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, 11 insertions, 10 deletions

diff --git a/modules/caddyhttp/fileserver/browse.go b/modules/caddyhttp/fileserver/browse.go
index d59010d..bdddc23 100644
--- a/modules/caddyhttp/fileserver/browse.go
+++ b/modules/caddyhttp/fileserver/browse.go
@@ -19,6 +19,7 @@ import (
 	_ "embed"
 	"encoding/json"
 	"fmt"
+	"io/fs"
 	"net/http"
 	"os"
 	"path"
@@ -80,7 +81,7 @@ func (fsrv *FileServer) serveBrowse(root, dirPath string, w http.ResponseWriter,
 	repl := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
 
 	// calling path.Clean here prevents weird breadcrumbs when URL paths are sketchy like /%2e%2e%2f
-	listing, err := fsrv.loadDirectoryContents(dir, root, path.Clean(r.URL.Path), repl)
+	listing, err := fsrv.loadDirectoryContents(dir.(fs.ReadDirFile), root, path.Clean(r.URL.Path), repl)
 	switch {
 	case os.IsPermission(err):
 		return caddyhttp.Error(http.StatusForbidden, err)
@@ -133,8 +134,8 @@ func (fsrv *FileServer) serveBrowse(root, dirPath string, w http.ResponseWriter,
 	return nil
 }
 
-func (fsrv *FileServer) loadDirectoryContents(dir *os.File, root, urlPath string, repl *caddy.Replacer) (browseTemplateContext, error) {
-	files, err := dir.Readdir(-1)
+func (fsrv *FileServer) loadDirectoryContents(dir fs.ReadDirFile, root, urlPath string, repl *caddy.Replacer) (browseTemplateContext, error) {
+	files, err := dir.ReadDir(10000) // TODO: this limit should probably be configurable
 	if err != nil {
 		return browseTemplateContext{}, err
 	}
@@ -201,25 +202,25 @@ func (fsrv *FileServer) makeBrowseTemplate(tplCtx *templateContext) (*template.T
 	return tpl, nil
 }
 
-// isSymlink return true if f is a symbolic link
-func isSymlink(f os.FileInfo) bool {
-	return f.Mode()&os.ModeSymlink != 0
-}
-
 // isSymlinkTargetDir returns true if f's symbolic link target
 // is a directory.
-func isSymlinkTargetDir(f os.FileInfo, root, urlPath string) bool {
+func (fsrv *FileServer) isSymlinkTargetDir(f fs.FileInfo, root, urlPath string) bool {
 	if !isSymlink(f) {
 		return false
 	}
 	target := caddyhttp.SanitizedPathJoin(root, path.Join(urlPath, f.Name()))
-	targetInfo, err := os.Stat(target)
+	targetInfo, err := fsrv.fileSystem.Stat(target)
 	if err != nil {
 		return false
 	}
 	return targetInfo.IsDir()
 }
 
+// isSymlink return true if f is a symbolic link.
+func isSymlink(f fs.FileInfo) bool {
+	return f.Mode()&os.ModeSymlink != 0
+}
+
 // templateContext powers the context used when evaluating the browse template.
 // It combines browse-specific features with the standard templates handler
 // features.