Setting ReactiveVal leads to Unhandled error in some cases

[wch]

The following app has a single button, which, when clicked, results in an Unhandled error.

from shiny import *


ui = page_fluid(
    input_action_button("go", "Go"),
)

def server(session: ShinySession):
    v = ReactiveVal(0)

    @reactive()
    def r():
        print("in reactive")
        return v()

    @observe()
    def _():
        session.input["go"]   # Take a dependency on the go button

        with isolate():
            print("in observer 1-1")
            r()
            print("in observer 1-2")
            val = v()
            print("v() is " + str(val))
            print("in observer 1-3")
            v(val + 1)
            print("in observer 1-4")


    @observe()
    def _():
        print("in observer 2-1")
        r()
        print("in observer 2-2")

app = ShinyApp(ui, server)

if __name__ == "__main__":
    app.run()

Before clicking the button, it prints this output:

in observer 1-1
in reactive
in observer 1-2
v() is 0
in observer 1-3
in observer 1-4
in observer 2-1
in reactive
in observer 2-2

When the button is clicked, this output shows up:

in observer 1-1
in observer 1-2
v() is 1
in observer 1-3
Unhandled error: 2763

So the error is happening when v(val + 1) is called.

Almost any change to the structure seems to make the error stop happening. For example, removing the first call to r() makes the error go away.