Add general `Rack::BadRequest`. (#2019)

Used to communicate a class of exceptions that represent 400 Bad Request
semantics.

7 files changed, 40 insertions, 8 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7fcc45a7..c852a84a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,10 @@ All notable changes to this project will be documented in this file. For info on
 
 - `rack.input` is now optional. ([#1997](https://github.com/rack/rack/pull/1997), [@ioquatix])
 
+### Changed
+
+- Improved handling of multipart requests. `rack.input` is now optional, and if missing, will raise an error which includes `module Rack::BadRequest`. Several other exceptions also include this module. ([#1997](https://github.com/rack/rack/pull/1997), [@ioquatix])
+
 ## [3.0.3] - 2022-12-07
 
 ### Fixed
diff --git a/lib/rack.rb b/lib/rack.rb
index 5b87ea1b..2dd8d3dc 100644
--- a/lib/rack.rb
+++ b/lib/rack.rb
@@ -15,6 +15,7 @@ require_relative 'rack/version'
 require_relative 'rack/constants'
 
 module Rack
+  autoload :BadRequest, "rack/bad_request"
   autoload :Builder, "rack/builder"
   autoload :BodyProxy, "rack/body_proxy"
   autoload :Cascade, "rack/cascade"
diff --git a/lib/rack/bad_request.rb b/lib/rack/bad_request.rb
new file mode 100644
index 00000000..9c675126
--- /dev/null
+++ b/lib/rack/bad_request.rb
@@ -0,0 +1,6 @@
+module Rack
+  # Represents a 400 Bad Request error when input data fails to meet the
+  # requirements.
+  module BadRequest
+  end
+end
diff --git a/lib/rack/multipart.rb b/lib/rack/multipart.rb
index 76b25a51..165b4db3 100644
--- a/lib/rack/multipart.rb
+++ b/lib/rack/multipart.rb
@@ -6,6 +6,8 @@ require_relative 'utils'
 require_relative 'multipart/parser'
 require_relative 'multipart/generator'
 
+require_relative 'bad_request'
+
 module Rack
   # A multipart form data parser, adapted from IOWA.
   #
@@ -14,6 +16,7 @@ module Rack
     MULTIPART_BOUNDARY = "AaB03x"
 
     class MissingInputError < StandardError
+      include BadRequest
     end
 
     class << self
diff --git a/lib/rack/multipart/parser.rb b/lib/rack/multipart/parser.rb
index ceafbc6b..c49c80e6 100644
--- a/lib/rack/multipart/parser.rb
+++ b/lib/rack/multipart/parser.rb
@@ -3,18 +3,28 @@
 require 'strscan'
 
 require_relative '../utils'
+require_relative '../bad_request'
 
 module Rack
   module Multipart
-    class MultipartPartLimitError < Errno::EMFILE; end
+    class MultipartPartLimitError < Errno::EMFILE
+      include BadRequest
+    end
 
     # Use specific error class when parsing multipart request
     # that ends early.
-    class EmptyContentError < ::EOFError; end
+    class EmptyContentError < ::EOFError
+      include BadRequest
+    end
 
     # Base class for multipart exceptions that do not subclass from
     # other exception classes for backwards compatibility.
-    class Error < StandardError; end
+    class BoundaryTooLongError < StandardError
+      include BadRequest
+    end
+
+    # Prefer to use the BoundaryTooLongError class or Rack::BadRequest.
+    Error = BoundaryTooLongError
 
     EOL = "\r\n"
     MULTIPART = %r|\Amultipart/.*boundary=\"?([^\";,]+)\"?|ni
@@ -96,7 +106,7 @@ module Rack
         if boundary.length > 70
           # RFC 1521 Section 7.2.1 imposes a 70 character maximum for the boundary.
           # Most clients use no more than 55 characters.
-          raise Error, "multipart boundary size too large (#{boundary.length} characters)"
+          raise BoundaryTooLongError, "multipart boundary size too large (#{boundary.length} characters)"
         end
 
         io = BoundedIO.new(io, content_length) if content_length
diff --git a/lib/rack/query_parser.rb b/lib/rack/query_parser.rb
index a89c4c7f..7c9621ed 100644
--- a/lib/rack/query_parser.rb
+++ b/lib/rack/query_parser.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require_relative 'bad_request'
+
 module Rack
   class QueryParser
     DEFAULT_SEP = /[&] */n
@@ -7,16 +9,22 @@ module Rack
 
     # ParameterTypeError is the error that is raised when incoming structural
     # parameters (parsed by parse_nested_query) contain conflicting types.
-    class ParameterTypeError < TypeError; end
+    class ParameterTypeError < TypeError
+      include BadRequest
+    end
 
     # InvalidParameterError is the error that is raised when incoming structural
     # parameters (parsed by parse_nested_query) contain invalid format or byte
     # sequence.
-    class InvalidParameterError < ArgumentError; end
+    class InvalidParameterError < ArgumentError
+      include BadRequest
+    end
 
     # ParamsTooDeepError is the error that is raised when params are recursively
     # nested over the specified limit.
-    class ParamsTooDeepError < RangeError; end
+    class ParamsTooDeepError < RangeError
+      include BadRequest
+    end
 
     def self.make_default(param_depth_limit)
       new Params, param_depth_limit
diff --git a/test/spec_multipart.rb b/test/spec_multipart.rb
index 7097528a..3149dd6b 100644
--- a/test/spec_multipart.rb
+++ b/test/spec_multipart.rb
@@ -38,7 +38,7 @@ describe Rack::Multipart do
     env = Rack::MockRequest.env_for("/", multipart_fixture(:content_type_and_no_filename, "A"*71))
     lambda {
       Rack::Multipart.parse_multipart(env)
-    }.must_raise Rack::Multipart::Error
+    }.must_raise Rack::Multipart::BoundaryTooLongError
   end
 
   it "raises a bad request exception if no body is given but content type indicates a multipart body" do