reverseproxy: Sync up `handleUpgradeResponse` with stdlib (#4664)

* reverseproxy: Sync up `handleUpgradeResponse` with stdlib

I had left this as a TODO for when we bump to minimum 1.17, but I should've realized it was under `internal` so it couldn't be used directly.

Copied the functions we needed for parity. Hopefully this is ok!

* Add tests and fix godoc comments

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

1 files changed, 57 insertions, 0 deletions

diff --git a/modules/caddyhttp/reverseproxy/ascii.go b/modules/caddyhttp/reverseproxy/ascii.go
new file mode 100644
index 0000000..75b8220
--- /dev/null
+++ b/modules/caddyhttp/reverseproxy/ascii.go
@@ -0,0 +1,57 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Most of the code in this file was initially borrowed from the Go
+// standard library and modified; It had this copyright notice:
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Original source, copied because the package was marked internal:
+// https://github.com/golang/go/blob/5c489514bc5e61ad9b5b07bd7d8ec65d66a0512a/src/net/http/internal/ascii/print.go
+
+package reverseproxy
+
+// asciiEqualFold is strings.EqualFold, ASCII only. It reports whether s and t
+// are equal, ASCII-case-insensitively.
+func asciiEqualFold(s, t string) bool {
+	if len(s) != len(t) {
+		return false
+	}
+	for i := 0; i < len(s); i++ {
+		if asciiLower(s[i]) != asciiLower(t[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+// asciiLower returns the ASCII lowercase version of b.
+func asciiLower(b byte) byte {
+	if 'A' <= b && b <= 'Z' {
+		return b + ('a' - 'A')
+	}
+	return b
+}
+
+// asciiIsPrint returns whether s is ASCII and printable according to
+// https://tools.ietf.org/html/rfc20#section-4.2.
+func asciiIsPrint(s string) bool {
+	for i := 0; i < len(s); i++ {
+		if s[i] < ' ' || s[i] > '~' {
+			return false
+		}
+	}
+	return true
+}