net/http: map FS Open errors just like Dir

When an http.FileServer is given a path like file1/file2 where file1
exists but file2 does not, the proper HTTP status should be
NotFound. Some OSes return a "not a directory" error instead, so this
must be mapped to NotFound.

That mapping was already being done for the Dir FileSystem
implementation, as discussed in #18984. But it wasn't for the
FS implementation.

This CL does the same mapping for FS, by generalizing the function
that did it for Dir.

Fixes #49552

Change-Id: I61d6aa8ef101158e9674707d44e653f5dedbd040
Reviewed-on: https://go-review.googlesource.com/c/go/+/376874
Trust: Jonathan Amsterdam <[email protected]>
Run-TryBot: Jonathan Amsterdam <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: jba

src/net/http/fs.go

@@ -42,20 +42,20 @@ import (
 // An empty Dir is treated as ".".
 type Dir string
 
-// mapDirOpenError maps the provided non-nil error from opening name
+// mapOpenError maps the provided non-nil error from opening name
 // to a possibly better non-nil error. In particular, it turns OS-specific errors
-// about opening files in non-directories into fs.ErrNotExist. See Issue 18984.
-func mapDirOpenError(originalErr error, name string) error {
+// about opening files in non-directories into fs.ErrNotExist. See Issues 18984 and 49552.
+func mapOpenError(originalErr error, name string, sep rune, stat func(string) (fs.FileInfo, error)) error {
 	if errors.Is(originalErr, fs.ErrNotExist) || errors.Is(originalErr, fs.ErrPermission) {
 		return originalErr
 	}
 
-	parts := strings.Split(name, string(filepath.Separator))
+	parts := strings.Split(name, string(sep))
 	for i := range parts {
 		if parts[i] == "" {
 			continue
 		}
-		fi, err := os.Stat(strings.Join(parts[:i+1], string(filepath.Separator)))
+		fi, err := stat(strings.Join(parts[:i+1], string(sep)))
 		if err != nil {
 			return originalErr
 		}
@@ -79,7 +79,7 @@ func (d Dir) Open(name string) (File, error) {
 	fullName := filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name)))
 	f, err := os.Open(fullName)
 	if err != nil {
-		return nil, mapDirOpenError(err, fullName)
+		return nil, mapOpenError(err, fullName, filepath.Separator, os.Stat)
 	}
 	return f, nil
 }
@@ -759,7 +759,9 @@ func (f ioFS) Open(name string) (File, error) {
 	}
 	file, err := f.fsys.Open(name)
 	if err != nil {
-		return nil, err
+		return nil, mapOpenError(err, name, '/', func(path string) (fs.FileInfo, error) {
+			return fs.Stat(f.fsys, path)
+		})
 	}
 	return ioFile{file}, nil
 }

src/net/http/fs_test.go

@@ -1244,10 +1244,19 @@ func TestLinuxSendfileChild(*testing.T) {
 	}
 }
 
-// Issue 18984: tests that requests for paths beyond files return not-found errors
+// Issues 18984, 49552: tests that requests for paths beyond files return not-found errors
 func TestFileServerNotDirError(t *testing.T) {
 	defer afterTest(t)
-	ts := httptest.NewServer(FileServer(Dir("testdata")))
+	t.Run("Dir", func(t *testing.T) {
+		testFileServerNotDirError(t, func(path string) FileSystem { return Dir(path) })
+	})
+	t.Run("FS", func(t *testing.T) {
+		testFileServerNotDirError(t, func(path string) FileSystem { return FS(os.DirFS(path)) })
+	})
+}
+
+func testFileServerNotDirError(t *testing.T, newfs func(string) FileSystem) {
+	ts := httptest.NewServer(FileServer(newfs("testdata")))
 	defer ts.Close()
 
 	res, err := Get(ts.URL + "/index.html/not-a-file")
@@ -1259,9 +1268,9 @@ func TestFileServerNotDirError(t *testing.T) {
 		t.Errorf("StatusCode = %v; want 404", res.StatusCode)
 	}
 
-	test := func(name string, dir Dir) {
+	test := func(name string, fsys FileSystem) {
 		t.Run(name, func(t *testing.T) {
-			_, err = dir.Open("/index.html/not-a-file")
+			_, err = fsys.Open("/index.html/not-a-file")
 			if err == nil {
 				t.Fatal("err == nil; want != nil")
 			}
@@ -1270,7 +1279,7 @@ func TestFileServerNotDirError(t *testing.T) {
 					errors.Is(err, fs.ErrNotExist))
 			}
 
-			_, err = dir.Open("/index.html/not-a-dir/not-a-file")
+			_, err = fsys.Open("/index.html/not-a-dir/not-a-file")
 			if err == nil {
 				t.Fatal("err == nil; want != nil")
 			}
@@ -1286,8 +1295,8 @@ func TestFileServerNotDirError(t *testing.T) {
 		t.Fatal("get abs path:", err)
 	}
 
-	test("RelativePath", Dir("testdata"))
-	test("AbsolutePath", Dir(absPath))
+	test("RelativePath", newfs("testdata"))
+	test("AbsolutePath", newfs(absPath))
 }
 
 func TestFileServerCleanPath(t *testing.T) {