Document how a source query goes into an image URL, and accept it (closes #255)

The query follows the source path after %3F. The image route now parses
the request's path decoded once instead of chi's wildcard, which holds
the path as the client wrote it whenever that is not Go's own encoding
of it, so a / ; or , written as %2F, %3B or %2C, as GenerateSignedURL
wrote them, hid the %3F and the request failed with 401. Now the ? and
every % in the source path or query must be percent-encoded and any
other character may be. GenerateSignedURL writes Go's own encoding of
the path. The README states the rule, with an example and its signature.

Model: opus-5-5

README.md

@@ -316,6 +316,20 @@ once, is refused with 400.
   both optional (values under Signature Specification). Both are part of what is
   cached, so each value of either is a separate cached image.
 
+A source URL's query follows its path after `%3F`, the encoded `?`:
+`/v1/image/<host>/<path>%3F<query>/<size>.<format>`. pixa decodes the
+percent-escapes in the image URL's path once, then reads the host up to the
+first `/`, the size and format after the last `/`, and between them the source
+path up to the first `?` and the source query after it. So the `?` before the
+query, any `?` in the query, and every `%` in the source path or query must be
+percent-encoded, as `%3F` and `%25`. Any other character may be written as
+itself or percent-encoded: `/`, `;`, `,`, `=` and `&` work either way. For
+example,
+`https://cdn.example.com/img?url=https://origin.example.com/my%20cat.jpg` at
+800x600 WebP is
+`/v1/image/cdn.example.com/img%3Furl=https://origin.example.com/my%2520cat.jpg/800x600.webp`,
+signed as shown under Signature Specification.
+
 A size with one side `0` has that side scaled to keep the source's proportions,
 and is refused with 400 when that side would come out above 8192, the most
 either side may be; so is a size with `fit=outside`, which scales the image to
@@ -535,6 +549,17 @@ For the same image at quality 40 with fit `contain`, the input ends in
 and the URL is
 `/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=5IwXUx6vf7yefhaUvFzgXZvG2o0Df4RJxPTK3pKq5VU=&exp=1704067200&q=40&fit=contain`.
 
+For a source URL with a query,
+`https://cdn.example.com/img?url=https://origin.example.com/my%20cat.jpg`, at
+800x600 WebP with the same key and expiration and the default quality and fit:
+
+1. Build input, with the query's `:` written as `%3A` and its `%` as `%25`:
+   `cdn.example.com:/img:url=https%3A//origin.example.com/my%2520cat.jpg:800:600:webp:1704067200:85:cover`
+2. The signature is `37aFa9GSVCZM1kRY9SzMOk2fOg_M7Wt5gKNR4xM-FYA=`
+3. URL, with the `?` before the query written as `%3F` and the query's `%` as
+   `%25` (see Routes):
+   `/v1/image/cdn.example.com/img%3Furl=https://origin.example.com/my%2520cat.jpg/800x600.webp?sig=37aFa9GSVCZM1kRY9SzMOk2fOg_M7Wt5gKNR4xM-FYA=&exp=1704067200`
+
 **Allowlist patterns:**
 
 - **Exact match**: `cdn.example.com` — matches only that host

TODO.md

@@ -30,6 +30,17 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 the README says how a source URL with a query is written into an
+  image URL, and the image route accepts it (closes #255): the query follows the
+  source path after `%3F`. pixa now reads the image URL's path decoded once, so
+  the `?` before the query, any `?` in it and every `%` in the source path or
+  query must be percent-encoded, and any other character may be. The route used
+  to parse chi's wildcard, which holds the path as the client wrote it whenever
+  that is not Go's own encoding of it; a `/`, `;` or `,` in the query written as
+  `%2F`, `%3B` or `%2C`, as `GenerateSignedURL` wrote them, kept the `%3F` from
+  being read as `?`, and the request failed with 401. `GenerateSignedURL` now
+  writes Go's own encoding of the path. The README has an example with a query
+  and its signature.
 - 2026-10-15 `max_concurrent_processing` defaults to half the CPUs pixa sees,
   rounded down, and at least 1 (closes #242): it defaulted to one image per CPU,
   and a container with no CPU limit sees every CPU of the host, so on a 48-CPU

internal/handlers/image.go

@@ -9,7 +9,6 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/go-chi/chi/v5"
 	"github.com/go-chi/chi/v5/middleware"
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
@@ -80,20 +79,21 @@ func (s *Handlers) HandleRobotsTxt() http.HandlerFunc {
 	}
 }
 
-// parseImageRequest parses the wildcard path and query parameters into
+// parseImageRequest parses the image URL's path and query parameters into
 // an ImageRequest. On invalid input it writes an error response and
 // returns false.
 func (s *Handlers) parseImageRequest(
 	w http.ResponseWriter, r *http.Request,
 ) (*imgcache.ImageRequest, bool) {
-	// Get the wildcard path from chi
-	pathParam := chi.URLParam(r, "*")
-
-	// Parse the URL path
-	parsed, err := imgcache.ParseImagePath(pathParam)
+	// The path is parsed decoded once, as r.URL.Path holds it, and not from
+	// chi's wildcard: chi routes on the path as the client wrote it whenever
+	// that is not Go's own encoding of it, so its wildcard would still hold
+	// the %3F before a source query when, say, a "/" in the query is written
+	// as %2F
+	parsed, err := imgcache.ParseImageURL(r.URL.Path)
 	if err != nil {
 		s.log.Warn("failed to parse image URL",
-			"path", pathParam,
+			"path", r.URL.Path,
 			"error", err,
 		)
 		s.respondError(w, "invalid image URL: "+err.Error(), http.StatusBadRequest)

internal/handlers/image_signature_internal_test.go

@@ -198,7 +198,7 @@ func TestHandleImage_GeneratedSignedURLWithSourceQueryVerifies(t *testing.T) {
 
 			signedURL, err := fix.service.GenerateSignedURL("", &imgcache.ImageRequest{
 				SourceHost:  signedHost,
-				SourcePath:  "/images/photo.jpg",
+				SourcePath:  photoPath,
 				SourceQuery: tt.query,
 				Size:        imgcache.Size{Width: 50, Height: 50},
 				Format:      imgcache.FormatJPEG,
@@ -226,7 +226,7 @@ func TestHandleImage_SourceQueryEncodedAnyWay(t *testing.T) {
 	signer := signature.New("test-signing-key-must-be-32-chars")
 	sig := signer.Sign(&signature.Request{
 		SourceHost:  signedHost,
-		SourcePath:  "/images/photo.jpg",
+		SourcePath:  photoPath,
 		SourceQuery: "url=https://origin.example.com/my%20cat.jpg",
 		Width:       50,
 		Height:      50,
@@ -237,23 +237,23 @@ func TestHandleImage_SourceQueryEncodedAnyWay(t *testing.T) {
 	})
 
 	tests := []struct {
-		name          string
-		writtenSource string
+		name         string
+		writtenQuery string
 	}{
 		{"only ? and % encoded",
-			"/images/photo.jpg%3Furl=https://origin.example.com/my%2520cat.jpg"},
+			"%3Furl=https://origin.example.com/my%2520cat.jpg"},
 		{"every reserved character encoded",
-			"/images/photo.jpg%3Furl%3Dhttps%3A%2F%2Forigin.example.com%2Fmy%2520cat.jpg"},
+			"%3Furl%3Dhttps%3A%2F%2Forigin.example.com%2Fmy%2520cat.jpg"},
 		{"lower-case hex digits",
-			"/images/photo.jpg%3furl=https://origin.example.com/my%2520cat.jpg"},
+			"%3furl=https://origin.example.com/my%2520cat.jpg"},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			target := fmt.Sprintf("/v1/image/%s%s/50x50.jpeg?sig=%s&exp=%d",
-				signedHost, tt.writtenSource, sig, expires.Unix())
+			target := fmt.Sprintf("/v1/image/%s%s%s/50x50.jpeg?sig=%s&exp=%d",
+				signedHost, photoPath, tt.writtenQuery, sig, expires.Unix())
 
 			status := getImage(t, setupTestHandler(t), target)
 			if status != http.StatusBadGateway {

internal/imgcache/urlparser.go

@@ -61,9 +61,8 @@ type ParsedURL struct {
 	Format ImageFormat
 }
 
-// ParseImagePath parses the path captured by chi's wildcard:
-// <host>/<path>/<size>.<format>, or <host>/<path>/<size> for JPEG XL
-// This is the primary entry point when using chi routing.
+// ParseImagePath parses an image URL's path after /v1/image/, decoded once:
+// <host>/<path>/<size>.<format>, or <host>/<path>/<size> for JPEG XL.
 // Examples:
 //   - cdn.example.com/photos/cat.jpg/800x600.webp
 //   - cdn.example.com/photos/cat.jpg/0x0.jpeg
@@ -79,9 +78,10 @@ func ParseImagePath(path string) (*ParsedURL, error) {
 	return parseImageComponents(path)
 }
 
-// ParseImageURL parses a full URL path like /v1/image/<host>/<path>/<size>.<format>,
-// or /v1/image/<host>/<path>/<size> for JPEG XL
-// Use ParseImagePath instead when working with chi's wildcard capture.
+// ParseImageURL parses an image URL's path, decoded once as r.URL.Path holds
+// it: /v1/image/<host>/<path>/<size>.<format>, or /v1/image/<host>/<path>/<size>
+// for JPEG XL. A source query follows the source path after a "?", which the
+// URL writes as %3F.
 func ParseImageURL(urlPath string) (*ParsedURL, error) {
 	// Remove the /v1/image/ prefix
 	const prefix = "/v1/image/"

internal/signature/signature.go

@@ -126,31 +126,20 @@ func (s *Signer) GenerateSignedURL(
 		sizeStr = fmt.Sprintf("%dx%d", req.Width, req.Height)
 	}
 
-	// Build the path.
-	// When a source query is present, it is embedded as a path segment
-	// (e.g. /host/path?query/size.fmt) so that the URL parser can extract
-	// it from the last-slash split. The "?" inside a path segment is
-	// percent-encoded by clients but chi delivers it decoded, which is
-	// exactly what the URL parser expects.
-	var path string
+	// Build the path. A source query follows the source path after a "?".
+	// The image route reads the path decoded once, so the whole path is
+	// written in Go's own percent-encoding of a URL path, which writes "?"
+	// as %3F and "%" as %25, and leaves "/", ";", ",", "=" and "&" as they
+	// are.
+	sourcePathAndQuery := req.SourcePath
 	if req.SourceQuery != "" {
-		path = fmt.Sprintf("/v1/image/%s%s%%3F%s/%s.%s",
-			req.SourceHost,
-			req.SourcePath,
-			url.PathEscape(req.SourceQuery),
-			sizeStr,
-			req.Format,
-		)
-	} else {
-		path = fmt.Sprintf("/v1/image/%s%s/%s.%s",
-			req.SourceHost,
-			req.SourcePath,
-			sizeStr,
-			req.Format,
-		)
+		sourcePathAndQuery += "?" + req.SourceQuery
 	}
 
-	return path, sig, exp
+	unescapedPath := fmt.Sprintf("/v1/image/%s%s/%s.%s",
+		req.SourceHost, sourcePathAndQuery, sizeStr, req.Format)
+
+	return (&url.URL{Path: unescapedPath}).EscapedPath(), sig, exp
 }
 
 // buildSignatureData creates the string to be signed.

internal/signature/signature_test.go

@@ -476,9 +476,10 @@ func TestGenerateSignedURL_WithoutQueryString(t *testing.T) {
 }
 
 // TestGenerateSignedURL_ReadmeQueryExample checks the README's example of a
-// source URL with a query, https://cdn.example.com/img?url=https://origin.example.com/my%20cat.jpg:
-// its signature with the README's key, and the path GenerateSignedURL writes
-// for it, with the "?" written as %3F and the "%" as %25.
+// source URL with a query, the image at
+// https://cdn.example.com/img?url=https://origin.example.com/my%20cat.jpg: its
+// signature with the README's key, and the path GenerateSignedURL writes for
+// it, with the "?" written as %3F and the "%" as %25.
 func TestGenerateSignedURL_ReadmeQueryExample(t *testing.T) {
 	t.Parallel()