Debugging a chain

Ignite chain debug command can help you find issues during development. It uses Delve debugger which enables you to interact with your blockchain app by controlling the execution of the process, evaluating variables, and providing information of thread / goroutine state, CPU register state and more.

Debug Command

The debug command requires that the blockchain app binary is build with debugging support by removing optimizations and inlining. A debug binary is built by default by the ignite chain serve command or can optionally be created using the --debug flag when running ignite chain init or ignite chain build sub-commands.

To start a debugging session in the terminal run:

ignite chain debug

The command runs your blockchan app in the background, attaches to it and launches a terminal debugger shell:

Type 'help' for list of commands.
(dlv)

At this point the blockchain app blocks execution, so you can set one or more breakpoints before continuing execution.

Use the break (alias b) command to set any number of breakpoints using, for example the <filename>:<line> notation:

(dlv) break x/hello/client/cli/query_say_hello.go:14

This command adds a breakpoint to the x/hello/client/cli/query_say_hello.go file at line 14.

Once all breakpoints are set resume blockchain execution using the continue (alias c) command:

(dlv) continue

The debugger will launch the shell and stop blockchain execution again when a breakpoint is triggered.

Within the debugger shell use the quit (alias q) or exit commands to stop the blockchain app and exit the debugger.

Debug Server

A debug server can optionally be started in cases where the default terminal client is not desirable. When the server starts it first runs the blockchain app, attaches to it and finally waits for a client connection. The default server address is tcp://127.0.0.1:30500 and it accepts both JSON-RPC or DAP client connections.

To start a debug server use the following flag:

ignite chain debug --server

To start a debug server with a custom address use the following flags:

ignite chain debug --server --server-address 127.0.0.1:30500

The debug server stops automatically when the client connection is closed.

Debugging Clients

Gdlv: Multiplatform Delve UI

Gdlv is a graphical frontend to Delve for Linux, Windows and macOS.

Using it as debugging client is straightforward as it doesn't require any configuration. Once the debug server is running and listening for client requests connect to it by running:

gdlv connect 127.0.0.1:30500

Setting breakpoints and continuing execution is done in the same way as Delve, by using the break and continue commands.

Visual Studio Code

Using Visual Studio Code as debugging client requires an initial configuration to allow it to connect to the debug server.

Make sure that the Go extension is installed.

VS Code debugging is configured using the launch.json file which is usually located inside the .vscode folder in your workspace.

You can use the following launch configuration to set up VS Code as debugging client:

launch.json

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Connect to Debug Server",
            "type": "go",
            "request": "attach",
            "mode": "remote",
            "remotePath": "${workspaceFolder}",
            "port": 30500,
            "host": "127.0.0.1"
        }
    ]
}

Alternatively it's possible to create a custom launch.json file from the "Run and Debug" panel. When prompted choose the Go debugger option labeled "Go: Connect to Server" and enter the debug host address and then the port number.

Example: Debugging a Blockchain App

In this short example we will be using Ignite CLI to create a new blockchain and a query to be able to trigger a debugging breakpoint when the query is called.

Create a new blockchain:

ignite scaffold chain hello

Scaffold a new query in the hello directory:

ignite scaffold query say-hello name --response name

The next step initializes the blockchain's data directory and compiles a debug binary:

ignite chain init --debug

Once the initialization finishes launch the debugger shell:

ignite chain debug

Within the debugger shell create a breakpoint that will be triggered when the SayHello function is called and then continue execution:

(dlv) break x/hello/keeper/query_say_hello.go:12
(dlv) continue

From a different terminal use the hellod binary to call the query:

hellod query hello say-hello bob

A debugger shell will be launched when the breakpoint is triggered:

     7:     "google.golang.org/grpc/codes"
     8:     "google.golang.org/grpc/status"
     9:     "hello/x/hello/types"
    10: )
    11:
=>  12: func (k Keeper) SayHello(goCtx context.Context, req *types.QuerySayHelloRequest) (*types.QuerySayHelloResponse, error) {
    13:     if req == nil {
    14:         return nil, status.Error(codes.InvalidArgument, "invalid request")
    15:     }
    16:
    17:     ctx := sdk.UnwrapSDKContext(goCtx)

From then on you can use Delve commands like next (alias n) or print (alias p) to control execution and print values. For example, to print the name argument value use the print command followed by "req.Name":

(dlv) print req.Name
"bob"

Finally, use quit (alias q) to stop the blockchain app and finish the debugging session.