Move HTTP Signature parsing code to its own class (#28932)

app/lib/signature_parser.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+class SignatureParser
+  class ParsingError < StandardError; end
+
+  # The syntax of this header is defined in:
+  # https://datatracker.ietf.org/doc/html/draft-cavage-http-signatures-12#section-4
+  # See https://datatracker.ietf.org/doc/html/rfc7235#appendix-C
+  # and https://datatracker.ietf.org/doc/html/rfc7230#section-3.2.6
+
+  # In addition, ignore a `Signature ` string prefix that was added by old versions
+  # of `node-http-signatures`
+
+  class SignatureParamsParser < Parslet::Parser
+    rule(:token)         { match("[0-9a-zA-Z!#$%&'*+.^_`|~-]").repeat(1).as(:token) }
+    rule(:quoted_string) { str('"') >> (qdtext | quoted_pair).repeat.as(:quoted_string) >> str('"') }
+    # qdtext and quoted_pair are not exactly according to spec but meh
+    rule(:qdtext)        { match('[^\\\\"]') }
+    rule(:quoted_pair)   { str('\\') >> any }
+    rule(:bws)           { match('\s').repeat }
+    rule(:param)         { (token.as(:key) >> bws >> str('=') >> bws >> (token | quoted_string).as(:value)).as(:param) }
+    rule(:comma)         { bws >> str(',') >> bws }
+    # Old versions of node-http-signature add an incorrect "Signature " prefix to the header
+    rule(:buggy_prefix)  { str('Signature ') }
+    rule(:params)        { buggy_prefix.maybe >> (param >> (comma >> param).repeat).as(:params) }
+    root(:params)
+  end
+
+  class SignatureParamsTransformer < Parslet::Transform
+    rule(params: subtree(:param)) do
+      (param.is_a?(Array) ? param : [param]).each_with_object({}) { |(key, value), hash| hash[key] = value }
+    end
+
+    rule(param: { key: simple(:key), value: simple(:val) }) do
+      [key, val]
+    end
+
+    rule(quoted_string: simple(:string)) do
+      string.to_s
+    end
+
+    rule(token: simple(:string)) do
+      string.to_s
+    end
+  end
+
+  def self.parse(raw_signature)
+    tree = SignatureParamsParser.new.parse(raw_signature)
+    SignatureParamsTransformer.new.apply(tree)
+  rescue Parslet::ParseFailed
+    raise ParsingError
+  end
+end