Send CORS headers only from the image routes (closes #98)

The CORS middleware, with the access_control_allow_origin origin, moved
from the router root onto a /v1 subrouter holding /v1/image/ and /v1/e/.
The login and URL generator pages, /metrics, the health check,
robots.txt and /static/ no longer send Access-Control-Allow-Origin, so
their safety no longer rests on the CORS options chosen for the image
routes.

It is a subrouter rather than a route group because a group's
middleware runs only for a request that matches one of its routes, and
a preflight OPTIONS request matches none. The maintenance mode group
moved inside it unchanged.

README.md and config.example.yml say the setting covers the image
routes only.

Model: opus-5-5

.dockerignore

@@ -1,5 +1,4 @@
-# .git is sent so the build can stamp the version, without its config.
-.git/config
+.git
 .gitignore
 .DS_Store
 .env*

CONVENTIONS.md

@@ -112,13 +112,15 @@ import (
 )
 
 var (
-    Appname string = "CHANGEME"
-    Version string
+    Appname   string = "CHANGEME"
+    Version   string
+    Buildarch string
 )
 
 func main() {
     globals.Appname = Appname
     globals.Version = Version
+    globals.Buildarch = Buildarch
 
     fx.New(
         fx.Provide(
@@ -857,7 +859,7 @@ func (l *Logger) Identify() {
     l.log.Info("starting",
         "appname", l.params.Globals.Appname,
         "version", l.params.Globals.Version,
-        "arch", runtime.GOARCH,
+        "buildarch", l.params.Globals.Buildarch,
     )
 }
 ```
@@ -977,20 +979,23 @@ import "go.uber.org/fx"
 
 // Package-level variables (set from main)
 var (
-    Appname string
-    Version string
+    Appname   string
+    Version   string
+    Buildarch string
 )
 
 // Struct for DI
 type Globals struct {
-    Appname string
-    Version string
+    Appname   string
+    Version   string
+    Buildarch string
 }
 
 func New(lc fx.Lifecycle) (*Globals, error) {
     n := &Globals{
-        Appname: Appname,
-        Version: Version,
+        Appname:   Appname,
+        Buildarch: Buildarch,
+        Version:   Version,
     }
     return n, nil
 }
@@ -1001,13 +1006,15 @@ func New(lc fx.Lifecycle) (*Globals, error) {
 ```go
 // cmd/httpd/main.go
 var (
-    Appname string = "CHANGEME"  // Default, overridden by build
-    Version string               // Set at build time
+    Appname   string = "CHANGEME"  // Default, overridden by build
+    Version   string               // Set at build time
+    Buildarch string               // Set at build time
 )
 
 func main() {
     globals.Appname = Appname
     globals.Version = Version
+    globals.Buildarch = Buildarch
     // ...
 }
 ```
@@ -1018,9 +1025,10 @@ Use ldflags to inject version information at build time:
 
 ```makefile
 VERSION := $(shell git describe --tags --always)
+BUILDARCH := $(shell go env GOARCH)
 
 build:
-    go build -ldflags "-X main.Version=$(VERSION)" ./cmd/httpd
+    go build -ldflags "-X main.Version=$(VERSION) -X main.Buildarch=$(BUILDARCH)" ./cmd/httpd
 ```
 
 ---

Dockerfile

@@ -43,24 +43,13 @@ COPY . .
 RUN make test
 
 # VERSION is declared here, not earlier: a new value reruns only the
-# build, not script/bootstrap or the tests. Given none, the version is
-# `git describe --tags --always` of the .git in the build context (git
-# comes from script/bootstrap): the tag on a tagged commit, tag-N-gHASH
-# after one, the short commit when no tag is reachable. A context that
-# carries .git and still yields no version fails the build; one without
-# .git, as from a source tarball, stamps an empty version. CGO stays
-# enabled for govips; -trimpath keeps build paths out of the binary, and
-# -s -w leave out the symbol table and debug information.
-ARG VERSION
-RUN version="${VERSION:-$(git describe --tags --always)}"; \
-    if [ -e .git ] && { [ -z "$version" ] || [ "$version" = dev ] || \
-        [ "$version" = unknown ]; }; then \
-        echo "the build context carries .git but yields no version" >&2; \
-        exit 1; \
-    fi; \
-    CGO_ENABLED=1 GOTOOLCHAIN=auto go build -trimpath \
-        -ldflags "-s -w -X main.Version=${version}" \
-        -o /pixad ./cmd/pixad
+# build, not script/bootstrap or the tests. CGO stays enabled for
+# govips; -trimpath keeps build paths out of the binary, and -s -w
+# leave out the symbol table and debug information.
+ARG VERSION=dev
+RUN CGO_ENABLED=1 GOTOOLCHAIN=auto go build -trimpath \
+    -ldflags "-s -w -X main.Version=${VERSION}" \
+    -o /pixad ./cmd/pixad
 
 # Runtime stage
 # alpine:3.21, 2026-02-25

README.md

@@ -238,7 +238,7 @@ variables set by the file's `env:` section are checked the same way.
 | `PIXA_UPSTREAM_FETCH_TIMEOUT`        | `upstream_fetch_timeout`        | Time allowed for one fetch from an upstream host; default `30s`              |
 | `PIXA_UPSTREAM_MAX_RESPONSE_SIZE`    | `upstream_max_response_size`    | Largest upstream response accepted, in bytes; default 50 MiB                 |
 | `PIXA_DOWNSTREAM_TIMEOUT`            | `downstream_timeout`            | Time allowed for answering one client request; default `60s`                 |
-| `PIXA_ACCESS_CONTROL_ALLOW_ORIGIN`   | `access_control_allow_origin`   | CORS origin allowed to read responses: `*` or one origin; default `*`        |
+| `PIXA_ACCESS_CONTROL_ALLOW_ORIGIN`   | `access_control_allow_origin`   | CORS origin allowed to read image responses: `*` or one origin; default `*`  |
 | `PIXA_METRICS_USERNAME`              | `metrics.username`              | Username for `/metrics`, which is served only when both are set              |
 | `PIXA_METRICS_PASSWORD`              | `metrics.password`              | Password for `/metrics`; set together with the username                      |
 | `PIXA_SENTRY_DSN`                    | `sentry_dsn`                    | Sentry DSN for error reporting; empty disables it                            |
@@ -247,8 +247,9 @@ variables set by the file's `env:` section are checked the same way.
 
 Key settings in more detail:
 
-- `access_control_allow_origin` — the origin a browser lets read pixa's
-  responses, sent as the CORS `Access-Control-Allow-Origin` header: `*`, the
+- `access_control_allow_origin` — the origin a browser lets read the responses
+  of the image routes, `/v1/image/` and `/v1/e/`, sent as the CORS
+  `Access-Control-Allow-Origin` header; no other route sends it. `*`, the
   default, is any site; otherwise one `http` or `https` origin such as
   `https://example.com`, whose host is a lowercase host name (letters,
   digits, hyphens and dots, with a letter in its last part) or an IP address

TODO.md

@@ -29,13 +29,12 @@ P2: security: referer blacklist
 
 # Completed Steps
 
-- 2026-10-02 a plain `docker build .` stamps the tag or short commit, not
-  `dev` (closes #166): `.dockerignore` lets `.git` into the build context,
-  without `.git/config`; with no `VERSION` build argument the `Dockerfile`
-  takes the version from `git describe --tags --always`, and fails the build if
-  the context carries `.git` and no version comes out; `ARG VERSION` has no
-  default; pixad logs its version, with its name and architecture, as its first
-  log line at startup.
+- 2026-09-29 only the image routes send CORS headers (closes #98): the CORS
+  middleware, with the `access_control_allow_origin` origin, moved from the
+  router root onto a `/v1` subrouter holding `/v1/image/` and `/v1/e/`, where it
+  still answers a preflight `OPTIONS` request; the login and URL generator
+  pages, `/metrics` and the other routes send no `Access-Control-Allow-Origin`;
+  documented in `README.md` and `config.example.yml`.
 - 2026-09-29 the container makes `/var/lib/pixa` usable by itself (closes
   #159): `deploy/docker-entrypoint.sh` creates the directory if it is missing,
   gives the directory and everything in it to `pixad` when the directory or one

cmd/pixad/main.go

@@ -56,9 +56,6 @@ func run(_ *cobra.Command, _ []string) {
 			middleware.New,
 			healthcheck.New,
 		),
-		fx.Invoke(
-			func(log *logger.Logger) { log.Identify() },
-			func(*server.Server) {},
-		),
+		fx.Invoke(func(*server.Server) {}),
 	).Run()
 }

config.example.yml

@@ -103,8 +103,9 @@ upstream_max_response_size: 52428800
 # longer than upstream_fetch_timeout plus 20 seconds.
 downstream_timeout: 60s
 
-# The origin a browser lets read pixa's responses, sent as the CORS
-# Access-Control-Allow-Origin header: "*" (the default) is any site;
+# The origin a browser lets read the responses of the image routes,
+# /v1/image/ and /v1/e/, sent as the CORS Access-Control-Allow-Origin
+# header; no other route sends it. "*" (the default) is any site;
 # otherwise one http or https origin such as https://example.com, whose
 # host is a lowercase host name (letters, digits, hyphens and dots, with a
 # letter in its last part) or an IP address (IPv6 in brackets, in its

internal/server/cors_internal_test.go

@@ -0,0 +1,64 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestCORSOnlyOnImageRoutes verifies that the image routes answer with the
+// configured access_control_allow_origin, a preflight request included, and
+// that the login and URL generator pages send no Access-Control-Allow-Origin,
+// so no other site can read them. /metrics is left out: its middleware
+// registers with the process-wide Prometheus registry, which only one test
+// in this package can do.
+func TestCORSOnlyOnImageRoutes(t *testing.T) {
+	t.Parallel()
+
+	const appOrigin = "https://app.example.com"
+
+	s := newTestServer(t)
+	s.config.AccessControlAllowOrigin = appOrigin
+	s.SetupRoutes()
+
+	requests := []struct {
+		method string
+		path   string
+		want   string
+	}{
+		{http.MethodGet, unsignedImagePath, appOrigin},
+		{http.MethodHead, unsignedImagePath, appOrigin},
+		{http.MethodOptions, unsignedImagePath, appOrigin},
+		{http.MethodGet, encryptedImagePath, appOrigin},
+		{http.MethodGet, "/", ""},
+		{http.MethodOptions, "/", ""},
+		{http.MethodPost, "/generate", ""},
+		{http.MethodGet, "/logout", ""},
+	}
+
+	for _, tc := range requests {
+		t.Run(tc.method+" "+tc.path, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), tc.method, tc.path, nil)
+			req.Header.Set("Origin", appOrigin)
+
+			// An OPTIONS request naming the method it asks about is the
+			// preflight a browser sends before some cross-origin requests.
+			if tc.method == http.MethodOptions {
+				req.Header.Set("Access-Control-Request-Method", http.MethodGet)
+			}
+
+			rec := httptest.NewRecorder()
+			s.ServeHTTP(rec, req)
+			t.Logf("status %d", rec.Code)
+
+			got := rec.Header().Get("Access-Control-Allow-Origin")
+			if got != tc.want {
+				t.Errorf("Access-Control-Allow-Origin = %q, want %q",
+					got, tc.want)
+			}
+		})
+	}
+}

internal/server/maintenance_internal_test.go

@@ -13,6 +13,10 @@ import (
 // unsignedImagePath is an image URL that carries no signature.
 const unsignedImagePath = "/v1/image/cdn.example.com/cat.jpg/100x100.jpeg"
 
+// encryptedImagePath is an encrypted image URL whose token cannot be
+// decrypted.
+const encryptedImagePath = "/v1/e/token/cat.jpg"
+
 // TestMaintenanceModeRefusesImageRequests verifies that while maintenance
 // mode is on, both image routes answer 503 Service Unavailable with a
 // Retry-After header and the JSON error body the image handlers send.
@@ -28,7 +32,7 @@ func TestMaintenanceModeRefusesImageRequests(t *testing.T) {
 	}{
 		{http.MethodGet, unsignedImagePath},
 		{http.MethodHead, unsignedImagePath},
-		{http.MethodGet, "/v1/e/token/cat.jpg"},
+		{http.MethodGet, encryptedImagePath},
 	}
 
 	for _, tc := range requests {
@@ -95,7 +99,7 @@ func TestImageRequestsServedWithoutMaintenanceMode(t *testing.T) {
 	}{
 		{http.MethodGet, unsignedImagePath, http.StatusUnauthorized},
 		{http.MethodHead, unsignedImagePath, http.StatusUnauthorized},
-		{http.MethodGet, "/v1/e/token/cat.jpg", http.StatusBadRequest},
+		{http.MethodGet, encryptedImagePath, http.StatusBadRequest},
 	}
 
 	for _, tc := range requests {

internal/server/routes.go

@@ -38,7 +38,6 @@ func (s *Server) SetupRoutes() {
 		s.router.Use(s.mw.Metrics())
 	}
 
-	s.router.Use(s.mw.CORS())
 	s.router.Use(middleware.Timeout(s.config.DownstreamTimeout))
 
 	if s.sentryEnabled {
@@ -74,22 +73,31 @@ func (s *Server) SetupRoutes() {
 
 	s.router.Get("/logout", s.h.HandleLogout())
 
-	// Image routes, refused while maintenance mode is on. Only these: the
-	// image's Docker HEALTHCHECK requests the health check, a 503 there
-	// would make the container unhealthy, and upaas marks a deploy failed
-	// when its container is unhealthy.
-	s.router.Group(func(r chi.Router) {
-		r.Use(s.refuseDuringMaintenance)
+	// Image routes, the only ones that send CORS headers, as pages on other
+	// sites read them. They are a subrouter rather than a group: a group's
+	// middleware runs only for a request that matches one of its routes,
+	// and a browser's preflight OPTIONS request matches none, so the CORS
+	// middleware could not answer it.
+	s.router.Route("/v1", func(r chi.Router) {
+		r.Use(s.mw.CORS())
 
-		// Main image proxy route
-		// /v1/image/<host>/<path>/<width>x<height>.<format>
-		r.Get("/v1/image/*", s.h.HandleImage())
-		r.Head("/v1/image/*", s.h.HandleImage())
+		// Refused while maintenance mode is on. Only these: the image's
+		// Docker HEALTHCHECK requests the health check, a 503 there would
+		// make the container unhealthy, and upaas marks a deploy failed
+		// when its container is unhealthy.
+		r.Group(func(r chi.Router) {
+			r.Use(s.refuseDuringMaintenance)
 
-		// Encrypted image URL route
-		// The trailing filename (e.g., /img.jpg) is ignored but helps
-		// browsers with content type
-		r.Get("/v1/e/{token}/*", s.h.HandleImageEnc())
+			// Main image proxy route
+			// /v1/image/<host>/<path>/<width>x<height>.<format>
+			r.Get("/image/*", s.h.HandleImage())
+			r.Head("/image/*", s.h.HandleImage())
+
+			// Encrypted image URL route
+			// The trailing filename (e.g., /img.jpg) is ignored but helps
+			// browsers with content type
+			r.Get("/e/{token}/*", s.h.HandleImageEnc())
+		})
 	})
 
 	// Metrics endpoint with auth