fileserver: Support virtual file systems (#4909)

* fileserver: Support virtual file systems (close #3720)

This change replaces the hard-coded use of os.Open() and os.Stat() with
the use of the new (Go 1.16) io/fs APIs, enabling virtual file systems.
It introduces a new module namespace, caddy.fs, for such file systems.

Also improve documentation for the file server. I realized it was one of
the first modules written for Caddy 2, and the docs hadn't really been
updated since!

* Virtualize FS for file matcher; minor tweaks

* Fix tests and rename dirFS -> osFS

(Since we do not use a root directory, it is dynamic.)

1 files changed, 7 insertions, 5 deletions

diff --git a/modules/caddyhttp/fileserver/matcher_test.go b/modules/caddyhttp/fileserver/matcher_test.go
index fd109e6..7734bf1 100644
--- a/modules/caddyhttp/fileserver/matcher_test.go
+++ b/modules/caddyhttp/fileserver/matcher_test.go
@@ -111,8 +111,9 @@ func TestFileMatcher(t *testing.T) {
 		},
 	} {
 		m := &MatchFile{
-			Root:     "./testdata",
-			TryFiles: []string{"{http.request.uri.path}", "{http.request.uri.path}/"},
+			fileSystem: osFS{},
+			Root:       "./testdata",
+			TryFiles:   []string{"{http.request.uri.path}", "{http.request.uri.path}/"},
 		}
 
 		u, err := url.Parse(tc.path)
@@ -213,9 +214,10 @@ func TestPHPFileMatcher(t *testing.T) {
 		},
 	} {
 		m := &MatchFile{
-			Root:      "./testdata",
-			TryFiles:  []string{"{http.request.uri.path}", "{http.request.uri.path}/index.php"},
-			SplitPath: []string{".php"},
+			fileSystem: osFS{},
+			Root:       "./testdata",
+			TryFiles:   []string{"{http.request.uri.path}", "{http.request.uri.path}/index.php"},
+			SplitPath:  []string{".php"},
 		}
 
 		u, err := url.Parse(tc.path)