THRIFT-6138: Give Ruby memory buffers private ownership

lib/rb/lib/thrift/transport/memory_buffer_transport.rb

@@ -23,13 +23,9 @@ module Thrift
   class MemoryBufferTransport < BaseTransport
     GARBAGE_BUFFER_SIZE = 4*(2**10) # 4kB
 
-    # If you pass a string to this, you should #dup that string
-    # unless you want it to be modified by #read and #write
-    #--
-    # this behavior is no longer required. If you wish to change it
-    # go ahead, just make sure the specs pass
+    # The transport copies the input buffer and keeps its own mutable storage.
     def initialize(buffer = nil)
-      @buf = buffer ? Bytes.force_binary_encoding(buffer) : Bytes.empty_byte_buffer
+      @buf = buffer ? Bytes.force_binary_encoding(buffer.dup) : Bytes.empty_byte_buffer
       @index = 0
     end
 

lib/rb/spec/base_transport_spec.rb

@@ -305,12 +305,26 @@
       expect(@buffer.to_s).to eq("memory")
     end
 
-    it "should accept a buffer on input and use it directly" do
-      s = +"this is a test"
-      @buffer = Thrift::MemoryBufferTransport.new(s)
-      expect(@buffer.read(4)).to eq("this")
-      s.slice!(-4..-1)
-      expect(@buffer.read(@buffer.available)).to eq(" is a ")
+    it "should privately own a buffer passed on input" do
+      source = +"this is a test"
+      @buffer = Thrift::MemoryBufferTransport.new(source)
+
+      expect(source.encoding).to eq(Encoding::UTF_8)
+      source.replace("caller changed")
+      @buffer.write("!")
+
+      expect(source).to eq("caller changed")
+      expect(@buffer.read(@buffer.available)).to eq("this is a test!".b)
+    end
+
+    it "should allow writes and resets after receiving a frozen buffer" do
+      @buffer = Thrift::MemoryBufferTransport.new("abc".b.freeze)
+
+      @buffer.write("d")
+      expect(@buffer.read(4)).to eq("abcd")
+
+      @buffer.reset_buffer("xy")
+      expect(@buffer.read(2)).to eq("xy")
     end
 
     it "should always remain open" do