fileserver: Only redirect if filename not rewritten (fix #4205)

This is the more correct implementation of  23dadc0 (#4179)... I think. This commit effectively undoes the revert in 8848df9, but with corrections to the logic.

We *do* need to use the original request path (the path the browser knows) for redirects, since they are external, and rewrites are only internal.

However, if the path was rewritten to a non-canonical path, we should not redirect to canonicalize that, since rewrites are intentional by the site owner. Canonicalizing the path involves modifying only the suffix (base element, or filename) of the path. Thus, if a rewrite involves only the prefix (like how handle_path strips a path prefix), then we can (hopefully!) safely redirect using the original URI since the filename was not rewritten.

So basically, if rewrites modify the filename, we should not canonicalize those requests. If rewrites only modify another part of the path (commonly a prefix), we should be OK to redirect.

modules/caddyhttp/fileserver/browse.go

@@ -42,15 +42,28 @@ func (fsrv *FileServer) serveBrowse(root, dirPath string, w http.ResponseWriter,
 		zap.String("path", dirPath),
 		zap.String("root", root))
 
-	// navigation on the client-side gets messed up if the
-	// URL doesn't end in a trailing slash because hrefs like
-	// "/b/c" on a path like "/a" end up going to "/b/c" instead
+	// Navigation on the client-side gets messed up if the
+	// URL doesn't end in a trailing slash because hrefs to
+	// "b/c" at path "/a" end up going to "/b/c" instead
 	// of "/a/b/c" - so we have to redirect in this case
-	if !strings.HasSuffix(r.URL.Path, "/") {
-		fsrv.logger.Debug("redirecting to trailing slash to preserve hrefs", zap.String("request_path", r.URL.Path))
-		r.URL.Path += "/"
-		http.Redirect(w, r, r.URL.String(), http.StatusMovedPermanently)
-		return nil
+	// so that the path is "/a/" and the client constructs
+	// relative hrefs "b/c" to be "/a/b/c".
+	//
+	// Only redirect if the last element of the path (the filename) was not
+	// rewritten; if the admin wanted to rewrite to the canonical path, they
+	// would have, and we have to be very careful not to introduce unwanted
+	// redirects and especially redirect loops! (Redirecting using the
+	// original URI is necessary because that's the URI the browser knows,
+	// we don't want to redirect from internally-rewritten URIs.)
+	// See https://github.com/caddyserver/caddy/issues/4205.
+	origReq := r.Context().Value(caddyhttp.OriginalRequestCtxKey).(http.Request)
+	if path.Base(origReq.URL.Path) == path.Base(r.URL.Path) {
+		if !strings.HasSuffix(origReq.URL.Path, "/") {
+			fsrv.logger.Debug("redirecting to trailing slash to preserve hrefs", zap.String("request_path", r.URL.Path))
+			origReq.URL.Path += "/"
+			http.Redirect(w, r, origReq.URL.String(), http.StatusMovedPermanently)
+			return nil
+		}
 	}
 
 	dir, err := fsrv.openFile(dirPath, w)

modules/caddyhttp/fileserver/staticfiles.go

@@ -20,6 +20,7 @@ import (
 	"mime"
 	"net/http"
 	"os"
+	"path"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -240,12 +241,26 @@ func (fsrv *FileServer) ServeHTTP(w http.ResponseWriter, r *http.Request, next c
 	// trailing slash - not enforcing this can break relative hrefs
 	// in HTML (see https://github.com/caddyserver/caddy/issues/2741)
 	if fsrv.CanonicalURIs == nil || *fsrv.CanonicalURIs {
-		if implicitIndexFile && !strings.HasSuffix(r.URL.Path, "/") {
-			fsrv.logger.Debug("redirecting to canonical URI (adding trailing slash for directory)", zap.String("path", r.URL.Path))
-			return redirect(w, r, r.URL.Path+"/")
-		} else if !implicitIndexFile && strings.HasSuffix(r.URL.Path, "/") {
-			fsrv.logger.Debug("redirecting to canonical URI (removing trailing slash for file)", zap.String("path", r.URL.Path))
-			return redirect(w, r, r.URL.Path[:len(r.URL.Path)-1])
+		// Only redirect if the last element of the path (the filename) was not
+		// rewritten; if the admin wanted to rewrite to the canonical path, they
+		// would have, and we have to be very careful not to introduce unwanted
+		// redirects and especially redirect loops!
+		// See https://github.com/caddyserver/caddy/issues/4205.
+		origReq := r.Context().Value(caddyhttp.OriginalRequestCtxKey).(http.Request)
+		if path.Base(origReq.URL.Path) == path.Base(r.URL.Path) {
+			if implicitIndexFile && !strings.HasSuffix(origReq.URL.Path, "/") {
+				to := origReq.URL.Path + "/"
+				fsrv.logger.Debug("redirecting to canonical URI (adding trailing slash for directory)",
+					zap.String("from_path", origReq.URL.Path),
+					zap.String("to_path", to))
+				return redirect(w, r, to)
+			} else if !implicitIndexFile && strings.HasSuffix(origReq.URL.Path, "/") {
+				to := origReq.URL.Path[:len(origReq.URL.Path)-1]
+				fsrv.logger.Debug("redirecting to canonical URI (removing trailing slash for file)",
+					zap.String("from_path", origReq.URL.Path),
+					zap.String("to_path", to))
+				return redirect(w, r, to)
+			}
 		}
 	}