README.md

@@ -139,12 +139,13 @@ path under `/v1/` answers 200, in maintenance mode too.
   401 without them; 404 when they are not set, as the route then does not exist.
 
 Every response carries an `X-Request-ID` header holding the request's ID, which
-a client can quote when reporting a problem: the request's own `X-Request-ID`
-when it sent one, as a reverse proxy in front of pixa may, otherwise one pixa
-makes up from its host name, a random string chosen at startup and a counter.
-pixa's log line for the request carries the same ID as `request_id`, and so do
-the lines it logs when it fetches, converts and serves an image; the fetch sends
-it to the upstream host as `X-Request-ID`.
+a client can quote when reporting a problem: the request's own `X-Request-ID`,
+as a reverse proxy in front of pixa may send, when it is at most 64 letters,
+digits, `-`, `_` or `.`; otherwise a random one pixa makes for the request,
+which tells nothing about the machine or the other requests. pixa's log line for
+the request carries the same ID as `request_id`, and so do the lines it logs
+when it fetches, converts and serves an image; the fetch sends it to the
+upstream host as `X-Request-ID`.
 
 Both `POST` routes accept only a form that pixa's own page served: the page puts
 a token in the form and sets a cookie to match, and a request without both is

TODO.md

@@ -30,9 +30,10 @@ P2: security: referer blacklist
 # Completed Steps
 
 - 2026-10-04 request IDs returned and passed on, and `/v1/e/` revalidates
-  (closes #84): a middleware right after chi's `RequestID` sets `X-Request-ID`
-  on every response from the ID `RequestID` stores in the request context, which
-  is the request's own `X-Request-ID` when it sent one; the upstream fetch sends
+  (closes #84): pixa's own `RequestID` middleware, in place of chi's, gives each
+  request an ID, its own `X-Request-ID` when that is at most 64 letters, digits,
+  `-`, `_` or `.` and a random one otherwise, stores it where chi's did and
+  sends it back as `X-Request-ID` on every response; the upstream fetch sends
   that ID, and the "upstream fetched", "image converted" and "image served" log
   lines carry it as `request_id`, a fetch shared by several requests carrying
   the first request's; `/v1/e/` sets `ETag`, answers a matching `If-None-Match`

internal/httpfetcher/request_id_internal_test.go

@@ -11,7 +11,7 @@ import (
 )
 
 // TestFetchSendsRequestID verifies that a fetch sends the ID of the request
-// it serves, which chi's RequestID middleware stores in the request context,
+// it serves, which the RequestID middleware stores in the request context,
 // to the upstream host as X-Request-Id, so the fetch can be found in that
 // host's logs.
 func TestFetchSendsRequestID(t *testing.T) {

internal/middleware/middleware.go

@@ -2,9 +2,12 @@
 package middleware
 
 import (
+	"context"
+	"crypto/rand"
 	"log/slog"
 	"net/http"
 	"net/netip"
+	"regexp"
 	"time"
 
 	basicauth "github.com/99designs/basicauth-go"
@@ -115,16 +118,28 @@ func (s *Middleware) RateLimit(
 		})
 }
 
-// RequestIDResponseHeader returns a middleware that sends the request's ID as
+// requestIDPattern is what a request's own X-Request-Id must look like to be
+// kept as its ID: 1 to 64 letters, digits, '-', '_' or '.'.
+var requestIDPattern = regexp.MustCompile(`^[A-Za-z0-9._-]{1,64}$`)
+
+// RequestID returns a middleware that gives each request an ID and sends it as
 // the X-Request-Id response header, so a client can quote it when reporting a
-// problem. The ID is the one chi's RequestID middleware stored in the request
-// context, so RequestID must run first.
-func (s *Middleware) RequestIDResponseHeader() func(http.Handler) http.Handler {
+// problem. The ID is the request's own X-Request-Id when that matches
+// requestIDPattern, and otherwise a random one, which tells nothing about the
+// machine or the traffic. It is stored in the request context under chi's
+// RequestIDKey, where the logging middleware, the handlers and the upstream
+// fetch read it.
+func (s *Middleware) RequestID() func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			w.Header().Set(middleware.RequestIDHeader,
-				middleware.GetReqID(r.Context()))
-			next.ServeHTTP(w, r)
+			id := r.Header.Get(middleware.RequestIDHeader)
+			if !requestIDPattern.MatchString(id) {
+				id = rand.Text()
+			}
+
+			w.Header().Set(middleware.RequestIDHeader, id)
+			ctx := context.WithValue(r.Context(), middleware.RequestIDKey, id)
+			next.ServeHTTP(w, r.WithContext(ctx))
 		})
 	}
 }

internal/server/routes.go

@@ -28,8 +28,7 @@ func (s *Server) SetupRoutes() {
 	s.router = chi.NewRouter()
 
 	s.router.Use(middleware.Recoverer)
-	s.router.Use(middleware.RequestID)
-	s.router.Use(s.mw.RequestIDResponseHeader())
+	s.router.Use(s.mw.RequestID())
 	s.router.Use(s.mw.ClientIP())
 	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())