Add tests for compatibility with GC.compact

@conninfo is not always defined in a describe context
So define the constants in a before block and ensure the connection is closed.

Moreover use rspec's :if filter to avoid indention
"if: false" skips the execution of before, after and it blocks but the describe block is always executed.

spec/pg/gc_compact_spec.rb

@@ -0,0 +1,103 @@
+# -*- rspec -*-
+# encoding: utf-8
+#
+# Tests to verify correct implementation of compaction callbacks in rb_data_type_t definitions.
+#
+# Compaction callbacks update moved VALUEs.
+# In ruby-2.7 they are invoked only while GC.compact or GC.verify_compaction_references.
+# Ruby constants are usually moved, but local variables are not.
+#
+# Effectiveness of the tests below should be verified by commenting the compact callback out like so:
+#
+#   static const rb_data_type_t pg_tmbo_type = {
+#     "PG::TypeMapByOid",
+#     {
+#       (void (*)(void*))pg_tmbo_mark,
+#       (void (*)(void*))-1,
+#       (size_t (*)(const void *))NULL,
+#   //    pg_compact_callback(pg_tmbo_compact),
+#     },
+#
+# This should result in a segmentation fault aborting the whole process.
+# Therefore the effectiveness of only one test can be verified per rspec run.
+
+require_relative '../helpers'
+
+describe "GC.compact", if: GC.respond_to?(:compact) do
+	before :all do
+		TM1 = Class.new(PG::TypeMapByClass) do
+			def conv_array(value)
+				PG::TextEncoder::JSON.new
+			end
+		end.new
+		TM1[Array] = :conv_array
+
+		E1 = PG::TextEncoder::JSON.new
+
+		TM2 = PG::TypeMapByClass.new
+		TM2.default_type_map = PG::TypeMapInRuby.new
+
+		TMBC = PG::TypeMapByColumn.new([PG::TextDecoder::Float.new])
+
+
+		CONN2 = PG.connect(@conninfo)
+		CONN2.type_map_for_results = PG::BasicTypeMapForResults.new(CONN2)
+
+		RES1 = CONN2.exec("SELECT 234")
+
+		TUP1 = RES1.tuple(0)
+
+		TM3 = PG::TypeMapByClass.new
+		CPYENC = PG::TextEncoder::CopyRow.new type_map: TM3
+		RECENC = PG::TextEncoder::Record.new type_map: TM3
+
+		# Use GC.verify_compaction_references instead of GC.compact .
+		# This has the advantage that all movable objects are actually moved.
+		# The downside is that it doubles the heap space of the Ruby process.
+		# Therefore we call it only once and do several tests afterwards.
+		GC.verify_compaction_references(toward: :empty, double_heap: true)
+	end
+
+	it "should compact PG::TypeMapByClass #328" do
+		res = @conn.exec_params("SELECT $1", [[5]], 0, TM1)
+		expect( res.getvalue(0, 0) ).to eq( "[5]" )
+	end
+
+	it "should compact PG::CompositeCoder #327" do
+		e2 = PG::TextEncoder::Array.new elements_type: E1
+		expect( e2.encode([5]) ).to eq("{5}")
+	end
+
+	it "should compact PG::TypeMap#default_type_map" do
+		expect( TM2.default_type_map ).to be_kind_of( PG::TypeMapInRuby )
+	end
+
+	it "should compact PG::TypeMapByColumn" do
+		res = CONN2.exec("SELECT 555")
+		expect( res.getvalue(0,0) ).to eq( 555 )
+	end
+
+	it "should compact PG::Connection" do
+		expect( TMBC.coders[0] ).to be_kind_of(PG::TextDecoder::Float)
+	end
+
+	it "should compact PG::Result" do
+		expect( RES1.getvalue(0,0) ).to eq( 234 )
+	end
+
+	it "should compact PG::Tuple" do
+		expect( TUP1[0] ).to eq( 234 )
+	end
+
+	it "should compact PG::CopyCoder" do
+		expect( CPYENC.encode([45]) ).to eq( "45\n" )
+	end
+
+	it "should compact PG::RecordCoder" do
+		expect( RECENC.encode([34]) ).to eq( '("34")' )
+	end
+
+	after :all do
+		CONN2.close
+	end
+end