Make all components into IO objects

By changing all components to use an IO object as a base, we can
implement a common IO interface for all components, which delegates to
the underlying IO object.

This enables streaming multipart data into the request body, avoiding
loading the whole multipart data into memory when File parts are backed
by File objects.

See httprb/http#409 for the new streaming API.

lib/http/form_data/composite_io.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module HTTP
+  module FormData
+    # Provides IO interface across multiple IO files.
+    class CompositeIO
+      # @param [Array<IO>] ios Array of IO objects
+      def initialize(*ios)
+        @ios = ios.flatten
+        @index = 0
+      end
+
+      # Reads and returns list of 
+      #
+      # @param [Integer] length Number of bytes to retrieve
+      # @param [String] outbuf String to be replaced with retrieved data
+      #
+      # @return [String]
+      def read(length = nil, outbuf = nil)
+        outbuf = outbuf.to_s.replace("")
+
+        while current_io
+          data = current_io.read(length)
+          outbuf << data.to_s
+          length -= data.to_s.length if length
+
+          break if length == 0
+
+          advance_io
+        end
+
+        return nil if length && outbuf.empty?
+
+        outbuf
+      end
+
+      def rewind
+        @ios.each(&:rewind)
+        @index = 0
+      end
+
+      def size
+        @size ||= @ios.map(&:size).inject(0, :+)
+      end
+
+      private
+
+      def current_io
+        @ios[@index]
+      end
+
+      def advance_io
+        @index += 1
+      end
+    end
+  end
+end

lib/http/form_data/file.rb

@@ -57,20 +57,6 @@ def initialize(path_or_io, opts = {})
           end
         end
       end
-
-      # Returns content size.
-      #
-      # @return [Integer]
-      def size
-        @io.size
-      end
-
-      # Returns content of the IO.
-      #
-      # @return [String]
-      def to_s
-        @io.read
-      end
     end
   end
 end

lib/http/form_data/multipart.rb

@@ -1,25 +1,27 @@
 # frozen_string_literal: true
 
 require "securerandom"
+require "stringio"
 
 require "http/form_data/multipart/param"
+require "http/form_data/readable"
+require "http/form_data/composite_io"
 
 module HTTP
   module FormData
     # `multipart/form-data` form data.
     class Multipart
+      include Readable
+
       # @param [#to_h, Hash] data form data key-value Hash
       def initialize(data)
-        @parts          = Param.coerce FormData.ensure_hash data
-        @boundary       = (Array.new(21, "-") << SecureRandom.hex(21)).join("")
-        @content_length = nil
-      end
+        parts = Param.coerce FormData.ensure_hash data
 
-      # Returns content to be used for HTTP request body.
-      #
-      # @return [String]
-      def to_s
-        head + @parts.map(&:to_s).join(glue) + tail
+        @boundary = ("-" * 21) << SecureRandom.hex(21)
+        @io = CompositeIO.new(
+          *parts.flat_map { |part| [StringIO.new(glue), part] },
+          StringIO.new(tail),
+        )
       end
 
       # Returns MIME type to be used for HTTP request `Content-Type` header.
@@ -34,30 +36,19 @@ def content_type
       #
       # @return [Integer]
       def content_length
-        unless @content_length
-          @content_length  = head.bytesize + tail.bytesize
-          @content_length += @parts.map(&:size).reduce(:+)
-          @content_length += (glue.bytesize * (@parts.count - 1))
-        end
-
-        @content_length
+        size
       end
 
       private
 
-      # @return [String]
-      def head
-        @head ||= "--#{@boundary}#{CRLF}"
-      end
-
       # @return [String]
       def glue
-        @glue ||= "#{CRLF}--#{@boundary}#{CRLF}"
+        @glue ||= "--#{@boundary}#{CRLF}"
       end
 
       # @return [String]
       def tail
-        @tail ||= "#{CRLF}--#{@boundary}--"
+        @tail ||= "--#{@boundary}--"
       end
     end
   end

lib/http/form_data/multipart/param.rb

@@ -1,34 +1,18 @@
 # frozen_string_literal: true
 
+require "stringio"
+
+require "http/form_data/readable"
+require "http/form_data/composite_io"
+
 module HTTP
   module FormData
     class Multipart
       # Utility class to represent multi-part chunks
       class Param
-        # @param [#to_s] name
-        # @param [FormData::File, FormData::Part, #to_s] value
-        def initialize(name, value)
-          @name = name.to_s
-
-          @part =
-            if value.is_a?(FormData::Part)
-              value
-            else
-              FormData::Part.new(value)
-            end
-
-          parameters = { :name => @name }
-          parameters[:filename] = @part.filename if @part.filename
-          parameters = parameters.map { |k, v| "#{k}=#{v.inspect}" }.join("; ")
-
-          @header = "Content-Disposition: form-data; #{parameters}"
+        include Readable
 
-          return unless @part.content_type
-
-          @header += "#{CRLF}Content-Type: #{@part.content_type}"
-        end
-
-        # Returns body part with headers and data.
+        # Initializes body part with headers and data.
         #
         # @example With {FormData::File} value
         #
@@ -44,15 +28,28 @@ def initialize(name, value)
         #   ixti
         #
         # @return [String]
-        def to_s
-          "#{@header}#{CRLF * 2}#{@part}"
-        end
+        # @param [#to_s] name
+        # @param [FormData::File, FormData::Part, #to_s] value
+        def initialize(name, value)
+          part =
+            if value.is_a?(FormData::Part)
+              value
+            else
+              FormData::Part.new(value)
+            end
 
-        # Calculates size of a part (headers + body).
-        #
-        # @return [Integer]
-        def size
-          @header.bytesize + (CRLF.bytesize * 2) + @part.size
+          parameters = { :name => name.to_s }
+          parameters[:filename] = part.filename if part.filename
+          parameters = parameters.map { |k, v| "#{k}=#{v.inspect}" }.join("; ")
+
+          header = String.new # rubocop:disable String/EmptyLiteral
+          header << "Content-Disposition: form-data; #{parameters}#{CRLF}"
+          header << "Content-Type: #{part.content_type}#{CRLF}" if part.content_type
+          header << CRLF
+
+          footer = CRLF.dup
+
+          @io = CompositeIO.new(StringIO.new(header), part, StringIO.new(footer))
         end
 
         # Flattens given `data` Hash into an array of `Param`'s.

lib/http/form_data/part.rb

@@ -1,5 +1,9 @@
 # frozen_string_literal: true
 
+require "stringio"
+
+require "http/form_data/readable"
+
 module HTTP
   module FormData
     # Represents a body part of multipart/form-data request.
@@ -9,30 +13,18 @@ module FormData
     #  body = "Message"
     #  FormData::Part.new body, :content_type => 'foobar.txt; charset="UTF-8"'
     class Part
+      include Readable
+
       attr_reader :content_type, :filename
 
       # @param [#to_s] body
       # @param [String] content_type Value of Content-Type header
       # @param [String] filename     Value of filename parameter
       def initialize(body, content_type: nil, filename: nil)
-        @body = body.to_s
+        @io = StringIO.new(body.to_s)
         @content_type = content_type
         @filename = filename
       end
-
-      # Returns content size.
-      #
-      # @return [Integer]
-      def size
-        @body.bytesize
-      end
-
-      # Returns content of a file of IO.
-      #
-      # @return [String]
-      def to_s
-        @body
-      end
     end
   end
 end

lib/http/form_data/readable.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module HTTP
+  module FormData
+    # Common behaviour for objects defined by an IO object.
+    module Readable
+      # Returns IO content.
+      #
+      # @return [String]
+      def to_s
+        rewind
+        read
+      end
+
+      # Reads and returns part of IO content.
+      #
+      # @param [Integer] length Number of bytes to retrieve
+      # @param [String] outbuf String to be replaced with retrieved data
+      #
+      # @return [String, nil]
+      def read(length = nil, outbuf = nil)
+        @io.read(length, outbuf)
+      end
+
+      # Returns IO size.
+      #
+      # @return [Integer]
+      def size
+        @io.size
+      end
+
+      # Rewinds the IO.
+      def rewind
+        @io.rewind
+      end
+    end
+  end
+end

spec/lib/http/form_data/composite_io_spec.rb

@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+RSpec.describe HTTP::FormData::CompositeIO do
+  let(:ios) { ["Hello", " ", "", "world", "!"].map { |string| StringIO.new(string) } }
+  subject(:composite_io) { HTTP::FormData::CompositeIO.new *ios }
+
+  describe "#read" do
+    it "reads all data" do
+      expect(composite_io.read).to eq "Hello world!"
+    end
+
+    it "reads partial data" do
+      expect(composite_io.read(3)).to eq "Hel"
+      expect(composite_io.read(2)).to eq "lo"
+      expect(composite_io.read(1)).to eq " "
+      expect(composite_io.read(6)).to eq "world!"
+    end
+
+    it "returns empty string when no data was retrieved" do
+      composite_io.read
+      expect(composite_io.read).to eq ""
+    end
+
+    it "returns nil when no partial data was retrieved" do
+      composite_io.read
+      expect(composite_io.read(3)).to eq nil
+    end
+
+    it "reads partial data with a buffer" do
+      outbuf = String.new
+      expect(composite_io.read(3, outbuf)).to eq "Hel"
+      expect(composite_io.read(2, outbuf)).to eq "lo"
+      expect(composite_io.read(1, outbuf)).to eq " "
+      expect(composite_io.read(6, outbuf)).to eq "world!"
+    end
+
+    it "fills the buffer with retrieved content" do
+      outbuf = String.new
+      composite_io.read(3, outbuf)
+      expect(outbuf).to eq "Hel"
+      composite_io.read(2, outbuf)
+      expect(outbuf).to eq "lo"
+      composite_io.read(1, outbuf)
+      expect(outbuf).to eq " "
+      composite_io.read(6, outbuf)
+      expect(outbuf).to eq "world!"
+    end
+
+    it "returns nil when no partial data was retrieved with a buffer" do
+      outbuf = String.new("content")
+      composite_io.read
+      expect(composite_io.read(3, outbuf)).to eq nil
+      expect(outbuf).to eq ""
+    end
+  end
+
+  describe "#rewind" do
+    it "rewinds all IOs" do
+      composite_io.read
+      composite_io.rewind
+      expect(composite_io.read).to eq "Hello world!"
+    end
+  end
+
+  describe "#size" do
+    it "returns sum of all IO sizes" do
+      expect(composite_io.size).to eq 12
+    end
+
+    it "returns 0 when there are no IOs" do
+      empty_composite_io = HTTP::FormData::CompositeIO.new
+      expect(empty_composite_io.size).to eq 0
+    end
+  end
+end