backport #2104 (#2121)

Return empty when parsing a multi-part POST with only one end delimiter.

Fixed: #2103

Sending the following request in a browser generates a request with
with only one end delimiter.

```javascript
const formData = new FormData();
const request = new Request('http://127.0.0.1:8080/', {
  method: 'POST',
  body: formData,
});
const response = fetch(request);
```

```
curl 'http://127.0.0.1:8080/' \
  -H 'Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryR1LC4tR6ayskIXJm' \
  --data-raw $'------WebKitFormBoundaryR1LC4tR6ayskIXJm--\r\n'
```

This request is not compliant RFC7578, but is generated by major browsers such as
FireFox and Chrome.
Supporting this request will cause the multipart parser to return an empty value.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 All notable changes to this project will be documented in this file. For info on how to format all future additions to this file please reference [Keep A Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## Unreleased
+
+- Return empty when parsing a multi-part POST with only one end delimiter. ([#2104](https://github.com/rack/rack/pull/2104), [@alpaca-tc])
+
 ## [2.2.8] - 2023-07-31
 
 - Regenerate SPEC ([#2102](https://github.com/rack/rack/pull/2102), [@skipkayhil](https://github.com/skipkayhil))

lib/rack/multipart/parser.rb

@@ -191,6 +191,7 @@ def initialize(boundary, tempfile, bufsize, query_parser)
 
         @sbuf = StringScanner.new("".dup)
         @body_regex = /(?:#{EOL})?#{Regexp.quote(@boundary)}(?:#{EOL}|--)/m
+        @end_boundary_size = boundary.bytesize + 6 # (-- at start, -- at finish, EOL at end)
         @rx_max_size = EOL.size + @boundary.bytesize + [EOL.size, '--'.size].max
         @head_regex = /(.*?#{EOL})#{EOL}/m
       end
@@ -231,7 +232,12 @@ def run_parser
       end
 
       def handle_fast_forward
-        if consume_boundary
+        tok = consume_boundary
+
+        if tok == :END_BOUNDARY && @sbuf.pos == @end_boundary_size && @sbuf.eos?
+          # stop parsing a buffer if a buffer is only an end boundary.
+          @state = :DONE
+        elsif tok
           @state = :MIME_HEAD
         else
           raise EOFError, "bad content body" if @sbuf.rest_size >= @bufsize

test/spec_request.rb

@@ -1009,6 +1009,24 @@ def initialize(*)
     f[:tempfile].size.must_equal 76
   end
 
+  it "parse multipart delimiter-only boundary" do
+    input = <<EOF
+--AaB03x--\r
+EOF
+    mr = Rack::MockRequest.env_for(
+      "/",
+      "CONTENT_TYPE" => "multipart/form-data, boundary=AaB03x",
+      "CONTENT_LENGTH" => input.size,
+      :input => input
+    )
+
+    req = make_request mr
+    req.query_string.must_equal ""
+    req.GET.must_be :empty?
+    req.POST.must_be :empty?
+    req.params.must_equal({})
+  end
+
   it "MultipartPartLimitError when request has too many multipart file parts if limit set" do
     begin
       data = 10000.times.map { "--AaB03x\r\nContent-Type: text/plain\r\nContent-Disposition: attachment; name=#{SecureRandom.hex(10)}; filename=#{SecureRandom.hex(10)}\r\n\r\ncontents\r\n" }.join("\r\n")