Fetch the source path and query URL-encoded, not joined as text (closes #288)

The source path, decoded, is fetched percent-encoded again by net/url,
so 100%25.png fetches /100%25.png, and my%20cat.png and my%2520cat.png
fetch different paths. The source query stays as the source URL writes
it; the characters a browser percent-encodes in a query, such as a
space, 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 for a
source URL its image URL would fetch written another way, such as with
%2F in the path, and shows the URL pixa would fetch. README.md says how
the source path and query are fetched.

Model: opus-5-5

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)