Test the generator refusals and a "'" in the source query

Through the router: a source query holding "'" must reach the upstream
as %27. The generator page must answer 400, show the URL pixa would
fetch and make no encrypted URL for a source path holding %2F next to a
space, a "|" or a letter outside ASCII, and for a source URL holding a
user name and password. It must also refuse a source path holding a ".."
segment, as /v1/image/ does. These fail on the branch; the fix follows.

Model: opus-5-5

README.md

@@ -331,6 +331,20 @@ writes one as `%23` is refused with 400. For example,
 `/v1/image/cdn.example.com/img%3Furl=https://origin.example.com/my%2520cat.jpg/800x600.webp`,
 signed as shown under Signature Specification.
 
+The source path is the path with its percent-escapes decoded, and pixa fetches
+it percent-encoded again: every character other than letters, digits, `-`, `.`,
+`_`, `~`, `/`, `$`, `&`, `+`, `,`, `:`, `;`, `=` and `@` is sent as a
+percent-escape. So `my%20cat.jpg` in the image URL is the source path
+`/my cat.jpg`, fetched as `/my%20cat.jpg`; `my%2520cat.jpg` is the source path
+`/my%20cat.jpg`, fetched as `/my%2520cat.jpg`; and `100%25.png` is fetched as
+`/100%25.png`. The source query is the source URL's query as that URL writes it,
+its percent-escapes included, and is sent as it is, except that the characters a
+browser percent-encodes in a query (a space, `"`, `<`, `>`, control characters
+and characters outside ASCII) and a `%` that does not begin a percent-escape are
+percent-encoded. pixa signs and caches the query in that form, so
+`img%3Fname=my%20cat` and `img%3Fname=my%2520cat` in the image URL are the same
+source query, `name=my%20cat`.
+
 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
@@ -445,7 +459,11 @@ nor change what it asks for.
    sent with an empty format, or none, also makes a JPEG XL URL. Width and
    height both empty or `0` keep the original size; if only one of them is empty
    or `0`, that side is scaled to keep the image's proportions. A URL whose
-   scaled side would come out above 8192 answers 400.
+   scaled side would come out above 8192 answers 400. So does a source URL that
+   the image URL would fetch written another way, such as one whose path holds
+   `%2F`, fetched as `/`, or `(`, fetched as `%28`, as pixa keeps the source
+   path decoded and fetches it percent-encoded again (see Routes); the page then
+   shows the URL pixa would fetch.
 3. The page shows the URL, `https://<host>/v1/e/<token>/img.<format>`, and when
    it expires. `<host>` is the host the page was opened on, and the URL starts
    with `http` instead while `debug` is on. The name after the token is ignored
@@ -507,11 +525,13 @@ Where:
 - `host` — source host as the URL writes it up to the first `/`, in lower case,
   keeping an IPv6 address's brackets and any `:` and port after it (e.g.
   `cdn.example.com`, `cdn.example.com:8443`, `[2001:db8::1]:8443`)
-- `path` — source path (e.g. `/photos/cat.jpg`), with each `%` written as `%25`
-  and each `:` as `%3A`, so `/photos/a:b.jpg` is signed as `/photos/a%3Ab.jpg`
-- `query` — source query string, empty string if none, with `%` and `:` written
-  as in `path`; neither then holds a `:`, so path `/a:b` with no query and path
-  `/a` with query `b:` are signed differently
+- `path` — source path, with its percent-escapes decoded as under Routes (e.g.
+  `/photos/cat.jpg`, or `/photos/my cat.jpg` for `/photos/my%20cat.jpg`), with
+  each `%` written as `%25` and each `:` as `%3A`, so `/photos/a:b.jpg` is
+  signed as `/photos/a%3Ab.jpg`
+- `query` — source query string as under Routes, empty string if none, with `%`
+  and `:` written as in `path`; neither then holds a `:`, so path `/a:b` with no
+  query and path `/a` with query `b:` are signed differently
 - `width` — requested width in pixels, `0` for original
 - `height` — requested height in pixels, `0` for original
 - `format` — output format, one of those listed under Routes, with `original`

TODO.md

@@ -30,6 +30,29 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 a thin source requested with `fit=cover` no longer makes libvips
+  build a huge image (closes #282): libvips' thumbnail scaled the whole source
+  to cover the requested size and copied that into memory before it cut out the
+  centre, so an 8192x64 source at 8192x8192 made it allocate about 24 GiB, and
+  an 8192x1 source answered 500. The image processor now first cuts the centre
+  of the source, widened to whole source pixels, and thumbnail scales and cuts
+  only that, by the same amount on both sides as before. What it copies into
+  memory is then larger than the output by less than one scaled source pixel on
+  each side. With the runtime image's libvips and JPEG output at 8192x8192,
+  resident memory peaked at about 242 MiB for the 8192x64 source and 431 MiB for
+  the 8192x1 one. The output can sit a fraction of an output pixel from where it
+  was.
+- 2026-10-16 the source URL a request fetches is built with `net/url` from the
+  source host, path and query, not joined as text (closes #288). The source
+  path, decoded, is fetched percent-encoded again, so `100%25.png` fetches
+  `/100%25.png` where it answered 401, and `my%20cat.png` and `my%2520cat.png`
+  fetch different paths where both fetched `/my%20cat.png` under two cache keys.
+  The source query stays as the source URL writes it; the characters a browser
+  percent-encodes in a query, such as a space, which was sent raw and answered
+  502, are percent-encoded when the image URL is read, so the query is signed
+  and cached as it is sent. The generator page answers 400, showing the URL pixa
+  would fetch, for a source URL its image URL would fetch written another way,
+  such as with `%2F` in the path.
 - 2026-10-15 a `/v1/image/` URL whose source path or source query holds a `#`,
   written as `%23`, is refused with 400 (closes #286): the source URL built from
   it ended at the `#` when fetched, so each text after it fetched the same

internal/handlers/auth.go

@@ -141,6 +141,22 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 			return
 		}
 
+		// The image URL must fetch the source URL as it was written. The
+		// token keeps the path decoded and pixa fetches it percent-encoded
+		// again, so a path written another way, such as with %2F, would fetch
+		// another URL. The scheme is not kept: only path and query are compared
+		sourceRequest := payload.ToImageRequest()
+		sourceRequest.AllowHTTP = s.config.AllowHTTP
+
+		fetchURL, err := url.Parse(sourceRequest.SourceURL())
+		if err != nil || fetchURL.RequestURI() != parsed.RequestURI() {
+			s.renderGeneratorWithForm(w, r, "Source URL cannot be fetched as "+
+				"written: pixa would fetch "+sourceRequest.SourceURL(),
+				r.Form, http.StatusBadRequest)
+
+			return
+		}
+
 		// Generate encrypted token
 		token, err := s.encGen.Generate(payload)
 		if err != nil {

internal/imageprocessor/fit_cover_internal_test.go

@@ -0,0 +1,268 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	"image/png"
+	"math"
+	"testing"
+)
+
+// TestImageProcessor_FitCoverServesThinSource verifies that FitCover serves
+// a thin source at the requested size. An 8192x1 source scaled to cover
+// 8192x8192 is 8192 times as long as the output on its long side. The centre
+// of an 8192x1 source with the proportions of 1x8192 is 1/8192 of a pixel
+// wide. Each case is also turned on its side.
+func TestImageProcessor_FitCoverServesThinSource(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                    string
+		inputWidth, inputHeight int
+		size                    Size
+	}{
+		{"8192x1 at 8192x8192", 8192, 1, Size{Width: 8192, Height: 8192}},
+		{"8192x1 at 1x8192", 8192, 1, Size{Width: 1, Height: 8192}},
+		{"1x8192 at 8192x8192", 1, 8192, Size{Width: 8192, Height: 8192}},
+		{"1x8192 at 8192x1", 1, 8192, Size{Width: 8192, Height: 1}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			input := createTestJPEG(t, tt.inputWidth, tt.inputHeight)
+
+			result, err := New(Params{}).Process(t.Context(),
+				bytes.NewReader(input),
+				&Request{Size: tt.size, Format: FormatJPEG, FitMode: FitCover})
+			if err != nil {
+				t.Fatalf("Process() error = %v", err)
+			}
+
+			defer func() { _ = result.Content.Close() }()
+
+			t.Logf("Process() made %dx%d", result.Width, result.Height)
+
+			if result.Width != tt.size.Width || result.Height != tt.size.Height {
+				t.Errorf("Process() made %dx%d, want %dx%d",
+					result.Width, result.Height, tt.size.Width, tt.size.Height)
+			}
+		})
+	}
+}
+
+// createBandsPNG creates a PNG of three equal bands, red, green and blue,
+// side by side when it is wider than tall and top to bottom otherwise.
+func createBandsPNG(t *testing.T, width, height int) []byte {
+	t.Helper()
+
+	bands := []color.RGBA{
+		{R: 255, A: 255},
+		{G: 255, A: 255},
+		{B: 255, A: 255},
+	}
+
+	img := image.NewRGBA(image.Rect(0, 0, width, height))
+	for y := range height {
+		for x := range width {
+			band := 3 * y / height
+			if width > height {
+				band = 3 * x / width
+			}
+
+			img.Set(x, y, bands[band])
+		}
+	}
+
+	return encodePNG(t, img)
+}
+
+// TestImageProcessor_FitCoverCropsTheCentre verifies that FitCover keeps the
+// centre of the source: a 300x100 source of red, green and blue bands side
+// by side, or a 100x300 one with them top to bottom, comes out at 50x50 as
+// its green band.
+func TestImageProcessor_FitCoverCropsTheCentre(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                    string
+		inputWidth, inputHeight int
+	}{
+		{"bands side by side", 300, 100},
+		{"bands top to bottom", 100, 300},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			input := createBandsPNG(t, tt.inputWidth, tt.inputHeight)
+
+			result, err := New(Params{}).Process(t.Context(),
+				bytes.NewReader(input),
+				&Request{
+					Size:    Size{Width: 50, Height: 50},
+					Format:  FormatPNG,
+					FitMode: FitCover,
+				})
+			if err != nil {
+				t.Fatalf("Process() error = %v", err)
+			}
+
+			defer func() { _ = result.Content.Close() }()
+
+			output, err := png.Decode(result.Content)
+			if err != nil {
+				t.Fatalf("failed to decode output: %v", err)
+			}
+
+			bounds := output.Bounds()
+			t.Logf("Process() made %dx%d", bounds.Dx(), bounds.Dy())
+
+			if bounds.Dx() != 50 || bounds.Dy() != 50 {
+				t.Fatalf("Process() made %dx%d, want 50x50",
+					bounds.Dx(), bounds.Dy())
+			}
+
+			// Resampling may blend a little of the bands beside the
+			// green one into the output's edges.
+			for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+				for x := bounds.Min.X; x < bounds.Max.X; x++ {
+					r, g, b, _ := output.At(x, y).RGBA()
+					if r>>8 > 64 || g>>8 < 192 || b>>8 > 64 {
+						t.Fatalf("output pixel %d,%d is %d,%d,%d, want green",
+							x, y, r>>8, g>>8, b>>8)
+					}
+				}
+			}
+		})
+	}
+}
+
+// The greys of createSquarePNG, and the grey halfway between them.
+const (
+	backgroundGrey = 192
+	squareGrey     = 64
+	halfwayGrey    = (backgroundGrey + squareGrey) / 2
+)
+
+// createSquarePNG creates a grey PNG side pixels square with a darker grey
+// square squareSide pixels wide in its centre.
+func createSquarePNG(t *testing.T, side, squareSide int) []byte {
+	t.Helper()
+
+	first := (side - squareSide) / 2
+	last := first + squareSide - 1
+
+	img := image.NewGray(image.Rect(0, 0, side, side))
+	for y := range side {
+		for x := range side {
+			grey := uint8(backgroundGrey)
+			if x >= first && x <= last && y >= first && y <= last {
+				grey = squareGrey
+			}
+
+			img.SetGray(x, y, color.Gray{Y: grey})
+		}
+	}
+
+	return encodePNG(t, img)
+}
+
+// squareLength returns how long the square is along greys, the greys of a
+// row or column of the output that crosses it: the distance between the
+// first and the last point where greys passes halfwayGrey, or 0 when there
+// is no such point. Resampling blurs the square's edges over several
+// pixels, and each point is placed between the two pixels on either side
+// of it in proportion to their greys, so the length is measured to a
+// fraction of a pixel.
+func squareLength(greys []float64) float64 {
+	first, last := -1.0, -1.0
+
+	for i := 1; i < len(greys); i++ {
+		before, after := greys[i-1], greys[i]
+		if (before > halfwayGrey) == (after > halfwayGrey) {
+			continue
+		}
+
+		point := float64(i-1) + (before-halfwayGrey)/(before-after)
+		if first < 0 {
+			first = point
+		}
+
+		last = point
+	}
+
+	return last - first
+}
+
+// TestImageProcessor_FitCoverKeepsProportions verifies that FitCover scales
+// both sides of the source by the same amount: a square in the centre of
+// the source comes out square. Its width and height are measured along the
+// output's middle row and column.
+func TestImageProcessor_FitCoverKeepsProportions(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name             string
+		side, squareSide int
+		size             Size
+	}{
+		{"16x16 at 100x60", 16, 8, Size{Width: 100, Height: 60}},
+		{"16x16 at 60x100", 16, 8, Size{Width: 60, Height: 100}},
+		{"10x10 at 100x35", 10, 2, Size{Width: 100, Height: 35}},
+		{"10x10 at 35x100", 10, 2, Size{Width: 35, Height: 100}},
+		{"64x64 at 300x100", 64, 16, Size{Width: 300, Height: 100}},
+		{"64x64 at 100x300", 64, 16, Size{Width: 100, Height: 300}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			input := createSquarePNG(t, tt.side, tt.squareSide)
+
+			result, err := New(Params{}).Process(t.Context(),
+				bytes.NewReader(input),
+				&Request{Size: tt.size, Format: FormatPNG, FitMode: FitCover})
+			if err != nil {
+				t.Fatalf("Process() error = %v", err)
+			}
+
+			defer func() { _ = result.Content.Close() }()
+
+			output, err := png.Decode(result.Content)
+			if err != nil {
+				t.Fatalf("failed to decode output: %v", err)
+			}
+
+			bounds := output.Bounds()
+			if bounds.Dx() != tt.size.Width || bounds.Dy() != tt.size.Height {
+				t.Fatalf("Process() made %dx%d, want %dx%d",
+					bounds.Dx(), bounds.Dy(), tt.size.Width, tt.size.Height)
+			}
+
+			row := make([]float64, bounds.Dx())
+			for x := range row {
+				r, _, _, _ := output.At(x, bounds.Dy()/2).RGBA()
+				row[x] = float64(r >> 8)
+			}
+
+			column := make([]float64, bounds.Dy())
+			for y := range column {
+				r, _, _, _ := output.At(bounds.Dx()/2, y).RGBA()
+				column[y] = float64(r >> 8)
+			}
+
+			width, height := squareLength(row), squareLength(column)
+			t.Logf("square is %.2f wide and %.2f high", width, height)
+
+			if width == 0 || math.Abs(width-height) > 0.5 {
+				t.Errorf("square is %.2f wide and %.2f high, want it square",
+					width, height)
+			}
+		})
+	}
+}

internal/imageprocessor/imageprocessor.go

@@ -475,8 +475,8 @@ func (p *ImageProcessor) resize(
 ) error {
 	switch fit {
 	case FitCover, "":
-		// Resize and crop to fill exact dimensions (default)
-		return img.Thumbnail(width, height, vips.InterestingCentre)
+		// Crop the centre and resize to fill exact dimensions (default)
+		return cropCentreAndResize(img, width, height)
 
 	case FitContain:
 		// Resize to fit within dimensions, maintaining aspect ratio
@@ -530,6 +530,38 @@ func (p *ImageProcessor) resize(
 	}
 }
 
+// cropCentreAndResize scales img by the same amount on both sides, the
+// least that makes it cover width x height, and cuts the centre width x
+// height out of it, as libvips' thumbnail does with InterestingCentre.
+// thumbnail alone scales the whole image and copies the result into memory
+// before it cuts, and for a thin image that is far larger than the image or
+// the output: 8192x64 at 8192x8192 is 1048576x8192, about 24 GiB. So the
+// centre is cut from img first, widened to whole pixels of img, which adds
+// less than one pixel of img on each side, and thumbnail scales and cuts
+// only that.
+func cropCentreAndResize(img *vips.ImageRef, width, height int) error {
+	imgW, imgH := img.Width(), img.Height()
+
+	// An image wider than the requested proportions keeps its height and
+	// loses width down to imgH*width/height, any other keeps its width and
+	// loses height down to imgW*height/width. Half of what is lost comes
+	// off each side, rounded down to whole pixels, so the part kept holds
+	// all of the centre.
+	left, top := 0, 0
+	if imgW*height > imgH*width {
+		left = (imgW*height - imgH*width) / (2 * height) //nolint:mnd // each side
+	} else {
+		top = (imgH*width - imgW*height) / (2 * width) //nolint:mnd // each side
+	}
+
+	err := img.ExtractArea(left, top, imgW-2*left, imgH-2*top)
+	if err != nil {
+		return err
+	}
+
+	return img.Thumbnail(width, height, vips.InterestingCentre)
+}
+
 const defaultQuality = 85
 
 // encode encodes an image to the specified format. govips gives pixa no way

internal/imgcache/imgcache.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"errors"
 	"io"
+	"net/url"
 	"time"
 )
 
@@ -86,9 +87,11 @@ func ValidateImageRequest(req *ImageRequest) error {
 type ImageRequest struct {
 	// SourceHost is the origin host (e.g., "cdn.example.com")
 	SourceHost string
-	// SourcePath is the path on the origin (e.g., "/photos/cat.jpg")
+	// SourcePath is the path on the origin with its percent-escapes decoded,
+	// as url.URL's Path holds it (e.g., "/photos/my cat.jpg")
 	SourcePath string
-	// SourceQuery is the optional query string for the origin URL
+	// SourceQuery is the optional query string for the origin URL as the URL
+	// writes it, percent-escapes included, as url.URL's RawQuery holds it
 	SourceQuery string
 	// Size is the requested output dimensions
 	Size Size
@@ -107,20 +110,25 @@ type ImageRequest struct {
 	AllowHTTP bool
 }
 
-// SourceURL returns the full upstream URL to fetch.
-// Uses http:// scheme when AllowHTTP is true, otherwise https://.
+// SourceURL returns the full upstream URL to fetch: the source path
+// percent-encoded again, as net/url writes a path, so each source path is one
+// upstream path, and the source query as it is, with the characters
+// escapeQuery encodes percent-encoded. Uses http:// scheme when AllowHTTP is
+// true, otherwise https://.
 func (r *ImageRequest) SourceURL() string {
 	scheme := "https"
 	if r.AllowHTTP {
 		scheme = "http"
 	}
 
-	url := scheme + "://" + r.SourceHost + r.SourcePath
-	if r.SourceQuery != "" {
-		url += "?" + r.SourceQuery
+	sourceURL := url.URL{
+		Scheme:   scheme,
+		Host:     r.SourceHost,
+		Path:     r.SourcePath,
+		RawQuery: escapeQuery(r.SourceQuery),
 	}
 
-	return url
+	return sourceURL.String()
 }
 
 // ImageResponse represents a processed image ready to serve

internal/imgcache/urlparser.go

@@ -51,6 +51,12 @@ var sizeFormatRegex = regexp.MustCompile(
 // percentEscapeRegex matches one percent-escape, such as "%2e"
 var percentEscapeRegex = regexp.MustCompile(`%[0-9A-Fa-f]{2}`)
 
+// queryEscapeRegex matches a percent-escape, or one character that a browser
+// percent-encodes in a URL's query: one outside the printable ASCII characters
+// "!" to "~", such as a space, a control character or a character outside
+// ASCII, or one of `"#<>`. A "%" that does not begin a percent-escape matches too.
+var queryEscapeRegex = regexp.MustCompile(`%[0-9A-Fa-f]{2}|[^!-~]|["#<>%]`)
+
 // hostRegex matches the forms the host of an image URL may take: a host name
 // or an IPv4 address (letters, digits, "-", "_" and "."), or an IPv6 address
 // in brackets (hex digits, ":" and "."), either one optionally followed by ":"
@@ -179,9 +185,12 @@ func parseImageComponents(remainder string) (*ParsedURL, error) {
 			`%w: the source path or query holds a "#"`, ErrInvalidPath)
 	}
 
-	// Extract query string if present in path
+	// Extract query string if present in path. It is the source URL's query
+	// as that URL writes it. A character a URL's query cannot hold as it is,
+	// such as a space written %20 in the image URL, is percent-encoded here,
+	// so the query is signed and cached as it is sent upstream
 	if qIndex := strings.Index(path, "?"); qIndex != -1 {
-		query = path[qIndex+1:]
+		query = escapeQuery(path[qIndex+1:])
 		path = path[:qIndex]
 	}
 
@@ -270,6 +279,20 @@ func decodePercentEscapes(s string) string {
 	})
 }
 
+// escapeQuery returns a source query as a URL's query holds it: each
+// character that queryEscapeRegex names, such as a space, percent-encoded,
+// and the rest, the query's percent-escapes included, kept as they are. The
+// result is unchanged by a second escapeQuery.
+func escapeQuery(query string) string {
+	return queryEscapeRegex.ReplaceAllStringFunc(query, func(match string) string {
+		if percentEscapeRegex.MatchString(match) {
+			return match
+		}
+
+		return url.PathEscape(match)
+	})
+}
+
 // parseSizeFormat parses strings like "800x600.webp", "orig.png" or "800x600"
 func parseSizeFormat(s string) (Size, ImageFormat, error) {
 	matches := sizeFormatRegex.FindStringSubmatch(s)

internal/server/source_url_internal_test.go

@@ -0,0 +1,371 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"regexp"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/session"
+)
+
+// percentSignPath is a source path holding a "%", which a URL writes %25.
+const percentSignPath = "/100%25.png"
+
+// generateFormPattern extracts the CSRF token rendered into the generator
+// page's form.
+var generateFormPattern = regexp.MustCompile(
+	`(?s)action="/generate".*?name="gorilla\.csrf\.Token" value="([^"]+)"`)
+
+// generatedURLPattern extracts the path of the encrypted URL the generator
+// page shows.
+var generatedURLPattern = regexp.MustCompile(`value="[^"]*(/v1/e/[^"]+)"`)
+
+// recordingUpstream is a test upstream that answers every request with a PNG
+// and records the request target, the path and query as sent, of each request
+// it receives.
+type recordingUpstream struct {
+	*httptest.Server
+
+	mu       sync.Mutex
+	received []string
+}
+
+// startRecordingUpstream starts a recordingUpstream, which is closed when the
+// test ends.
+func startRecordingUpstream(t *testing.T) *recordingUpstream {
+	t.Helper()
+
+	source := encodeTestPNG(t, 64, 48)
+	upstream := &recordingUpstream{}
+
+	upstream.Server = httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			upstream.mu.Lock()
+			upstream.received = append(upstream.received, r.RequestURI)
+			upstream.mu.Unlock()
+
+			w.Header().Set("Content-Type", "image/png")
+			_, _ = w.Write(source)
+		}))
+	t.Cleanup(upstream.Close)
+
+	return upstream
+}
+
+// requestTargets returns the request targets the upstream has received, in
+// the order it received them.
+func (u *recordingUpstream) requestTargets() []string {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	return slices.Clone(u.received)
+}
+
+// pageOrigin is the origin of the login and generator pages below, opened
+// over HTTPS; httptest gives every request the host example.com.
+const pageOrigin = "https://example.com"
+
+// openRoot sends GET / to s over HTTPS, with cookie when it is not nil, and
+// returns the page: the login page, or the generator page for a login session.
+func openRoot(
+	t *testing.T, s *Server, cookie *http.Cookie,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet,
+		pageOrigin+"/", nil)
+	if cookie != nil {
+		req.AddCookie(cookie)
+	}
+
+	page := httptest.NewRecorder()
+	s.ServeHTTP(page, req)
+
+	return page
+}
+
+// submitForm posts form to path on s as a browser submits it from page over
+// HTTPS: with the CSRF token tokenPattern finds in page, the cookies page set,
+// cookie when it is not nil, and a Referer naming the page, which pixa needs
+// while debug is off, as it is in startImageProxy's config.
+func submitForm(
+	t *testing.T, s *Server, page *httptest.ResponseRecorder,
+	tokenPattern *regexp.Regexp, path string, form url.Values,
+	cookie *http.Cookie,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	match := tokenPattern.FindStringSubmatch(page.Body.String())
+	if match == nil {
+		t.Fatalf("the page has no form for %s with a CSRF token", path)
+	}
+
+	form.Set("gorilla.csrf.Token", match[1])
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodPost,
+		pageOrigin+path, strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("Referer", pageOrigin+"/")
+
+	for _, c := range page.Result().Cookies() {
+		req.AddCookie(c)
+	}
+
+	if cookie != nil {
+		req.AddCookie(cookie)
+	}
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, req)
+
+	return rec
+}
+
+// postGenerate logs in to s with the signing key, submits the generator page's
+// form with sourceURL, and returns the response.
+func postGenerate(
+	t *testing.T, s *Server, sourceURL string,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	login := submitForm(t, s, openRoot(t, s, nil), csrfFieldPattern, "/",
+		url.Values{"key": {testSigningKey}}, nil)
+
+	var sessionCookie *http.Cookie
+
+	for _, c := range login.Result().Cookies() {
+		if c.Name == session.CookieName {
+			sessionCookie = c
+		}
+	}
+
+	if sessionCookie == nil {
+		t.Fatalf("logging in answered %d and set no session cookie", login.Code)
+	}
+
+	rec := submitForm(t, s, openRoot(t, s, sessionCookie), generateFormPattern,
+		"/generate", url.Values{"url": {sourceURL}}, sessionCookie)
+	t.Logf("POST /generate with url %s: %d", sourceURL, rec.Code)
+
+	return rec
+}
+
+// TestImageURLFetchesItsSourcePathAndQuery requests image URLs whose source
+// query holds a space, written percent-encoded, or a "'", and whose source
+// path holds a "%", written percent-encoded, and checks the one request the
+// upstream receives: the source path percent-encoded again, and the source
+// query with the space written %20 and the "'" written %27, as a browser
+// writes them in a query.
+func TestImageURLFetchesItsSourcePathAndQuery(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		// pathAndQuery is the source path and query as the image URL writes
+		// them
+		pathAndQuery string
+		// wantRequest is the path and query the upstream must receive
+		wantRequest string
+	}{
+		{"space in the query", "/photo.png%3Fname=my%20cat",
+			"/photo.png?name=my%20cat"},
+		{"apostrophe in the query", "/photo.png%3Fname=it's",
+			"/photo.png?name=it%27s"},
+		{"percent sign in the path", percentSignPath, percentSignPath},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			upstream := startRecordingUpstream(t)
+			s, _, _ := startImageProxy(t, upstream.Server)
+
+			getImage(t, s, "/v1/image/"+upstreamHost+tt.pathAndQuery+"/orig.png")
+
+			got := upstream.requestTargets()
+			t.Logf("upstream received %q", got)
+
+			if !slices.Equal(got, []string{tt.wantRequest}) {
+				t.Errorf("upstream received %q, want [%q]", got, tt.wantRequest)
+			}
+		})
+	}
+}
+
+// TestSourcePathsHoldingSpaceAndPercent20AreFetchedApart requests twice each
+// of the source paths /my cat.png and /my%20cat.png, which the image URL
+// writes my%20cat.png and my%2520cat.png. Each is fetched once, as a request
+// of its own, and its second request is served from the cache.
+func TestSourcePathsHoldingSpaceAndPercent20AreFetchedApart(t *testing.T) {
+	t.Parallel()
+
+	upstream := startRecordingUpstream(t)
+	s, _, _ := startImageProxy(t, upstream.Server)
+
+	imageURLs := []string{
+		"/v1/image/" + upstreamHost + "/my%20cat.png/orig.png",
+		"/v1/image/" + upstreamHost + "/my%2520cat.png/orig.png",
+	}
+
+	for _, imageURL := range imageURLs {
+		getImage(t, s, imageURL)
+	}
+
+	for _, imageURL := range imageURLs {
+		cacheStatus := getImage(t, s, imageURL).Header().Get("X-Pixa-Cache")
+		if cacheStatus != string(imgcache.CacheHit) {
+			t.Errorf("second GET %s: X-Pixa-Cache = %q, want HIT",
+				imageURL, cacheStatus)
+		}
+	}
+
+	got := upstream.requestTargets()
+	t.Logf("upstream received %q", got)
+
+	want := []string{"/my%20cat.png", "/my%2520cat.png"}
+	if !slices.Equal(got, want) {
+		t.Errorf("upstream received %q, want %q", got, want)
+	}
+}
+
+// TestSourceQueryWrittenTwoWaysIsOneUpstreamRequest requests the source query
+// name=my%20cat written as the README says, name=my%2520cat, and with the
+// space written %20, name=my%20cat. Both are one source query: the upstream
+// receives one request, and the second image URL is served from the cache.
+func TestSourceQueryWrittenTwoWaysIsOneUpstreamRequest(t *testing.T) {
+	t.Parallel()
+
+	upstream := startRecordingUpstream(t)
+	s, _, _ := startImageProxy(t, upstream.Server)
+
+	getImage(t, s, "/v1/image/"+upstreamHost+"/photo.png%3Fname=my%2520cat/orig.png")
+
+	second := getImage(t, s,
+		"/v1/image/"+upstreamHost+"/photo.png%3Fname=my%20cat/orig.png")
+	if got := second.Header().Get("X-Pixa-Cache"); got != string(imgcache.CacheHit) {
+		t.Errorf("second X-Pixa-Cache = %q, want HIT", got)
+	}
+
+	got := upstream.requestTargets()
+	t.Logf("upstream received %q", got)
+
+	want := []string{"/photo.png?name=my%20cat"}
+	if !slices.Equal(got, want) {
+		t.Errorf("upstream received %q, want %q", got, want)
+	}
+}
+
+// TestGeneratedURLFetchesTheSourceURLAsEntered makes an encrypted URL on the
+// generator page for source URLs holding %20, %25 and %23 in the path or the
+// query, requests it, and checks that the upstream receives the path and
+// query exactly as entered.
+func TestGeneratedURLFetchesTheSourceURLAsEntered(t *testing.T) {
+	t.Parallel()
+
+	// The names hold no "%": each test's state directory is named after it
+	// and goes into the database URL, which decodes percent-escapes
+	tests := []struct {
+		name         string
+		pathAndQuery string
+	}{
+		{"space in the path", "/my%20cat.png"},
+		{"percent sign in the path", percentSignPath},
+		{"number sign in the path", "/a%23b.png"},
+		{"all three in the query", "/photo.png?name=my%20cat%25%23"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			upstream := startRecordingUpstream(t)
+			s, _, _ := startImageProxy(t, upstream.Server)
+
+			rec := postGenerate(t, s, "https://"+upstreamHost+tt.pathAndQuery)
+
+			match := generatedURLPattern.FindStringSubmatch(rec.Body.String())
+			if rec.Code != http.StatusOK || match == nil {
+				t.Fatalf("POST /generate: %d with no encrypted URL; body %s",
+					rec.Code, rec.Body.String())
+			}
+
+			getImage(t, s, match[1])
+
+			got := upstream.requestTargets()
+			t.Logf("upstream received %q", got)
+
+			if !slices.Equal(got, []string{tt.pathAndQuery}) {
+				t.Errorf("upstream received %q, want [%q]", got, tt.pathAndQuery)
+			}
+		})
+	}
+}
+
+// TestGeneratorRefusesSourceURLItCannotFetchAsWritten submits source URLs the
+// generator page must make no encrypted URL for, and checks that it answers
+// 400 and shows why. pixa keeps the source path decoded and fetches it
+// percent-encoded again, so a path holding %2F would be fetched with a "/"
+// there, whatever else the path holds. pixa sends no user name or password,
+// and no "?" with no query after it. For these the page shows the URL pixa
+// would fetch. A source path holding a ".." segment is refused, as
+// /v1/image/ refuses it.
+func TestGeneratorRefusesSourceURLItCannotFetchAsWritten(t *testing.T) {
+	t.Parallel()
+
+	// The test config sets allow_http, so pixa would fetch over plain HTTP
+	const fetchedFrom = "http://" + upstreamHost
+
+	tests := []struct {
+		name string
+		// sourceURL is the source URL after "https://"
+		sourceURL string
+		// wantShown is what the page must show: the URL pixa would fetch, or
+		// why it makes no encrypted URL
+		wantShown string
+	}{
+		{"encoded slash", upstreamHost + "/a%2Fb.png",
+			fetchedFrom + "/a/b.png"},
+		{"encoded slash and a space", upstreamHost + "/a%2Fb c.png",
+			fetchedFrom + "/a/b%20c.png"},
+		{"encoded slash and a bar", upstreamHost + "/a%2Fb|c.png",
+			fetchedFrom + "/a/b%7Cc.png"},
+		{"encoded slash and a letter outside ASCII", upstreamHost + "/café%2Fx.png",
+			fetchedFrom + "/caf%C3%A9/x.png"},
+		{"user name and password", "user:pass@" + upstreamHost + "/a.png",
+			fetchedFrom + "/a.png"},
+		{"question mark with no query", upstreamHost + "/a.png?",
+			fetchedFrom + "/a.png"},
+		{"dot dot segment", upstreamHost + "/a/../b.png",
+			"cannot hold a .. segment"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			s, _, _ := startImageProxy(t, startRecordingUpstream(t).Server)
+
+			rec := postGenerate(t, s, "https://"+tt.sourceURL)
+			body := rec.Body.String()
+
+			if rec.Code != http.StatusBadRequest {
+				t.Errorf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+			}
+
+			if strings.Contains(body, "/v1/e/") {
+				t.Error("the page shows an encrypted URL")
+			}
+
+			if !strings.Contains(body, tt.wantShown) {
+				t.Errorf("the page does not show %s; body %s", tt.wantShown, body)
+			}
+		})
+	}
+}