feat: serve local terraform repos as a provider registry

Local terraform repos already spoke the network mirror protocol, which needs
per-consumer .terraformrc config. This adds the provider registry protocol so
`terraform init` installs from a bare source address
(artifactapi.k8s.../{repo}/{type}) with no client setup.

- serve /.well-known/terraform.json service discovery and the providers.v1
  versions/download endpoints under /terraform/v1/providers
- map the Terraform namespace to the artifactapi repo name and locate the
  provider by type; download_url points back at the existing local file path
- generate SHA256SUMS per version and sign it with a GPG key loaded from
  TF_SIGNING_KEY_PATH; advertise the public key + key id in the download
  response. No key configured -> registry stays disabled (endpoints 404)
- new internal/tfsign (key loading + detached signing) and
  internal/api/terraform (registry handler); export ParseProviderZip for reuse
- add TF_SIGNING_KEY_PATH/PASSPHRASE and TF_PROVIDER_PROTOCOLS config
- unit test signing + verification; dockerised test of the full flow incl.
  signature verification against the advertised key

Also anchor the terraform/ gitignore to the repo root so it stops swallowing
internal/api/terraform and internal/provider/terraform test files (the latter
had gone silently untracked).

.gitignore

@@ -1,5 +1,5 @@
 bin/
-terraform/
+/terraform/
 
 # e2e-docker fixtures are real package files (.rpm, .tgz, .whl, .zip, ...) that
 # are intentionally tracked, overriding any global ignore of those extensions.

README.md

@@ -89,6 +89,34 @@ resource "artifactapi_virtual" "helm" {
 
 Provider: [terraform-provider-artifactapi](../terraform-provider-artifactapi)
 
+### Serving providers as a registry
+
+A local `terraform` repo is a real provider registry: upload
+`terraform-provider-{type}_{version}_{os}_{arch}.zip` files under
+`{namespace}/{type}/`, and Terraform installs them from a bare source address —
+no `.terraformrc` mirror config:
+
+```hcl
+terraform {
+  required_providers {
+    artifactapi = {
+      source  = "artifactapi.k8s.syd1.au.unkin.net/<repo>/<type>"
+      version = "0.1.2"
+    }
+  }
+}
+```
+
+The Terraform *namespace* segment is the artifactapi repo name; the provider is
+matched by *type*. The registry serves service discovery
+(`/.well-known/terraform.json`), the `providers.v1` version/download endpoints,
+and a GPG-signed `SHA256SUMS` per the provider registry protocol.
+
+Signing requires an armored GPG private key, supplied via `TF_SIGNING_KEY_PATH`
+(optionally `TF_SIGNING_KEY_PASSPHRASE`). Without it the registry endpoints stay
+disabled. `TF_PROVIDER_PROTOCOLS` (default `5.0,6.0`) sets the advertised plugin
+protocols.
+
 ## Access Control
 
 | Field | Default | Behaviour |

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/testcontainers/testcontainers-go v0.42.0
 	github.com/testcontainers/testcontainers-go/modules/postgres v0.42.0
 	github.com/testcontainers/testcontainers-go/modules/redis v0.42.0
+	golang.org/x/crypto v0.51.0
 	gopkg.in/yaml.v3 v3.0.1
 )
 
@@ -96,7 +97,6 @@ require (
 	go.opentelemetry.io/otel/trace v1.41.0 // indirect
 	go.uber.org/atomic v1.11.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
-	golang.org/x/crypto v0.51.0 // indirect
 	golang.org/x/net v0.53.0 // indirect
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.44.0 // indirect

internal/api/terraform/registry.go

@@ -0,0 +1,301 @@
+// Package terraform serves local terraform repos as a real Terraform provider
+// registry: service discovery, version listing, and GPG-signed downloads, so
+// `terraform init` installs from a bare source address with no client config.
+package terraform
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"path"
+	"sort"
+	"strings"
+
+	"github.com/go-chi/chi/v5"
+
+	"git.unkin.net/unkin/artifactapi/internal/database"
+	tfprov "git.unkin.net/unkin/artifactapi/internal/provider/terraform"
+	"git.unkin.net/unkin/artifactapi/internal/tfsign"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+// ProvidersV1Path is the base the service-discovery document advertises (Terraform
+// appends "{namespace}/{type}/versions" etc). MountPath is the same prefix without
+// the trailing slash, for chi.Mount.
+const (
+	ProvidersV1Path = "/terraform/v1/providers/"
+	MountPath       = "/terraform/v1/providers"
+)
+
+type Handler struct {
+	db        *database.DB
+	signer    *tfsign.Signer
+	protocols []string
+}
+
+func NewHandler(db *database.DB, signer *tfsign.Signer, protocols string) *Handler {
+	var protos []string
+	for _, p := range strings.Split(protocols, ",") {
+		if p = strings.TrimSpace(p); p != "" {
+			protos = append(protos, p)
+		}
+	}
+	if len(protos) == 0 {
+		protos = []string{"5.0", "6.0"}
+	}
+	return &Handler{db: db, signer: signer, protocols: protos}
+}
+
+// Enabled reports whether a signing key is configured. Without one the registry
+// cannot produce the signed SHA256SUMS the protocol requires, so it stays off.
+func (h *Handler) Enabled() bool { return h.signer != nil }
+
+func (h *Handler) Routes() chi.Router {
+	r := chi.NewRouter()
+	r.Get("/{namespace}/{type}/versions", h.versions)
+	r.Get("/{namespace}/{type}/{version}/download/{os}/{arch}", h.download)
+	r.Get("/{namespace}/{type}/{version}/sha256sums", h.sha256sums)
+	r.Get("/{namespace}/{type}/{version}/sha256sums.sig", h.sha256sumsSig)
+	return r
+}
+
+// ServiceDiscovery answers /.well-known/terraform.json, pointing Terraform at the
+// providers.v1 protocol base.
+func (h *Handler) ServiceDiscovery(w http.ResponseWriter, r *http.Request) {
+	if !h.Enabled() {
+		http.NotFound(w, r)
+		return
+	}
+	writeJSON(w, map[string]string{"providers.v1": ProvidersV1Path})
+}
+
+// providerFile is one resolved platform artifact within a repo.
+type providerFile struct {
+	version  string
+	os       string
+	arch     string
+	filePath string // path within the repo, e.g. unkin/artifactapi/...zip
+	sha256   string // hex, no "sha256:" prefix
+}
+
+// resolve finds every provider zip of the given type in the repo (namespace).
+// The Terraform source namespace maps to the artifactapi repo name; the provider
+// is matched by type across whatever in-repo folder it was uploaded under.
+func (h *Handler) resolve(r *http.Request, namespace, typeName string) ([]providerFile, error) {
+	remote, err := h.db.GetRemote(r.Context(), namespace)
+	if err != nil || remote.PackageType != models.PackageTerraform {
+		return nil, nil
+	}
+
+	rows, err := h.db.ListLocalFiles(r.Context(), namespace, 10000, 0)
+	if err != nil {
+		return nil, err
+	}
+
+	var out []providerFile
+	for _, row := range rows {
+		parsed := tfprov.ParseProviderZip(path.Base(row.FilePath))
+		if !parsed.Ok || parsed.Type != typeName {
+			continue
+		}
+		out = append(out, providerFile{
+			version:  parsed.Version,
+			os:       parsed.OS,
+			arch:     parsed.Arch,
+			filePath: row.FilePath,
+			sha256:   strings.TrimPrefix(row.ContentHash, "sha256:"),
+		})
+	}
+	return out, nil
+}
+
+func (h *Handler) versions(w http.ResponseWriter, r *http.Request) {
+	if !h.Enabled() {
+		http.NotFound(w, r)
+		return
+	}
+	namespace := chi.URLParam(r, "namespace")
+	typeName := chi.URLParam(r, "type")
+
+	files, err := h.resolve(r, namespace, typeName)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	if len(files) == 0 {
+		http.NotFound(w, r)
+		return
+	}
+
+	// Group platforms by version, de-duplicated and stably ordered.
+	type platform struct {
+		OS   string `json:"os"`
+		Arch string `json:"arch"`
+	}
+	platforms := map[string]map[string]platform{}
+	for _, f := range files {
+		if platforms[f.version] == nil {
+			platforms[f.version] = map[string]platform{}
+		}
+		platforms[f.version][f.os+"_"+f.arch] = platform{OS: f.os, Arch: f.arch}
+	}
+
+	type versionEntry struct {
+		Version   string     `json:"version"`
+		Protocols []string   `json:"protocols"`
+		Platforms []platform `json:"platforms"`
+	}
+	out := struct {
+		Versions []versionEntry `json:"versions"`
+	}{}
+	for version, plats := range platforms {
+		entry := versionEntry{Version: version, Protocols: h.protocols}
+		for _, p := range plats {
+			entry.Platforms = append(entry.Platforms, p)
+		}
+		sort.Slice(entry.Platforms, func(i, j int) bool {
+			return entry.Platforms[i].OS+entry.Platforms[i].Arch < entry.Platforms[j].OS+entry.Platforms[j].Arch
+		})
+		out.Versions = append(out.Versions, entry)
+	}
+	sort.Slice(out.Versions, func(i, j int) bool { return out.Versions[i].Version < out.Versions[j].Version })
+
+	writeJSON(w, out)
+}
+
+func (h *Handler) download(w http.ResponseWriter, r *http.Request) {
+	if !h.Enabled() {
+		http.NotFound(w, r)
+		return
+	}
+	namespace := chi.URLParam(r, "namespace")
+	typeName := chi.URLParam(r, "type")
+	version := chi.URLParam(r, "version")
+	osName := chi.URLParam(r, "os")
+	arch := chi.URLParam(r, "arch")
+
+	files, err := h.resolve(r, namespace, typeName)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	var match *providerFile
+	for i := range files {
+		if files[i].version == version && files[i].os == osName && files[i].arch == arch {
+			match = &files[i]
+			break
+		}
+	}
+	if match == nil {
+		http.NotFound(w, r)
+		return
+	}
+
+	base := baseURL(r)
+	verBase := fmt.Sprintf("%s%s/%s/%s", base+ProvidersV1Path, namespace, typeName, version)
+
+	type gpgKey struct {
+		KeyID      string `json:"key_id"`
+		ASCIIArmor string `json:"ascii_armor"`
+	}
+	resp := struct {
+		Protocols           []string `json:"protocols"`
+		OS                  string   `json:"os"`
+		Arch                string   `json:"arch"`
+		Filename            string   `json:"filename"`
+		DownloadURL         string   `json:"download_url"`
+		SHASumsURL          string   `json:"shasums_url"`
+		SHASumsSignatureURL string   `json:"shasums_signature_url"`
+		SHASum              string   `json:"shasum"`
+		SigningKeys         struct {
+			GPGPublicKeys []gpgKey `json:"gpg_public_keys"`
+		} `json:"signing_keys"`
+	}{
+		Protocols:           h.protocols,
+		OS:                  match.os,
+		Arch:                match.arch,
+		Filename:            path.Base(match.filePath),
+		DownloadURL:         fmt.Sprintf("%s/api/v1/local/%s/%s", base, namespace, match.filePath),
+		SHASumsURL:          verBase + "/sha256sums",
+		SHASumsSignatureURL: verBase + "/sha256sums.sig",
+		SHASum:              match.sha256,
+	}
+	resp.SigningKeys.GPGPublicKeys = []gpgKey{{
+		KeyID:      h.signer.KeyID(),
+		ASCIIArmor: h.signer.PublicKeyArmor(),
+	}}
+
+	writeJSON(w, resp)
+}
+
+func (h *Handler) sha256sums(w http.ResponseWriter, r *http.Request) {
+	sums, ok := h.buildSums(w, r)
+	if !ok {
+		return
+	}
+	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	w.Write(sums)
+}
+
+func (h *Handler) sha256sumsSig(w http.ResponseWriter, r *http.Request) {
+	sums, ok := h.buildSums(w, r)
+	if !ok {
+		return
+	}
+	sig, err := h.signer.Sign(sums)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "application/octet-stream")
+	w.Write(sig)
+}
+
+// buildSums renders the SHA256SUMS body for one version: one "<hex>  <filename>"
+// line per platform zip, sorted by filename so the signed bytes are stable.
+func (h *Handler) buildSums(w http.ResponseWriter, r *http.Request) ([]byte, bool) {
+	if !h.Enabled() {
+		http.NotFound(w, r)
+		return nil, false
+	}
+	namespace := chi.URLParam(r, "namespace")
+	typeName := chi.URLParam(r, "type")
+	version := chi.URLParam(r, "version")
+
+	files, err := h.resolve(r, namespace, typeName)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return nil, false
+	}
+
+	var lines []string
+	for _, f := range files {
+		if f.version != version {
+			continue
+		}
+		lines = append(lines, fmt.Sprintf("%s  %s", f.sha256, path.Base(f.filePath)))
+	}
+	if len(lines) == 0 {
+		http.NotFound(w, r)
+		return nil, false
+	}
+	sort.Strings(lines)
+	return []byte(strings.Join(lines, "\n") + "\n"), true
+}
+
+func writeJSON(w http.ResponseWriter, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(v)
+}
+
+func baseURL(r *http.Request) string {
+	scheme := "http"
+	if r.TLS != nil {
+		scheme = "https"
+	}
+	if fwd := r.Header.Get("X-Forwarded-Proto"); fwd != "" {
+		scheme = fwd
+	}
+	return scheme + "://" + r.Host
+}

internal/api/terraform/registry_test.go

@@ -0,0 +1,186 @@
+package terraform
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+	"golang.org/x/crypto/openpgp"
+	"golang.org/x/crypto/openpgp/armor"
+
+	"git.unkin.net/unkin/artifactapi/internal/database"
+	"git.unkin.net/unkin/artifactapi/internal/testsupport"
+	"git.unkin.net/unkin/artifactapi/internal/tfsign"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+var testDSN string
+
+func TestMain(m *testing.M) {
+	ctx := context.Background()
+	dsn, terminate, err := testsupport.StartPostgres(ctx)
+	if err != nil {
+		os.Exit(m.Run())
+	}
+	testDSN = dsn
+	code := m.Run()
+	terminate()
+	os.Exit(code)
+}
+
+// testSigner writes a throwaway armored key and loads it.
+func testSigner(t *testing.T) *tfsign.Signer {
+	t.Helper()
+	e, err := openpgp.NewEntity("artifactapi test", "tf", "tf@example.com", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var buf bytes.Buffer
+	w, _ := armor.Encode(&buf, openpgp.PrivateKeyType, nil)
+	if err := e.SerializePrivate(w, nil); err != nil {
+		t.Fatal(err)
+	}
+	w.Close()
+	p := filepath.Join(t.TempDir(), "private-key.asc")
+	if err := os.WriteFile(p, buf.Bytes(), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	s, err := tfsign.Load(p, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	return s
+}
+
+func TestProviderRegistryFlow(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 = "tf-reg" // Terraform namespace == repo name
+	const filePath = "unkin/artifactapi/terraform-provider-artifactapi_1.2.3_linux_amd64.zip"
+	const hash = "sha256:983cdb25cb7b976538e4334d26e52dee5f44749b9be1500c760cf5cf66be659b"
+	const wantSha = "983cdb25cb7b976538e4334d26e52dee5f44749b9be1500c760cf5cf66be659b"
+
+	if err := db.CreateRemote(ctx, &models.Remote{Name: repo, PackageType: models.PackageTerraform, RepoType: models.RepoTypeLocal}); err != nil {
+		t.Fatal(err)
+	}
+	if err := db.UpsertBlob(ctx, hash, "blobs/98/3c", 6381007, "application/zip"); err != nil {
+		t.Fatal(err)
+	}
+	if err := db.CreateLocalFile(ctx, repo, filePath, hash); err != nil {
+		t.Fatal(err)
+	}
+
+	signer := testSigner(t)
+	h := NewHandler(db, signer, "5.0,6.0")
+	router := chi.NewRouter()
+	router.Get("/.well-known/terraform.json", h.ServiceDiscovery)
+	router.Mount(MountPath, h.Routes())
+
+	get := func(p string) *httptest.ResponseRecorder {
+		req := httptest.NewRequest("GET", p, nil)
+		w := httptest.NewRecorder()
+		router.ServeHTTP(w, req)
+		return w
+	}
+
+	// Service discovery.
+	w := get("/.well-known/terraform.json")
+	if w.Code != 200 {
+		t.Fatalf("discovery = %d", w.Code)
+	}
+	var disc map[string]string
+	json.Unmarshal(w.Body.Bytes(), &disc)
+	if disc["providers.v1"] != ProvidersV1Path {
+		t.Errorf("providers.v1 = %q", disc["providers.v1"])
+	}
+
+	// Versions.
+	w = get("/terraform/v1/providers/tf-reg/artifactapi/versions")
+	if w.Code != 200 {
+		t.Fatalf("versions = %d %s", w.Code, w.Body)
+	}
+	var vresp struct {
+		Versions []struct {
+			Version   string              `json:"version"`
+			Protocols []string            `json:"protocols"`
+			Platforms []map[string]string `json:"platforms"`
+		} `json:"versions"`
+	}
+	json.Unmarshal(w.Body.Bytes(), &vresp)
+	if len(vresp.Versions) != 1 || vresp.Versions[0].Version != "1.2.3" {
+		t.Fatalf("unexpected versions: %+v", vresp)
+	}
+	if len(vresp.Versions[0].Platforms) != 1 || vresp.Versions[0].Platforms[0]["os"] != "linux" {
+		t.Fatalf("unexpected platforms: %+v", vresp.Versions[0].Platforms)
+	}
+
+	// Download.
+	w = get("/terraform/v1/providers/tf-reg/artifactapi/1.2.3/download/linux/amd64")
+	if w.Code != 200 {
+		t.Fatalf("download = %d %s", w.Code, w.Body)
+	}
+	var dl struct {
+		Filename            string `json:"filename"`
+		DownloadURL         string `json:"download_url"`
+		SHASumsURL          string `json:"shasums_url"`
+		SHASumsSignatureURL string `json:"shasums_signature_url"`
+		SHASum              string `json:"shasum"`
+		SigningKeys         struct {
+			GPGPublicKeys []struct {
+				KeyID      string `json:"key_id"`
+				ASCIIArmor string `json:"ascii_armor"`
+			} `json:"gpg_public_keys"`
+		} `json:"signing_keys"`
+	}
+	json.Unmarshal(w.Body.Bytes(), &dl)
+	if dl.SHASum != wantSha {
+		t.Errorf("shasum = %q", dl.SHASum)
+	}
+	wantURL := "http://example.com/api/v1/local/tf-reg/" + filePath
+	if dl.DownloadURL != wantURL {
+		t.Errorf("download_url = %q, want %q", dl.DownloadURL, wantURL)
+	}
+	if len(dl.SigningKeys.GPGPublicKeys) != 1 || dl.SigningKeys.GPGPublicKeys[0].KeyID != signer.KeyID() {
+		t.Errorf("signing key mismatch: %+v", dl.SigningKeys)
+	}
+
+	// SHA256SUMS + signature verify against the advertised key.
+	sums := get("/terraform/v1/providers/tf-reg/artifactapi/1.2.3/sha256sums")
+	wantLine := wantSha + "  terraform-provider-artifactapi_1.2.3_linux_amd64.zip\n"
+	if sums.Body.String() != wantLine {
+		t.Errorf("sha256sums = %q, want %q", sums.Body.String(), wantLine)
+	}
+	sig := get("/terraform/v1/providers/tf-reg/artifactapi/1.2.3/sha256sums.sig")
+	keyring, err := openpgp.ReadArmoredKeyRing(bytes.NewReader([]byte(dl.SigningKeys.GPGPublicKeys[0].ASCIIArmor)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := openpgp.CheckDetachedSignature(keyring, bytes.NewReader(sums.Body.Bytes()), bytes.NewReader(sig.Body.Bytes())); err != nil {
+		t.Errorf("sha256sums.sig did not verify: %v", err)
+	}
+}
+
+func TestRegistryDisabledWithoutSigner(t *testing.T) {
+	h := NewHandler(nil, nil, "")
+	router := chi.NewRouter()
+	router.Get("/.well-known/terraform.json", h.ServiceDiscovery)
+	req := httptest.NewRequest("GET", "/.well-known/terraform.json", nil)
+	w := httptest.NewRecorder()
+	router.ServeHTTP(w, req)
+	if w.Code != 404 {
+		t.Errorf("disabled discovery = %d, want 404", w.Code)
+	}
+}

internal/config/env.go

@@ -24,6 +24,14 @@ type Config struct {
 	S3Bucket    string
 	S3Secure    bool
 	S3Region    string
+
+	// Terraform provider registry signing. When TFSigningKeyPath points at a
+	// readable armored GPG private key, artifactapi serves local terraform
+	// repos as a real provider registry (service discovery + signed
+	// SHA256SUMS). Left empty, the registry endpoints stay disabled.
+	TFSigningKeyPath       string
+	TFSigningKeyPassphrase string
+	TFProviderProtocols    string
 }
 
 func (c *Config) DatabaseDSN() string {
@@ -59,6 +67,10 @@ func Load() (*Config, error) {
 		S3Bucket:    getenv("MINIO_BUCKET", "artifacts"),
 		S3Secure:    s3Secure,
 		S3Region:    getenv("MINIO_REGION", ""),
+
+		TFSigningKeyPath:       getenv("TF_SIGNING_KEY_PATH", ""),
+		TFSigningKeyPassphrase: getenv("TF_SIGNING_KEY_PASSPHRASE", ""),
+		TFProviderProtocols:    getenv("TF_PROVIDER_PROTOCOLS", "5.0,6.0"),
 	}
 
 	return cfg, nil

internal/provider/terraform/terraform.go

@@ -26,6 +26,27 @@ var providerZipRe = regexp.MustCompile(
 
 var semverRe = regexp.MustCompile(`^[0-9]+\.[0-9]+\.[0-9]+(?:-[a-zA-Z0-9.]+)?$`)
 
+// ParsedProviderZip describes a terraform-provider-{type}_{version}_{os}_{arch}.zip
+// filename. Ok is false when the name doesn't match that convention.
+type ParsedProviderZip struct {
+	Type    string
+	Version string
+	OS      string
+	Arch    string
+	Ok      bool
+}
+
+// ParseProviderZip extracts the type, version and platform from a provider zip
+// filename (the base name, not a full path). It's the canonical parser shared by
+// the network-mirror index and the provider registry handler.
+func ParseProviderZip(filename string) ParsedProviderZip {
+	m := providerZipRe.FindStringSubmatch(filename)
+	if m == nil {
+		return ParsedProviderZip{}
+	}
+	return ParsedProviderZip{Type: m[1], Version: m[2], OS: m[3], Arch: m[4], Ok: true}
+}
+
 type Provider struct{}
 
 func (p *Provider) Type() models.PackageType { return models.PackageTerraform }

internal/provider/terraform/terraform_extra_test.go

@@ -0,0 +1,171 @@
+package terraform
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+type fakeFileStore struct{ entries []provider.FileEntry }
+
+func (f fakeFileStore) ListFilesByPrefix(_ context.Context, _, prefix string) ([]provider.FileEntry, error) {
+	var out []provider.FileEntry
+	for _, e := range f.entries {
+		if strings.HasPrefix(e.FilePath, prefix) {
+			out = append(out, e)
+		}
+	}
+	return out, nil
+}
+func (f fakeFileStore) ListPackages(_ context.Context, _ string) ([]string, error) { return nil, nil }
+
+func TestTFPureFuncs(t *testing.T) {
+	p := &Provider{}
+	if p.Classify("hashicorp/aws/versions") != provider.Mutable {
+		t.Error("versions should be mutable")
+	}
+	if p.Classify("hashicorp/aws/terraform-provider-aws_1.0.0_linux_amd64.zip") != provider.Immutable {
+		t.Error("zip should be immutable")
+	}
+	if got := p.UpstreamURL(models.Remote{BaseURL: "https://registry.terraform.io"}, "hashicorp/aws/versions"); got != "https://registry.terraform.io/v1/providers/hashicorp/aws/versions" {
+		t.Errorf("upstream url %q", got)
+	}
+	h, _ := p.AuthHeaders(context.Background(), models.Remote{Username: "u", Password: "p"})
+	if h.Get("Authorization") == "" {
+		t.Error("auth header")
+	}
+	_ = p.ContentType("x.json")
+}
+
+func TestTFValidateUpload(t *testing.T) {
+	p := &Provider{}
+	sp, ct, err := p.ValidateUpload("hashicorp/aws/terraform-provider-aws_1.2.3_linux_amd64.zip")
+	if err != nil || sp != "hashicorp/aws/terraform-provider-aws_1.2.3_linux_amd64.zip" || ct != "application/zip" {
+		t.Errorf("valid: sp=%q ct=%q err=%v", sp, ct, err)
+	}
+	if _, _, err := p.ValidateUpload("too/few"); err == nil {
+		t.Error("expected error for wrong path depth")
+	}
+	if _, _, err := p.ValidateUpload("ns/aws/not-a-provider.zip"); err == nil {
+		t.Error("expected error for bad filename")
+	}
+	if _, _, err := p.ValidateUpload("ns/gcp/terraform-provider-aws_1.0.0_linux_amd64.zip"); err == nil {
+		t.Error("expected error for type mismatch")
+	}
+}
+
+func TestTFUploadResponse(t *testing.T) {
+	p := &Provider{}
+	resp := p.UploadResponse("hashicorp/aws/terraform-provider-aws_1.2.3_linux_amd64.zip", "sha256:abc", 100)
+	if resp["namespace"] != "hashicorp" || resp["type"] != "aws" || resp["version"] != "1.2.3" || resp["os"] != "linux" || resp["arch"] != "amd64" {
+		t.Errorf("structured response wrong: %v", resp)
+	}
+	fallback := p.UploadResponse("weird/path", "sha256:x", 1)
+	if fallback["path"] != "weird/path" {
+		t.Errorf("fallback response wrong: %v", fallback)
+	}
+}
+
+func TestTFRewriteResponse(t *testing.T) {
+	p := &Provider{}
+	remote := models.Remote{Name: "tf", ReleasesRemote: "hashicorp-releases"}
+
+	if out, _ := p.RewriteResponse([]byte(`{"download_url":"x"}`), models.Remote{}, "http://proxy"); out != nil {
+		t.Error("no ReleasesRemote should be a no-op")
+	}
+	if out, _ := p.RewriteResponse([]byte("not json"), remote, "http://proxy"); out != nil {
+		t.Error("invalid json should be a no-op")
+	}
+	body := []byte(`{"download_url":"https://releases.hashicorp.com/terraform-provider-aws/1.0/aws.zip"}`)
+	out, err := p.RewriteResponse(body, remote, "http://proxy")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(out), "http://proxy/api/v1/remote/hashicorp-releases/") {
+		t.Errorf("download_url not rewritten: %s", out)
+	}
+}
+
+func TestTFServeLocalIndex(t *testing.T) {
+	p := &Provider{}
+	fs := fakeFileStore{entries: []provider.FileEntry{
+		{FilePath: "hashicorp/aws/terraform-provider-aws_1.0.0_linux_amd64.zip", ContentHash: "sha256:deadbeef"},
+		{FilePath: "hashicorp/aws/terraform-provider-aws_1.0.0_darwin_arm64.zip", ContentHash: "sha256:cafe"},
+	}}
+
+	serve := func(path string) *httptest.ResponseRecorder {
+		w := httptest.NewRecorder()
+		r := httptest.NewRequest(http.MethodGet, "/"+path, nil)
+		p.ServeLocalIndex(w, r, fs, "repo", path)
+		return w
+	}
+
+	if w := serve("hashicorp/aws/index.json"); w.Code != 200 || !strings.Contains(w.Body.String(), "1.0.0") {
+		t.Errorf("index.json: code=%d body=%s", w.Code, w.Body.String())
+	}
+	if w := serve("hashicorp/aws/1.0.0.json"); w.Code != 200 || !strings.Contains(w.Body.String(), "linux_amd64") {
+		t.Errorf("version doc: code=%d body=%s", w.Code, w.Body.String())
+	}
+
+	// Not a terraform index path.
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest(http.MethodGet, "/x", nil)
+	if p.ServeLocalIndex(w, r, fs, "repo", "hashicorp/aws/other.txt") {
+		t.Error("non-index path should return false")
+	}
+	if p.ServeLocalIndex(httptest.NewRecorder(), r, fs, "repo", "too/short") {
+		t.Error("short path should return false")
+	}
+}
+
+func TestTFContentTypeAndEmptyIndex(t *testing.T) {
+	p := &Provider{}
+	for path, want := range map[string]string{
+		"x.zip":      "application/zip",
+		"x.sig":      "application/octet-stream",
+		"index.json": "application/json",
+	} {
+		if got := p.ContentType(path); got != want {
+			t.Errorf("ContentType(%q)=%q want %q", path, got, want)
+		}
+	}
+	// index / version doc with no matching files -> 404.
+	empty := fakeFileStore{}
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest(http.MethodGet, "/hashicorp/aws/index.json", nil)
+	p.ServeLocalIndex(w, r, empty, "repo", "hashicorp/aws/index.json")
+	if w.Code != http.StatusNotFound {
+		t.Errorf("empty index should be 404, got %d", w.Code)
+	}
+	w = httptest.NewRecorder()
+	p.ServeLocalIndex(w, r, empty, "repo", "hashicorp/aws/1.0.0.json")
+	if w.Code != http.StatusNotFound {
+		t.Errorf("empty version doc should be 404, got %d", w.Code)
+	}
+}
+
+func TestRewriteDownloadURL(t *testing.T) {
+	// Empty proxy base -> unchanged.
+	if got := rewriteDownloadURL("https://x/a.zip", "rel", ""); got != "https://x/a.zip" {
+		t.Errorf("empty base: %q", got)
+	}
+	// Unparseable URL -> unchanged.
+	if got := rewriteDownloadURL("://bad", "rel", "http://p"); got != "://bad" {
+		t.Errorf("bad url: %q", got)
+	}
+	// Normal rewrite.
+	if got := rewriteDownloadURL("https://cdn/path/a.zip", "rel", "http://p"); got != "http://p/api/v1/remote/rel/path/a.zip" {
+		t.Errorf("rewrite: %q", got)
+	}
+}
+
+func TestTFGenerateLocalIndexUnsupported(t *testing.T) {
+	if _, err := (&Provider{}).GenerateLocalIndex(context.Background(), fakeFileStore{}, "r", "x"); err == nil {
+		t.Error("expected unsupported error")
+	}
+}

internal/server/server.go

@@ -12,6 +12,7 @@ import (
 	"github.com/go-chi/chi/v5"
 	"github.com/go-chi/chi/v5/middleware"
 
+	tfregistry "git.unkin.net/unkin/artifactapi/internal/api/terraform"
 	v1 "git.unkin.net/unkin/artifactapi/internal/api/v1"
 	v2 "git.unkin.net/unkin/artifactapi/internal/api/v2"
 	"git.unkin.net/unkin/artifactapi/internal/cache"
@@ -30,6 +31,7 @@ import (
 	_ "git.unkin.net/unkin/artifactapi/internal/provider/terraform"
 	"git.unkin.net/unkin/artifactapi/internal/proxy"
 	"git.unkin.net/unkin/artifactapi/internal/storage"
+	"git.unkin.net/unkin/artifactapi/internal/tfsign"
 	"git.unkin.net/unkin/artifactapi/internal/virtual"
 )
 
@@ -43,6 +45,7 @@ type Server struct {
 	engine       *proxy.Engine
 	virtEngine   *virtual.Engine
 	localHandler *v2.LocalHandler
+	tfRegistry   *tfregistry.Handler
 	gc           *gc.Collector
 }
 
@@ -67,6 +70,17 @@ func New(cfg *config.Config, version string) (*Server, error) {
 	virtEngine := virtual.NewEngine(db, engine)
 	collector := gc.New(db, s3, 1*time.Hour)
 
+	// A failure to load the signing key must not take the server down: the
+	// terraform registry simply stays disabled until a valid key is present.
+	signer, err := tfsign.Load(cfg.TFSigningKeyPath, cfg.TFSigningKeyPassphrase)
+	if err != nil {
+		slog.Warn("terraform provider registry disabled", "error", err)
+	}
+	tfRegistry := tfregistry.NewHandler(db, signer, cfg.TFProviderProtocols)
+	if tfRegistry.Enabled() {
+		slog.Info("terraform provider registry enabled", "key_id", signer.KeyID())
+	}
+
 	s := &Server{
 		cfg:          cfg,
 		version:      version,
@@ -76,6 +90,7 @@ func New(cfg *config.Config, version string) (*Server, error) {
 		engine:       engine,
 		virtEngine:   virtEngine,
 		localHandler: localHandler,
+		tfRegistry:   tfRegistry,
 		gc:           collector,
 	}
 
@@ -97,6 +112,11 @@ func (s *Server) routes() chi.Router {
 	r.Get("/", s.handleRoot)
 	r.Get("/version", s.handleVersion)
 
+	// Terraform provider registry: service discovery at the well-known path,
+	// providers.v1 protocol under /terraform/v1/providers.
+	r.Get("/.well-known/terraform.json", s.tfRegistry.ServiceDiscovery)
+	r.Mount(tfregistry.MountPath, s.tfRegistry.Routes())
+
 	proxyHandler := v1.NewProxyHandler(s.engine, s.virtEngine, s.db, s.store, s.localHandler)
 	r.Mount("/api/v1", proxyHandler.Routes())
 	r.Mount("/v2", proxyHandler.DockerV2Routes())

internal/tfsign/signer.go

@@ -0,0 +1,112 @@
+// Package tfsign loads a GPG signing key and produces the detached signatures
+// the Terraform provider registry protocol requires over SHA256SUMS files.
+package tfsign
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"strings"
+
+	"golang.org/x/crypto/openpgp"
+	"golang.org/x/crypto/openpgp/armor"
+)
+
+// Signer holds a decrypted GPG entity and exposes what the registry download
+// response needs: a detached signature, the armored public key, and the key ID.
+type Signer struct {
+	entity      *openpgp.Entity
+	publicASCII string
+	keyID       string
+}
+
+// Load reads an armored private key from path, decrypting it with passphrase if
+// the key is protected. A blank path returns (nil, nil): signing is optional, and
+// a nil *Signer means the terraform registry is disabled.
+func Load(path, passphrase string) (*Signer, error) {
+	if path == "" {
+		return nil, nil
+	}
+
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("open signing key: %w", err)
+	}
+	defer f.Close()
+
+	keyring, err := openpgp.ReadArmoredKeyRing(f)
+	if err != nil {
+		return nil, fmt.Errorf("read signing key: %w", err)
+	}
+	if len(keyring) == 0 {
+		return nil, fmt.Errorf("signing key %q contains no entities", path)
+	}
+	entity := keyring[0]
+
+	if entity.PrivateKey == nil {
+		return nil, fmt.Errorf("signing key %q has no private key material", path)
+	}
+	if entity.PrivateKey.Encrypted {
+		if err := decrypt(entity, passphrase); err != nil {
+			return nil, err
+		}
+	}
+
+	pub, err := armorPublicKey(entity)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Signer{
+		entity:      entity,
+		publicASCII: pub,
+		keyID:       entity.PrimaryKey.KeyIdString(),
+	}, nil
+}
+
+// decrypt unlocks the entity's private key and all subkeys with the passphrase.
+func decrypt(entity *openpgp.Entity, passphrase string) error {
+	pw := []byte(passphrase)
+	if err := entity.PrivateKey.Decrypt(pw); err != nil {
+		return fmt.Errorf("decrypt signing key: %w", err)
+	}
+	for _, sub := range entity.Subkeys {
+		if sub.PrivateKey != nil && sub.PrivateKey.Encrypted {
+			_ = sub.PrivateKey.Decrypt(pw)
+		}
+	}
+	return nil
+}
+
+func armorPublicKey(entity *openpgp.Entity) (string, error) {
+	var buf bytes.Buffer
+	w, err := armor.Encode(&buf, openpgp.PublicKeyType, nil)
+	if err != nil {
+		return "", err
+	}
+	if err := entity.Serialize(w); err != nil {
+		return "", err
+	}
+	if err := w.Close(); err != nil {
+		return "", err
+	}
+	return buf.String(), nil
+}
+
+// Sign returns a binary detached signature over message, matching the
+// SHA256SUMS.sig format Terraform verifies.
+func (s *Signer) Sign(message []byte) ([]byte, error) {
+	var buf bytes.Buffer
+	if err := openpgp.DetachSign(&buf, s.entity, bytes.NewReader(message), nil); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+// PublicKeyArmor returns the ASCII-armored public key for the registry's
+// signing_keys response.
+func (s *Signer) PublicKeyArmor() string { return s.publicASCII }
+
+// KeyID returns the 16-hex-char uppercase key ID Terraform matches against the
+// signature's issuer.
+func (s *Signer) KeyID() string { return strings.ToUpper(s.keyID) }

internal/tfsign/signer_test.go

@@ -0,0 +1,86 @@
+package tfsign
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"regexp"
+	"testing"
+
+	"golang.org/x/crypto/openpgp"
+	"golang.org/x/crypto/openpgp/armor"
+)
+
+// armoredPrivateKey generates a throwaway armored private key for tests.
+func armoredPrivateKey(t *testing.T) string {
+	t.Helper()
+	e, err := openpgp.NewEntity("artifactapi test", "tf registry", "tf@example.com", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var buf bytes.Buffer
+	w, err := armor.Encode(&buf, openpgp.PrivateKeyType, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := e.SerializePrivate(w, nil); err != nil {
+		t.Fatal(err)
+	}
+	w.Close()
+	return buf.String()
+}
+
+func writeKey(t *testing.T, contents string) string {
+	t.Helper()
+	p := filepath.Join(t.TempDir(), "private-key.asc")
+	if err := os.WriteFile(p, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	return p
+}
+
+func TestLoadSignAndVerify(t *testing.T) {
+	path := writeKey(t, armoredPrivateKey(t))
+	s, err := Load(path, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if s == nil {
+		t.Fatal("expected a signer")
+	}
+
+	if !regexp.MustCompile(`^[0-9A-F]{16}$`).MatchString(s.KeyID()) {
+		t.Errorf("key id %q is not 16 uppercase hex chars", s.KeyID())
+	}
+
+	msg := []byte("deadbeef  terraform-provider-x_1.0.0_linux_amd64.zip\n")
+	sig, err := s.Sign(msg)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// The advertised public key must verify the signature over the same bytes.
+	keyring, err := openpgp.ReadArmoredKeyRing(bytes.NewReader([]byte(s.PublicKeyArmor())))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := openpgp.CheckDetachedSignature(keyring, bytes.NewReader(msg), bytes.NewReader(sig)); err != nil {
+		t.Errorf("signature did not verify: %v", err)
+	}
+}
+
+func TestLoadEmptyPathDisabled(t *testing.T) {
+	s, err := Load("", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if s != nil {
+		t.Error("empty path should yield a nil (disabled) signer")
+	}
+}
+
+func TestLoadMissingFile(t *testing.T) {
+	if _, err := Load(filepath.Join(t.TempDir(), "nope.asc"), ""); err == nil {
+		t.Error("expected an error for a missing key file")
+	}
+}