mojo/devtools/common/README.md

# Devtools

Unopinionated tools for **running**, **debugging** and **testing** Mojo apps.

## Install

```
git clone https://github.com/domokit/devtools.git
```

## Contents

Devtools offers the following tools:

 - `mojo_run` - shell runner
 - `mojo_test` - apptest runner
 - `mojo_debug` - debugger supporting interactive tracing and debugging of a
   running mojo shell

Additionally, `remote_adb_setup` script helps to configure adb on a remote
machine to communicate with a device attached to a local machine, forwarding the
ports used by `mojo_run`.

### Runner

`mojo_run` allows you to run a Mojo shell either on the host, or on an attached
Android device.

```sh
mojo_run APP_URL  # Run on the host.
mojo_run APP_URL --android  # Run on Android device.
mojo_run "APP_URL APP_ARGUMENTS"  # Run an app with startup arguments
```

Unless running within a Mojo checkout, we need to indicate the path to the shell
binary:

```sh
mojo_run --shell-path path/to/shell/binary APP_URL
```

Some applications are meant to be run embedded in a **window manager**. To run
these, you can pass the app url using the `--embed` flag. This will run the
window manager and pass the given url to it:

```sh
mojo_run --embed APP_URL [--android]
```

By default, `mojo_run` uses https://core.mojoapps.io/kiosk_wm.mojo as the window
manager. You can pass a different window manager url using the
`--window-manager` flag to override this.

### Debugger

`mojo_debug` allows you to interactively inspect a running shell, collect
performance traces and attach a gdb debugger.

#### Tracing
[Performance
traces](https://www.chromium.org/developers/how-tos/trace-event-profiling-tool)
can either be collected by Mojo Shell during its startup, or collected
interactively by `mojo_debug`.

To trace the Mojo Shell startup, use the `--trace-startup` flag:

```sh
mojo_run --trace-startup APP_URL [--android]
```

In order to collect traces interactively through `mojo_debug`, make sure that
the app being inspected was run with `--debugger` switch. E.g.:

```sh
mojo_run --debugger APP_URL [--android]
```

While Mojo Shell is running, tracing can be started and stopped by these two
commands respectively:

```sh
mojo_debug tracing start
mojo_debug tracing stop [result.json]
```

Trace files can be then loaded using the trace viewer in Chrome available at
`about://tracing`.

#### GDB
It is possible to inspect a Mojo Shell process using GDB. The `mojo_debug`
script can be used to launch GDB and attach it to a running shell process
(android only):

```sh
mojo_debug gdb attach
```

Once started, GDB will first stop the Mojo Shell execution, then load symbols
from loaded Mojo applications. Please note that this initial step can take some
time (up to several minutes in the worst case).

After each execution pause, GDB will update the set of loaded symbols based on
the selected thread only. If you need symbols for all threads, use the
`update-symbols` GDB command:
```sh
(gdb) update-symbols
```

If you only want to update symbols for the current selected thread (for example,
after changing threads), use the `current` option:
```sh
(gdb) update-symbols current
```

If you want to debug the startup of your application, you can pass
`--wait-for-debugger` to `mojo_run` to have the Mojo Shell stop and wait to be
attached by `gdb` before continuing.

#### Android crash stacks
When Mojo shell crashes on Android ("Unfortunately, Mojo shell has stopped.")
due to a crash in native code, `mojo_debug` can be used to find and symbolize
the stack trace present in the device log:

```sh
mojo_debug device stack
```

## Development

The library is canonically developed [in the mojo
repository](https://github.com/domokit/mojo/tree/master/mojo/devtools/common),
https://github.com/domokit/devtools is a mirror allowing to consume it
separately.