Avoid relocation of strings by moving the content to heap memory

.gitignore

@@ -21,5 +21,6 @@ Gemfile.lock
 types_log
 *.gem
 embed-test.rb.log
+gc_compact.rb.log
 spec/ffi/embed-test/ext/Makefile
 spec/ffi/embed-test/ext/embed_test.bundle

ext/ffi_c/Call.c

@@ -91,6 +91,23 @@ static inline void* getPointer(VALUE value, int type);
 
 static ID id_to_ptr, id_map_symbol, id_to_native;
 
+static VALUE rbffi_pin_string_content(VALUE str)
+{
+#ifdef RSTRING_EMBED_LEN_MAX
+    if (likely(RB_TYPE_P(str, T_STRING)) && !unlikely(RB_OBJ_FROZEN(str))){
+        /* Expand the string capacity so we aren't using embedded string memory
+         * but heap memory for the string content.
+         * This is to avoid relocation of the content by GC.compact
+         */
+        long expand_len = RSTRING_EMBED_LEN_MAX + 1 - RSTRING_LEN(str);
+        if (likely(expand_len > 0)) {
+            rb_str_modify_expand( str, expand_len );
+        }
+    }
+#endif
+    return str;
+}
+
 void
 rbffi_SetupCallParams(int argc, VALUE* argv, int paramCount, Type** paramTypes,
         FFIStorage* paramStorage, void** ffiValues,
@@ -298,7 +315,9 @@ rbffi_SetupCallParams(int argc, VALUE* argv, int paramCount, Type** paramTypes,
                     param->ptr = NULL;
 
                 } else {
-                    param->ptr = StringValueCStr(argv[argidx]);
+                    VALUE str = argv[argidx];
+                    rbffi_pin_string_content(str);
+                    param->ptr = StringValueCStr(str);
                 }
 
                 ADJ(param, ADDRESS);
@@ -435,6 +454,7 @@ getPointer(VALUE value, int type)
 
     } else if (type == T_STRING) {
 
+        rbffi_pin_string_content(value);
         return StringValuePtr(value);
 
     } else if (type == T_NIL) {

spec/ffi/fixtures/StringTest.c

@@ -32,3 +32,6 @@ string_null(void)
     return NULL;
 }
 
+void store_string(char* str, char **ptr) {
+    *ptr = str;
+}

spec/ffi/gc_compact.rb

@@ -0,0 +1,44 @@
+# -*- encoding: utf-8 -*-
+#
+# This file is part of ruby-ffi.
+# For licensing, see LICENSE.SPECS
+#
+
+require File.expand_path(File.join(File.dirname(__FILE__), "spec_helper"))
+
+# These specs are not run as regular rspec file, but as a separate process from string_spec.rb.
+# That's due to GC.verify_compaction_references is very memory consuming since it doubles the needed memory with each call.
+# So it's intentionally that the file name miss the "_spec" prefix.
+
+if GC.respond_to?(:compact)
+  describe "GC.compact" do
+    module GcCompactTestLib
+      extend FFI::Library
+      ffi_lib TestLibrary::PATH
+      attach_function :store_string, [ :string, :pointer ], :void
+      attach_function :store_pointer, :store_string, [ :pointer, :pointer ], :void
+    end
+
+    A = []
+    F = []
+    [0, 1, 23, 24].each_with_index do |len, idx|
+      A[idx] = "a" * len
+      F[idx] = ("a" * len).freeze
+
+      it "preserves a #{len} character string" do
+        mem = FFI::MemoryPointer.new :pointer, 3
+        GcCompactTestLib.store_string(A[idx], mem)
+        GcCompactTestLib.store_pointer(A[idx], mem + 1*FFI::TYPE_POINTER.size)
+        GcCompactTestLib.store_string(F[idx], mem + 2*FFI::TYPE_POINTER.size)
+
+        GC.verify_compaction_references(toward: :empty, double_heap: true)
+
+        expect( mem.get_pointer(0*FFI::TYPE_POINTER.size).read_string ).to eq("a" * len)
+        expect( mem.get_pointer(1*FFI::TYPE_POINTER.size).read_string ).to eq("a" * len)
+        expect( mem.get_pointer(2*FFI::TYPE_POINTER.size).read_string ).to eq("a" * len)
+        expect( A[idx] ).to eq("a" * len)
+        expect( F[idx] ).to eq("a" * len)
+      end
+    end
+  end
+end

spec/ffi/string_spec.rb

@@ -23,6 +23,11 @@ module StrLibTest
     expect(StrLibTest.pointer_buffer_equals(str + "a", str + "b", str.bytesize + 1)).to eq(0)
   end
 
+  it "Frozen String can be passed to :pointer and :string argument" do
+    str = "string buffer".freeze
+    expect(StrLibTest.pointer_buffer_equals(str, str, str.bytesize)).to eq(1)
+  end
+
   it "Poison null byte raises error" do
     s = "123\0abc"
     expect{ StrLibTest.pointer_buffer_equals("", s, 0) }.to raise_error(ArgumentError)
@@ -98,4 +103,12 @@ module StrLibTest
     ptrary.write_array_of_pointer(ary)
     expect { ptrary.get_array_of_string(-1) }.to raise_error(IndexError)
   end
+
+  it "with GC.compact" do
+    skip "GC.compact is unavailable" unless GC.respond_to?(:compact)
+
+    out = external_run("rspec", "gc_compact.rb")
+    expect(out).to match(/ 0 failures/)
+  end
+
 end