Hopwatch - a debugging tool for Go

Hopwatch is an experimental tool that can help debugging Go programs. Unlike most debuggers, hopwatch requires you to insert function calls at points of interest in your program. At these program locations, you can tell Hopwatch to display variable values and suspend the program (or goroutine). Hopwatch uses Websockets to exchange commands between your program and the debugger running in a HTML5 page.

Using Hopwatch

Basically, there are two functions Display and Break. Calling Display will print the variable,value pairs in the debugger page. Calling Break will suspend the program until you tell the debugger to Resume. The Display function takes one or more pairs of (variable name,value). The Break function takes one or more boolean values to be conditionally. Below is a test example.

	package main

	import "github.com/emicklei/hopwatch"

	func main() {
		for i := 0; i < 6; i++ {
			j := i * i
			hopwatch.Display("i",i, "j", j ).Break(j > 10)

Starting your Go program that includes a Hopwatch function call waits for a connection to the debugger page. You can disable Hopwatch by passing the command line parameters -hopwatch false.

2012/12/14 17:24:47 [hopwatch] open http://localhost:23456/hopwatch.html ...
2012/12/14 17:24:47 [hopwatch] no browser connection, wait for it ...

Once the debugger is connected, it can receive Display and Break command requests from your Go program. You can look at the stack trace, resume a suspended program or disconnect from your program.

Hopwatch debugger page

I made a short movie (1.5Mb .mov) that demonstrates using hopwatch in a multiple goroutines program. This example is included in the tests folder of the hopwatch package.

hopwatch with source

What's next?

Creating this tool has been fun and educative. Hopwatch uses two channels to communicate between the sendLoop and receiveLoop goroutines and uses the go.net Websocket package to talk to Javascript in a browser. As a tool Hopwatch may be useful (perhaps for remote debugging?) but ofcourse it lacks the important features of a real debugger (GDB) such as setting breakpoints at runtime (and therefore without source modification). If you have suggestions then please leave a comment on the announcement in the go-nuts discussion group.

The github project is hopwatch.

Update 2020-12-23: because tests are now exectuted in the background, hopwatch no longer works without changes.

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