mojo_debug allows you to interactively inspect a running shell, collect performance traces and attach a gdb debugger.
Performance traces 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:
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.:
mojo_run --debugger APP_URL [--android]
While Mojo Shell is running, tracing can be started and stopped by these two commands respectively:
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.
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):
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:
(gdb) update-symbols
If you only want to update symbols for the current selected thread (for example, after changing threads), use the current option:
(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.
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:
mojo_debug device stack