Keep only a short, plain request ID; make others up at random

pixa's own RequestID middleware replaces chi's and the response-header
middleware. It keeps a request's own X-Request-Id only when it is at
most 64 letters, digits, '-', '_' or '.', so an over-long or odd value
is never sent back or upstream, and otherwise makes a random ID with
crypto/rand, which tells nothing about the host or how many requests
pixa served. It stores the ID under chi's RequestIDKey, where the
logging middleware, the handlers and the fetcher read it.

Model: opus-5-5

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())