Mutex
+	asked := map[string]int{}
+	return &Engine{
+		mergeTTL: time.Minute,
+		getRemote: func(_ context.Context, name string) (*models.Remote, error) {
+			rt := models.RepoTypeRemote
+			if name == "internal" {
+				rt = models.RepoTypeLocal
+			}
+			return &models.Remote{Name: name, RepoType: rt}, nil
+		},
+		goMember: func(_ context.Context, name, path string) (*GoMember, error) {
+			mu.Lock()
+			asked[name+" "+path]++
+			mu.Unlock()
+			if name == fail {
+				return nil, errors.New("upstream down")
+			}
+			if d, ok := docs[name+" "+path]; ok {
+				return &GoMember{RemoteName: name, Body: []byte(d)}, nil
+			}
+			return &GoMember{RemoteName: name}, nil
+		},
+	}, asked
+}
+
+func goVirt() models.Virtual {
+	return models.Virtual{Name: "go", PackageType: models.PackageGoProxy, Members: []string{"internal", "goproxy"}}
+}
+
+func TestGoProxyRedirectsToFirstOwner(t *testing.T) {
+	e, asked := goEngine(map[string]string{
+		"internal example.unkin.net/!hello/@v/list":                       "v0.1.0\nv0.2.0\n",
+		"goproxy example.unkin.net/!hello/@v/list":                        "v0.2.0\nv0.3.0-rc.1\n",
+		"goproxy golang.org/x/text/@v/list":                               "v0.1.0\n",
+		"goproxy example.unkin.net/!hello/@v/v0.0.0-20260101-abcdef.info": "{}",
+		"goproxy example.com/notags/@v/list":                              "",
+		"goproxy example.com/notags/@latest":                              "{}",
+	}, "")
+	virt := goVirt()
+	body, loc, err := e.GoProxy(context.Background(), virt, "example.unkin.net/!hello/@v/list", "")
+	if err != nil || loc != "" || string(body) != "v0.1.0\nv0.2.0\nv0.3.0-rc.1\n" {
+		t.Fatalf("list = %q %q %v", body, loc, err)
+	}
+	for path, want := range map[string]string{
+		"example.unkin.net/!hello/@v/v0.2.0.zip":                 "/api/v1/local/internal/example.unkin.net/!hello/@v/v0.2.0.zip",
+		"example.unkin.net/!hello/@v/v0.3.0-rc.1.mod":            "/api/v1/remote/goproxy/example.unkin.net/!hello/@v/v0.3.0-rc.1.mod",
+		"example.unkin.net/!hello/@latest":                       "/api/v1/local/internal/example.unkin.net/!hello/@v/v0.2.0.info",
+		"example.unkin.net/!hello/@v/v0.0.0-20260101-abcdef.zip": "/api/v1/remote/goproxy/example.unkin.net/!hello/@v/v0.0.0-20260101-abcdef.zip",
+		"golang.org/x/text/@v/v0.1.0.info":                       "/api/v1/remote/goproxy/golang.org/x/text/@v/v0.1.0.info",
+		"example.com/notags/@latest":                             "/api/v1/remote/goproxy/example.com/notags/@latest",
+	} {
+		_, loc, err := e.GoProxy(context.Background(), virt, path, "")
+		if err != nil || loc != want {
+			t.Errorf("%s -> %q %v, want %q", path, loc, err, want)
+		}
+	}
+	for _, path := range []string{
+		"example.unkin.net/!hello/@v/v9.9.9.zip",
+		"example.unkin.net/Hello/@v/list",
+		"example.unkin.net/!hello/@v/v0.1.0.tar",
+		"example.unkin.net/!hello/@v/../v0.1.0.zip",
+		"example.unkin.net/!hello",
+		"missing.example/mod/@v/list",
+		"missing.example/mod/@latest",
+	} {
+		if _, _, err := e.GoProxy(context.Background(), virt, path, ""); !errors.Is(err, ErrNotFound) {
+			t.Errorf("%s: want ErrNotFound, got %v", path, err)
+		}
+	}
+	if n := asked["internal example.unkin.net/!hello/@v/list"]; n != 1 {
+		t.Fatalf("merged list must be cached, internal asked %d times", n)
+	}
+	if n := asked["goproxy missing.example/mod/@v/list"]; n != 1 {
+		t.Fatalf("a module no member has must be cached, goproxy asked %d times", n)
+	}
+}
+
+func TestGoProxyFollowsOwnerChange(t *testing.T) {
+	docs := map[string]string{"internal example.com/mod/@v/list": "v0.1.0\n"}
+	e, _ := goEngine(docs, "")
+	e.mergeTTL = 0
+	zip := func() string {
+		t.Helper()
+		_, loc, err := e.GoProxy(context.Background(), goVirt(), "example.com/mod/@v/v0.1.0.zip", "")
+		if err != nil {
+			t.Fatal(err)
+		}
+		return loc
+	}
+	if loc := zip(); loc != "/api/v1/local/internal/example.com/mod/@v/v0.1.0.zip" {
+		t.Fatalf("before move: %q", loc)
+	}
+	delete(docs, "internal example.com/mod/@v/list")
+	docs["goproxy example.com/mod/@v/list"] = "v0.1.0\n"
+	if loc := zip(); loc != "/api/v1/remote/goproxy/example.com/mod/@v/v0.1.0.zip" {
+		t.Fatalf("same list, new owner: redirect still %q", loc)
+	}
+}
+
+func TestGoProxyFailsClosed(t *testing.T) {
+	e, _ := goEngine(map[string]string{"internal example.unkin.net/hello/@v/list": "v0.1.0\n"}, "goproxy")
+	for _, path := range []string{"example.unkin.net/hello/@v/list", "example.unkin.net/hello/@latest", "example.unkin.net/hello/@v/v0.1.0.zip"} {
+		if _, _, err := e.GoProxy(context.Background(), goVirt(), path, ""); err == nil || errors.Is(err, ErrNotFound) {
+			t.Errorf("%s: want member error, got %v", path, err)
+		}
+	}
+}
+
+func TestGoProxyLocalMemberNotFound(t *testing.T) {
+	ctx := context.Background()
+	dsn, terminate, err := testsupport.StartPostgres(ctx)
+	if err != nil {
+		t.Skip("Docker unavailable")
+	}
+	defer terminate()
+	db, err := database.New(dsn)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	local := models.Remote{Name: "internal", PackageType: models.PackageGoProxy, RepoType: models.RepoTypeLocal}
+	if err := db.CreateRemote(ctx, &local); err != nil {
+		t.Fatal(err)
+	}
+
+	e := NewEngine(db, nil)
+	if _, err := e.fetchMemberPath(ctx, local, "example.com/mod/@latest"); !errors.Is(err, ErrNotFound) {
+		t.Fatalf("local 404 must be ErrNotFound, got %v", err)
+	}
+	virt := models.Virtual{Name: "go", PackageType: models.PackageGoProxy, Members: []string{"internal"}}
+	if _, _, err := e.GoProxy(ctx, virt, "example.com/mod/@latest", ""); !errors.Is(err, ErrNotFound) {
+		t.Fatalf("module only absent from a local member must be not-found, got %v", err)
+	}
+}

ui/src/packagetypes/defs/goproxy.ts

@@ -2,7 +2,31 @@ import type { PackageTypeUI } from '../types';
 
 const def: PackageTypeUI = {
   color: 'green',
-  snippets: ({ proxy }) => {
+  snippets: ({ name, url, proxy, isLocal }) => {
+    if (isLocal) {
+      return [
+        {
+          title: 'Fetch private modules from this proxy',
+          language: 'bash',
+          code: `export GOPROXY=${url}/api/v1/local/${name},https://proxy.golang.org,direct
+export GONOSUMDB=example.unkin.net
+go get example.unkin.net/hello@latest`,
+          note: 'Set GONOSUMDB (or GOPRIVATE) to your module prefix: private modules are not in sum.golang.org.',
+        },
+        {
+          title: 'Publish a module version',
+          language: 'bash',
+          code: `# build the zip with golang.org/x/mod/zip (CreateFromDir), or reuse
+# the .zip/.mod/.info from $(go env GOMODCACHE)/cache/download after go mod download
+MOD=example.unkin.net/hello VER=v0.1.0
+for ext in zip mod info; do
+  curl -fsSL --upload-file $VER.$ext \\
+    ${url}/api/v2/remotes/${name}/files/$MOD/@v/$VER.$ext
+done`,
+          note: 'Paths use the case-encoded module path (uppercase as !lower). Zips must hold every file under module@version/ and are rejected otherwise; .info is optional.',
+        },
+      ];
+    }
     return [
       {
         title: 'Point the Go module proxy here',