celFileMatcherMacroExpander() does not accept "try_files" map entry with a Comprehension value

[effleurager]

Looking at the documentation for the file matcher, the example CEL expression uses an array of strings to specify a list of files to search for. Seeming like the only alternative to globbing for Windows users, I was surprised to find building a list failed to compile with the following error:

provision http.matchers.expression: compiling CEL program: ERROR: <input>:1:6: matcher requires either a map or string literal argument

As a toy example, this seems like it should be a valid Caddyfile:

domain.tld {
  root * C:/path/to/folder
  @autosave `file({
    "try_files": [0,1,2,3,4,5,7,8,9].map(i, {path} + "_autosave_" + string(i) + ".sav"),
    "try_policy": "most_recently_modified"
  })`
  rewrite @autosave {file_match.relative}
  file_server
}

Given the severe restriction of globbing being unavailable, this seems like an important fallback to have?

[francislavoie]

Due to how matchers are provisioned (once at config load time, not on every request), they need to have static values as input (except for placeholders). I'm not sure it would be possible to implement this. From a type system standpoint I don't see how the MatchFile struct could take a dynamic try_files input fed by CEL.

In your case, can't you just list out all 10 permutations you want to try? That should work, no? The config is verbose yes, but it should work.

[effleurager]

While testing, I discovered a bug with the try_policy value not being assigned:

caddy/modules/caddyhttp/fileserver/matcher.go

Lines 192 to 195 in 48ce47f

	var try_policy string
	if len(values["try_policy"]) > 0 {
	root = values["try_policy"][0]
	}

I'm preparing a PR, but I'm wondering if I should add a test for CELLibrary specifically or just add an addition to expressionTests?

caddy/modules/caddyhttp/fileserver/matcher_test.go

Lines 283 to 292 in 48ce47f

	var expressionTests = []struct {
	name string
	expression *caddyhttp.MatchExpression
	urlTarget string
	httpMethod string
	httpHeader *http.Header
	wantErr bool
	wantResult bool
	clientCertificate []byte
	}{