net/http: add ServeFileFS, FileServerFS, NewFileTransportFS

These new apis are analogous to ServeFile, FileServer and NewFileTransport respectively. The main difference is that these functions operate on an fs.FS.

Fixes #51971

Change-Id: Ie56b245b795eeb7edf613657578592306945469b
GitHub-Last-Rev: 26e75c0368f155a2299fbdcb72f47036b71a5e06
GitHub-Pull-Request: golang/go#61641
Reviewed-on: https://go-review.googlesource.com/c/go/+/513956
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

4 files changed, 157 insertions, 3 deletions

diff --git a/src/net/http/filetransport.go b/src/net/http/filetransport.go
index 94684b07a1..2a9e9b02ba 100644
--- a/src/net/http/filetransport.go
+++ b/src/net/http/filetransport.go
@@ -7,6 +7,7 @@ package http
 import (
 	"fmt"
 	"io"
+	"io/fs"
 )
 
 // fileTransport implements RoundTripper for the 'file' protocol.
@@ -31,6 +32,24 @@ func NewFileTransport(fs FileSystem) RoundTripper {
 	return fileTransport{fileHandler{fs}}
 }
 
+// NewFileTransportFS returns a new RoundTripper, serving the provided
+// file system fsys. The returned RoundTripper ignores the URL host in its
+// incoming requests, as well as most other properties of the
+// request.
+//
+// The typical use case for NewFileTransportFS is to register the "file"
+// protocol with a Transport, as in:
+//
+//	fsys := os.DirFS("/")
+//	t := &http.Transport{}
+//	t.RegisterProtocol("file", http.NewFileTransportFS(fsys))
+//	c := &http.Client{Transport: t}
+//	res, err := c.Get("file:///etc/passwd")
+//	...
+func NewFileTransportFS(fsys fs.FS) RoundTripper {
+	return NewFileTransport(FS(fsys))
+}
+
 func (t fileTransport) RoundTrip(req *Request) (resp *Response, err error) {
 	// We start ServeHTTP in a goroutine, which may take a long
 	// time if the file is large. The newPopulateResponseWriter
diff --git a/src/net/http/filetransport_test.go b/src/net/http/filetransport_test.go
index 77fc8eeccf..b3e3301e10 100644
--- a/src/net/http/filetransport_test.go
+++ b/src/net/http/filetransport_test.go
@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+	"testing/fstest"
 )
 
 func checker(t *testing.T) func(string, error) {
@@ -62,3 +63,44 @@ func TestFileTransport(t *testing.T) {
 	}
 	res.Body.Close()
 }
+
+func TestFileTransportFS(t *testing.T) {
+	check := checker(t)
+
+	fsys := fstest.MapFS{
+		"index.html": {Data: []byte("index.html says hello")},
+	}
+
+	tr := &Transport{}
+	tr.RegisterProtocol("file", NewFileTransportFS(fsys))
+	c := &Client{Transport: tr}
+
+	for fname, mfile := range fsys {
+		urlstr := "file:///" + fname
+		res, err := c.Get(urlstr)
+		check("Get "+urlstr, err)
+		if res.StatusCode != 200 {
+			t.Errorf("for %s, StatusCode = %d, want 200", urlstr, res.StatusCode)
+		}
+		if res.ContentLength != -1 {
+			t.Errorf("for %s, ContentLength = %d, want -1", urlstr, res.ContentLength)
+		}
+		if res.Body == nil {
+			t.Fatalf("for %s, nil Body", urlstr)
+		}
+		slurp, err := io.ReadAll(res.Body)
+		res.Body.Close()
+		check("ReadAll "+urlstr, err)
+		if string(slurp) != string(mfile.Data) {
+			t.Errorf("for %s, got content %q, want %q", urlstr, string(slurp), "Bar")
+		}
+	}
+
+	const badURL = "file://../no-exist.txt"
+	res, err := c.Get(badURL)
+	check("Get "+badURL, err)
+	if res.StatusCode != 404 {
+		t.Errorf("for %s, StatusCode = %d, want 404", badURL, res.StatusCode)
+	}
+	res.Body.Close()
+}
diff --git a/src/net/http/fs.go b/src/net/http/fs.go
index 41e0b43ac8..c605fe3aca 100644
--- a/src/net/http/fs.go
+++ b/src/net/http/fs.go
@@ -741,6 +741,40 @@ func ServeFile(w ResponseWriter, r *Request, name string) {
 	serveFile(w, r, Dir(dir), file, false)
 }
 
+// ServeFileFS replies to the request with the contents
+// of the named file or directory from the file system fsys.
+//
+// If the provided file or directory name is a relative path, it is
+// interpreted relative to the current directory and may ascend to
+// parent directories. If the provided name is constructed from user
+// input, it should be sanitized before calling ServeFile.
+//
+// As a precaution, ServeFile will reject requests where r.URL.Path
+// contains a ".." path element; this protects against callers who
+// might unsafely use filepath.Join on r.URL.Path without sanitizing
+// it and then use that filepath.Join result as the name argument.
+//
+// As another special case, ServeFile redirects any request where r.URL.Path
+// ends in "/index.html" to the same path, without the final
+// "index.html". To avoid such redirects either modify the path or
+// use ServeContent.
+//
+// Outside of those two special cases, ServeFile does not use
+// r.URL.Path for selecting the file or directory to serve; only the
+// file or directory provided in the name argument is used.
+func ServeFileFS(w ResponseWriter, r *Request, fsys fs.FS, name string) {
+	if containsDotDot(r.URL.Path) {
+		// Too many programs use r.URL.Path to construct the argument to
+		// serveFile. Reject the request under the assumption that happened
+		// here and ".." may not be wanted.
+		// Note that name might not contain "..", for example if code (still
+		// incorrectly) used filepath.Join(myDir, r.URL.Path).
+		Error(w, "invalid URL path", StatusBadRequest)
+		return
+	}
+	serveFile(w, r, FS(fsys), name, false)
+}
+
 func containsDotDot(v string) bool {
 	if !strings.Contains(v, "..") {
 		return false
@@ -850,13 +884,23 @@ func FS(fsys fs.FS) FileSystem {
 //
 //	http.Handle("/", http.FileServer(http.Dir("/tmp")))
 //
-// To use an fs.FS implementation, use http.FS to convert it:
-//
-//	http.Handle("/", http.FileServer(http.FS(fsys)))
+// To use an fs.FS implementation, use http.FileServerFS instead.
 func FileServer(root FileSystem) Handler {
 	return &fileHandler{root}
 }
 
+// FileServerFS returns a handler that serves HTTP requests
+// with the contents of the file system fsys.
+//
+// As a special case, the returned file server redirects any request
+// ending in "/index.html" to the same path, without the final
+// "index.html".
+//
+//	http.Handle("/", http.FileServerFS(fsys))
+func FileServerFS(root fs.FS) Handler {
+	return FileServer(FS(root))
+}
+
 func (f *fileHandler) ServeHTTP(w ResponseWriter, r *Request) {
 	upath := r.URL.Path
 	if !strings.HasPrefix(upath, "/") {
diff --git a/src/net/http/fs_test.go b/src/net/http/fs_test.go
index 3fb9e01235..bb96d2ca68 100644
--- a/src/net/http/fs_test.go
+++ b/src/net/http/fs_test.go
@@ -26,6 +26,7 @@ import (
 	"runtime"
 	"strings"
 	"testing"
+	"testing/fstest"
 	"time"
 )
 
@@ -1559,3 +1560,51 @@ func testFileServerMethods(t *testing.T, mode testMode) {
 		}
 	}
 }
+
+func TestFileServerFS(t *testing.T) {
+	filename := "index.html"
+	contents := []byte("index.html says hello")
+	fsys := fstest.MapFS{
+		filename: {Data: contents},
+	}
+	ts := newClientServerTest(t, http1Mode, FileServerFS(fsys)).ts
+	defer ts.Close()
+
+	res, err := ts.Client().Get(ts.URL + "/" + filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+	b, err := io.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal("reading Body:", err)
+	}
+	if s := string(b); s != string(contents) {
+		t.Errorf("for path %q got %q, want %q", filename, s, contents)
+	}
+	res.Body.Close()
+}
+
+func TestServeFileFS(t *testing.T) {
+	filename := "index.html"
+	contents := []byte("index.html says hello")
+	fsys := fstest.MapFS{
+		filename: {Data: contents},
+	}
+	ts := newClientServerTest(t, http1Mode, HandlerFunc(func(w ResponseWriter, r *Request) {
+		ServeFileFS(w, r, fsys, filename)
+	})).ts
+	defer ts.Close()
+
+	res, err := ts.Client().Get(ts.URL + "/" + filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+	b, err := io.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal("reading Body:", err)
+	}
+	if s := string(b); s != string(contents) {
+		t.Errorf("for path %q got %q, want %q", filename, s, contents)
+	}
+	res.Body.Close()
+}