Make PG::Coder and derivations friendly to GC.compact

This way the VALUE references may be relocated.
Since self is always referenced indirectly, marking simple coders can be omitted entirely.

This partly reverts 7c1756f which was introduced as a simple fix for GC.compact compatbility.

ext/extconf.rb

@@ -80,6 +80,7 @@
 have_func 'PQresultMemorySize' # since PostgreSQL-12
 have_func 'timegm'
 have_func 'rb_gc_adjust_memory_usage' # since ruby-2.4
+have_func 'rb_gc_mark_movable' # since ruby-2.7
 
 # unistd.h confilicts with ruby/win32.h when cross compiling for win32 and ruby 1.9.1
 have_header 'unistd.h'

ext/pg.h

@@ -78,6 +78,14 @@ typedef long suseconds_t;
 #define RARRAY_AREF(a, i) (RARRAY_PTR(a)[i])
 #endif
 
+#ifdef HAVE_RB_GC_MARK_MOVABLE
+#define pg_compact_callback(x) ((void (*)(void*))(x))
+#else
+#define rb_gc_mark_movable(x) rb_gc_mark(x)
+#define rb_gc_location(x) (x)
+#define pg_compact_callback(x) {(x)}
+#endif
+
 #define PG_ENC_IDX_BITS 28
 
 /* The data behind each PG::Connection object */
@@ -306,7 +314,7 @@ VALUE pg_obj_to_i                                      _(( VALUE ));
 VALUE pg_tmbc_allocate                                 _(( void ));
 void pg_coder_init_encoder                             _(( VALUE ));
 void pg_coder_init_decoder                             _(( VALUE ));
-void pg_coder_mark                                     _(( t_pg_coder * ));
+void pg_coder_compact                                  _(( t_pg_coder * ));
 char *pg_rb_str_ensure_capa                            _(( VALUE, long, char *, char ** ));
 
 #define PG_RB_STR_ENSURE_CAPA( str, expand_len, curr_ptr, end_ptr ) \

ext/pg_coder.c

@@ -62,23 +62,24 @@ pg_coder_init_decoder( VALUE self )
 }
 
 void
-pg_coder_mark(t_pg_coder *this)
+pg_coder_compact(t_pg_coder *this)
 {
-	rb_gc_mark(this->coder_obj);
+	this->coder_obj = rb_gc_location(this->coder_obj);
 }
 
 static void
-pg_composite_coder_mark(t_pg_composite_coder *this)
+pg_composite_coder_compact(t_pg_composite_coder *this)
 {
-	pg_coder_mark(&this->comp);
+	pg_coder_compact(&this->comp);
 }
 
 const rb_data_type_t pg_coder_type = {
 	"PG::Coder",
 	{
-		(void (*)(void*))pg_coder_mark,
+		(void (*)(void*))NULL,
 		(void (*)(void*))-1,
 		(size_t (*)(const void *))NULL,
+		pg_compact_callback(pg_coder_compact),
 	},
 	0,
 	0,
@@ -99,9 +100,10 @@ pg_simple_encoder_allocate( VALUE klass )
 static const rb_data_type_t pg_composite_coder_type = {
 	"PG::CompositeCoder",
 	{
-		(void (*)(void*))pg_composite_coder_mark,
+		(void (*)(void*))NULL,
 		(void (*)(void*))-1,
 		(size_t (*)(const void *))NULL,
+		pg_compact_callback(pg_composite_coder_compact),
 	},
 	&pg_coder_type,
 	0,

ext/pg_copy_coder.c

@@ -23,9 +23,16 @@ typedef struct {
 static void
 pg_copycoder_mark( t_pg_copycoder *this )
 {
-	pg_coder_mark(&this->comp);
-	rb_gc_mark(this->typemap);
-	rb_gc_mark(this->null_string);
+	rb_gc_mark_movable(this->typemap);
+	rb_gc_mark_movable(this->null_string);
+}
+
+static void
+pg_copycoder_compact( t_pg_copycoder *this )
+{
+	pg_coder_compact(&this->comp);
+	this->typemap = rb_gc_location(this->typemap);
+	this->null_string = rb_gc_location(this->null_string);
 }
 
 static const rb_data_type_t pg_copycoder_type = {
@@ -34,6 +41,7 @@ static const rb_data_type_t pg_copycoder_type = {
 		(void (*)(void*))pg_copycoder_mark,
 		(void (*)(void*))-1,
 		(size_t (*)(const void *))NULL,
+		pg_compact_callback(pg_copycoder_compact),
 	},
 	&pg_coder_type,
 	0,

ext/pg_record_coder.c

@@ -18,8 +18,14 @@ typedef struct {
 static void
 pg_recordcoder_mark( t_pg_recordcoder *this )
 {
-	pg_coder_mark(&this->comp);
-	rb_gc_mark(this->typemap);
+	rb_gc_mark_movable(this->typemap);
+}
+
+static void
+pg_recordcoder_compact( t_pg_recordcoder *this )
+{
+	pg_coder_compact(&this->comp);
+	this->typemap = rb_gc_location(this->typemap);
 }
 
 static const rb_data_type_t pg_recordcoder_type = {
@@ -28,6 +34,7 @@ static const rb_data_type_t pg_recordcoder_type = {
 		(void (*)(void*))pg_recordcoder_mark,
 		(void (*)(void*))-1,
 		(size_t (*)(const void *))NULL,
+		pg_compact_callback(pg_recordcoder_compact),
 	},
 	&pg_coder_type,
 	0,