Fix a GC compaction issue with busy_handler

Previous discussion in sparklemotion#458

Storing `VALUE self` as context for `rb_sqlite3_busy_handler` is unsafe
because as of Ruby 2.7, the GC compactor may move objects around
which can lead to this reference pointing to either another random
object or to garbage.

Instead we can store the callback reference inside the malloced
struct (`sqlite3Ruby`) which can't possibly move, and then inside
the handler, get the callback reference from that struct.

This however requires to define a mark function for the database
object, and while I was at it, I implemented compaction support
for it so we don't pin that proc.

ext/sqlite3/database.c

@@ -12,6 +12,13 @@
 
 VALUE cSqlite3Database;
 
+static void
+database_mark(void *ctx)
+{
+    sqlite3RubyPtr c = (sqlite3RubyPtr)ctx;
+    rb_gc_mark(c->busy_handler);
+}
+
 static void
 deallocate(void *ctx)
 {
@@ -31,15 +38,13 @@ database_memsize(const void *ctx)
 }
 
 static const rb_data_type_t database_type = {
-    "SQLite3::Backup",
-    {
-        NULL,
-        deallocate,
-        database_memsize,
+    .wrap_struct_name = "SQLite3::Backup",
+    .function = {
+        .dmark = database_mark,
+        .dfree = deallocate,
+        .dsize = database_memsize,
     },
-    0,
-    0,
-    RUBY_TYPED_WB_PROTECTED, // Not freed immediately because the dfree function do IOs.
+    .flags = RUBY_TYPED_WB_PROTECTED, // Not freed immediately because the dfree function do IOs.
 };
 
 static VALUE
@@ -202,10 +207,11 @@ trace(int argc, VALUE *argv, VALUE self)
 }
 
 static int
-rb_sqlite3_busy_handler(void *ctx, int count)
+rb_sqlite3_busy_handler(void *context, int count)
 {
-    VALUE self = (VALUE)(ctx);
-    VALUE handle = rb_iv_get(self, "@busy_handler");
+    sqlite3RubyPtr ctx = (sqlite3RubyPtr)context;
+
+    VALUE handle = ctx->busy_handler;
     VALUE result = rb_funcall(handle, rb_intern("call"), 1, INT2NUM(count));
 
     if (Qfalse == result) { return 0; }
@@ -240,11 +246,13 @@ busy_handler(int argc, VALUE *argv, VALUE self)
     rb_scan_args(argc, argv, "01", &block);
 
     if (NIL_P(block) && rb_block_given_p()) { block = rb_block_proc(); }
-
-    rb_iv_set(self, "@busy_handler", block);
+    ctx->busy_handler = block;
 
     status = sqlite3_busy_handler(
-                 ctx->db, NIL_P(block) ? NULL : rb_sqlite3_busy_handler, (void *)self);
+        ctx->db, 
+        NIL_P(block) ? NULL : rb_sqlite3_busy_handler,
+        (void *)ctx
+    );
 
     CHECK(ctx->db, status);
 

ext/sqlite3/database.h

@@ -5,6 +5,7 @@
 
 struct _sqlite3Ruby {
     sqlite3 *db;
+    VALUE busy_handler;
 };
 
 typedef struct _sqlite3Ruby sqlite3Ruby;