using byteslice instead of slice for buffer manipulation

The code from which the buffer and string handling was adapted,
inherited also the usage of slice!, which is not the appropriate tool
for when dealing with byte streams. Ruby doesn't have an equivalent
String#byteslice! yet, so we adapted other known workarounds (see
https://bugs.ruby-lang.org/issues/13626) for our code. This means that:

- For HTTPX::Buffer, we add a shift! method, as all usage shift the
buffer part which has been written to the socket;
- For HTTP1 parser and Chunker, we have to double the usage of
byteslice. In the cases where it results in nil, we might needlessly
initialize an empty string, however this is the lesser of evils, until a
proper backport is in place.

lib/httpx/buffer.rb

@@ -16,8 +16,6 @@ module HTTPX
 
     def_delegator :@buffer, :bytesize
 
-    def_delegator :@buffer, :slice!
-
     def_delegator :@buffer, :clear
 
     def_delegator :@buffer, :replace
@@ -32,5 +30,9 @@ module HTTPX
     def full?
       @buffer.bytesize >= @limit
     end
+
+    def shift!(fin)
+      @buffer = @buffer.byteslice(fin..-1)
+    end
   end
 end

lib/httpx/io/tcp.rb

@@ -87,7 +87,7 @@ module HTTPX
 
       def write(buffer)
         siz = @io.write_nonblock(buffer)
-        buffer.slice!(0, siz)
+        buffer.shift!(siz)
         siz
       rescue ::IO::WaitWritable
         0
@@ -108,7 +108,7 @@ module HTTPX
         return 0 if siz == :wait_writable
         return if siz.nil?
 
-        buffer.slice!(0, siz)
+        buffer.shift!(siz)
         siz
       end
     end

lib/httpx/io/udp.rb

@@ -30,7 +30,7 @@ module HTTPX
 
     def write(buffer)
       siz = @io.send(buffer, 0, @host, @port)
-      buffer.slice!(0, siz)
+      buffer.shift!(siz)
       siz
     end
 

lib/httpx/parser/http1.rb

@@ -67,7 +67,8 @@ module HTTPX
         @status_code = code.to_i
         raise(Error, "wrong status code (#{@status_code})") unless (100..599).cover?(@status_code)
 
-        @buffer.slice!(0, idx + 1)
+        # @buffer.slice!(0, idx + 1)
+        @buffer = @buffer.byteslice((idx + 1)..-1)
         nextstate(:headers)
       end
 
@@ -116,7 +117,8 @@ module HTTPX
             @observer.on_data(chunk)
           end
         elsif @content_length
-          data = @buffer.slice!(0, @content_length)
+          data = @buffer.byteslice(0, @content_length)
+          @buffer = @buffer.byteslice(@content_length..-1) || "".b
           @content_length -= data.bytesize
           @observer.on_data(data)
           data.clear

lib/httpx/transcoder/chunker.rb

@@ -60,7 +60,8 @@ module HTTPX::Transcoder
             return unless index && index.positive?
 
             # Read hex-length
-            hexlen = @buffer.slice!(0, index)
+            hexlen = @buffer.byteslice(0, index)
+            @buffer = @buffer.byteslice(index..-1) || "".b
             hexlen[/\h/] || raise(Error, "wrong chunk size line: #{hexlen}")
             @chunk_length = hexlen.hex
             # check if is last chunk
@@ -72,7 +73,7 @@ module HTTPX::Transcoder
             return if @buffer.bytesize < crlf_size
             raise Error, "wrong chunked encoding format" unless @buffer.start_with?(CRLF * (crlf_size / 2))
 
-            @buffer.slice!(0, crlf_size)
+            @buffer = @buffer.byteslice(crlf_size..-1)
             if @chunk_length.nil?
               nextstate(:length)
             else
@@ -81,8 +82,10 @@ module HTTPX::Transcoder
               nextstate(:data)
             end
           when :data
-            @chunk_buffer << (slice = @buffer.slice!(0, @chunk_length))
-            @chunk_length -= slice.bytesize
+            chunk = @buffer.byteslice(0, @chunk_length)
+            @buffer = @buffer.byteslice(@chunk_length..-1) || "".b
+            @chunk_buffer << chunk
+            @chunk_length -= chunk.bytesize
             if @chunk_length.zero?
               yield @chunk_buffer unless @chunk_buffer.empty?
               @chunk_buffer.clear