Debugging on OpenHarmony

It's recommended to read the general advice from the Debugging section first. This section will only cover the differences that need to be considered on OpenHarmony.

Starting servoshell from the commandline

When debugging, it can often be useful to start servo from the commandline instead of clicking the app icon, since we can pass parameters from the commandline.

# Run this command to see which parameters can be passed to an ohos app
hdc shell aa start --help
# Start servo from the commandline
hdc shell aa start -a EntryAbility -b org.servo.servo
# --ps=<arg> <value> can be used to pass arguments with values to servoshell.
# The space between arg and value is mandatory when using `--ps`, meaning `--ps=--log-filter warn` is
# correct while `--ps=--log-filter=warn` is not.
# For pure flags without a value use `--psn <flag>`
hdc shell aa start -a EntryAbility -b org.servo.servo --ps=--log-filter "warn"
# Use `-U <url>` to let servo load a custom URL.
hdc shell aa start -a EntryAbility -b org.servo.servo -U https://servo.org

Logging

On OpenHarmony devices log messages can be saved and accessed via the hilog service:

# See the hilog help for a full list of arguments supported by hilog
hdc shell hilog --help
# View all logs (very verbose, includes all other apps)
hdc shell hilog
# All servo and Spidermonkey related logs
hdc shell hilog --domain=0xE0C3,0xE0C4
# Only Rust code from servo
hdc shell hilog --domain=0xE0C3
# Only spidermonkey C++ code
hdc shell hilog --domain=0xE0C4
# Filter by domain and log level
hdc shell hilog --domain=0xE0C3 --level=ERROR

Log level

Whether log statements are visible depends on multiple conditions:

1. The compile-time max log level of the log crate. See log compile-time filters. Note: The release_max_level_<level> features of the log crate check if debug_assertions = false is set to determine the release filters should apply or not.
2. The runtime log global filter per module filters set in servoshell. Since environment variables aren't an option to customize the log level, servoshell has the --log-filter option on ohos targets, which allows customizing the log filter of the log crate. By default servoshell sets a log filter which hides log statements from many crates, so you likely will need to set a custom log-filter if you aren't seeing the logs from the crate you are debugging.
3. The hilog base log filter. hdc shell hilog --base-level=<log_level>. Can be combined with --domain and --tag options to customize which logs are saved.
4. The hilog log filter when displaying logs: hdc shell hilog --level=<level>

Most of the time option 2 and/or 4 should be used, since they allow quick changes with recompiling.

Hilog domains

hilog allows setting a custom integer called "domain" (between 0 and 0xFFFF) when logging, which allows developers to easily filter logs using the domain. For Rust code in servo logged via the log crate we set 0xE0C3 as the domain and for Spidermonkey C++ code we set 0xE0C4. These values are somewhat arbitrarily chosen.

Hilog privacy feature

hilog has a privacy feature, which by default hides values in logs (e.g. from %d or %s substitutions). Log statements from Rust are generally unaffected by this, since the string formatting is done on the Rust side. If you encounter this issue when viewing C/C++ logs, you can temporarily turn off the privacy feature by running:

hdc shell hilog -p off

Devtools and port forwards

You can enable the devtools and connect to them remotely with Firefox. It is easiest to do this with the command line via hdc shell aa start -a EntryAbility -b org.servo.servo --psn=--devtools=1234 To connect to an instance of Servo you have to forward the port with hdc fport tcp:1234 tcp:1234. You should see a message that the forward succeeded. Now you can connect to the devtools using localhost:1234.