fileserver: Fix `file` matcher with empty `try_files` (#4147)

* fileserver: Fix `file` matcher with empty `try_files`

Fixes https://github.com/caddyserver/caddy/issues/4146

If `TryFiles` is empty, we fill it with `r.URL.Path`. In this case, this is `/`. Then later, in `prepareFilePath()`, we run the replacer (which turns `{path}` into `/` at that point) but `file` remains the original value (and the placeholder is still the placeholder there).

So then `strings.HasSuffix(file, "/")` will be `false` for the placeholder, but `true` for the empty `TryFiles` codepath, because `file` was `/` due to being set to the actual request value beforehand.

This means that `suffix` becomes `//` in that case, so after `sanitizedPathJoin`, it becomes `./`, so `strictFileExists`'s `strings.HasSuffix(file, separator)` codepath will return true.

I think we should change the `m.TryFiles == nil` codepath to `m.TryFiles = []string{"{http.request.uri.path}"}` for consistency. (And maybe consider hoisting this to `Provision` cause there's no point doing this on every request). I don't think this "optimization" of directly using `r.URL.Path` is so valuable, cause it causes this edgecase with directories.

* Update modules/caddyhttp/fileserver/matcher.go

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

1 files changed, 5 insertions, 7 deletions

diff --git a/modules/caddyhttp/fileserver/matcher.go b/modules/caddyhttp/fileserver/matcher.go
index 44ef9ed..064398c 100644
--- a/modules/caddyhttp/fileserver/matcher.go
+++ b/modules/caddyhttp/fileserver/matcher.go
@@ -139,6 +139,11 @@ func (m *MatchFile) Provision(_ caddy.Context) error {
 	if m.Root == "" {
 		m.Root = "{http.vars.root}"
 	}
+	// if list of files to try was omitted entirely, assume URL path
+	// (use placeholder instead of r.URL.Path; see issue #4146)
+	if m.TryFiles == nil {
+		m.TryFiles = []string{"{http.request.uri.path}"}
+	}
 	return nil
 }
 
@@ -174,13 +179,6 @@ func (m MatchFile) selectFile(r *http.Request) (matched bool) {
 
 	root := repl.ReplaceAll(m.Root, ".")
 
-	// if list of files to try was omitted entirely,
-	// assume URL path
-	if m.TryFiles == nil {
-		// m is not a pointer, so this is safe
-		m.TryFiles = []string{r.URL.Path}
-	}
-
 	// common preparation of the file into parts
 	prepareFilePath := func(file string) (suffix, fullpath, remainder string) {
 		suffix, remainder = m.firstSplit(path.Clean(repl.ReplaceAll(file, "")))