raise(Exception): Setup callstack on raise only if not already set. F…

…ixes #4482.

It's useful for reraising of exceptions in `rescue` blocks.

As proposed by #4482 (comment).

/cc @asterite, @straight-shoota.

spec/std/raise_spec.cr

@@ -0,0 +1,25 @@
+require "spec"
+
+struct CallStack # allow clone and equality
+  def_clone
+  def_equals @callstack
+end
+
+describe "raise" do
+  it "should throw exception with existing callstack if already set" do
+    ex = Exception.new "with callstack"
+    ex.callstack = CallStack.new
+    callstack_to_match = ex.callstack.clone
+    new_ex = expect_raises Exception, "with callstack" do
+      raise ex
+    end
+    new_ex.callstack.should eq(callstack_to_match)
+  end
+
+  it "should throw exception with new callstack if not already set" do
+    new_ex = expect_raises Exception, "without callstack" do
+      raise Exception.new("without callstack")
+    end
+    new_ex.callstack.should_not be_nil
+  end
+end

src/raise.cr

@@ -175,16 +175,21 @@ fun __crystal_get_exception(unwind_ex : LibUnwind::Exception*) : UInt64
   unwind_ex.value.exception_object
 end
 
-def raise(ex : Exception) : NoReturn
-  ex.callstack = CallStack.new
+# Raises the *exception*.
+#
+# Note that exception callstack will only be set if it's not already set.
+# It's useful for reraising of exceptions in `rescue` blocks.
+def raise(exception : Exception) : NoReturn
+  exception.callstack ||= CallStack.new
   unwind_ex = Pointer(LibUnwind::Exception).malloc
   unwind_ex.value.exception_class = LibC::SizeT.zero
   unwind_ex.value.exception_cleanup = LibC::SizeT.zero
-  unwind_ex.value.exception_object = ex.object_id
-  unwind_ex.value.exception_type_id = ex.crystal_type_id
+  unwind_ex.value.exception_object = exception.object_id
+  unwind_ex.value.exception_type_id = exception.crystal_type_id
   __crystal_raise(unwind_ex)
 end
 
+# Raises an Exception with the *message*.
 def raise(message : String) : NoReturn
   raise Exception.new(message)
 end