Make tracing available on all platforms, with or without perfetto (#33301)

* Make tracing available on all platforms

Signed-off-by: Delan Azabani <dazabani@igalia.com>

* Gate perfetto support behind its own feature

Signed-off-by: Delan Azabani <dazabani@igalia.com>

---------

Signed-off-by: Delan Azabani <dazabani@igalia.com>

ports/servoshell/Cargo.toml

@@ -48,7 +48,8 @@ multiview = ["libservo/multiview"]
 native-bluetooth = ["libservo/native-bluetooth"]
 profilemozjs = ["libservo/profilemozjs"]
 refcell_backtrace = ["libservo/refcell_backtrace"]
-tracing = ["dep:tracing", "dep:tracing-subscriber", "dep:tracing-perfetto"]
+tracing = ["dep:tracing", "dep:tracing-subscriber"]
+tracing-perfetto = ["tracing", "dep:tracing-perfetto"]
 webdriver = ["libservo/webdriver"]
 webgl_backtrace = ["libservo/webgl_backtrace"]
 
@@ -62,6 +63,9 @@ mime_guess = { workspace = true }
 url = { workspace = true }
 servo-media = { workspace = true }
 tokio = { workspace = true }
+tracing = { workspace = true, optional = true }
+tracing-subscriber = { workspace = true, optional = true }
+tracing-perfetto = { workspace = true, optional = true }
 
 [target.'cfg(target_os = "android")'.dependencies]
 android_logger = "0.14"
@@ -111,9 +115,6 @@ raw-window-handle = "0.6"
 shellwords = "1.0.0"
 surfman = { workspace = true, features = ["sm-x11", "sm-raw-window-handle-06"] }
 tinyfiledialogs = "3.0"
-tracing = { workspace = true, optional = true }
-tracing-subscriber = { workspace = true, features = ["env-filter"], optional = true }
-tracing-perfetto = { workspace = true, optional = true }
 webxr = { git = "https://github.com/servo/webxr", features = ["ipc", "glwindow", "headless"] }
 winit = "0.29.10"
 

ports/servoshell/desktop/cli.rs

@@ -8,42 +8,13 @@ use getopts::Options;
 use log::error;
 use servo::config::opts::{self, ArgumentParsingResult};
 use servo::servo_config::pref;
-#[cfg(feature = "tracing")]
-use tracing::subscriber::set_global_default;
-#[cfg(feature = "tracing")]
-use tracing_perfetto::PerfettoLayer;
-#[cfg(feature = "tracing")]
-use tracing_subscriber::layer::SubscriberExt;
-#[cfg(feature = "tracing")]
-use tracing_subscriber::util::SubscriberInitExt;
-#[cfg(feature = "tracing")]
-use tracing_subscriber::EnvFilter;
 
 use crate::desktop::app::App;
 use crate::panic_hook;
 
 pub fn main() {
     crate::crash_handler::install();
-
-    #[cfg(feature = "tracing")]
-    {
-        let file = std::fs::File::create("servo.pftrace").unwrap();
-        let perfetto_layer = PerfettoLayer::new(std::sync::Mutex::new(file));
-
-        // Set up a custom tracing subscriber and PerfettoLayer for performance tracing.
-        // The servo.pftrace file can be uploaded to https://ui.perfetto.dev for analysis.
-        let env_filter_layer = EnvFilter::from_default_env();
-        let subscriber = tracing_subscriber::registry()
-            .with(env_filter_layer)
-            .with(perfetto_layer);
-
-        // Set the subscriber as the global default for tracing. This means it will be used
-        // throughout the entire program unless a thread-local subscriber is set.
-        // Can only be set once; subsequent attempts will fail.
-        // <https://docs.rs/tracing/0.1.40/src/tracing/subscriber.rs.html#30>
-        set_global_default(subscriber).expect("Failed to set global default subscriber");
-    }
-
+    crate::init_tracing();
     crate::resources::init();
 
     // Parse the command line options and store them globally

ports/servoshell/egl/android/simpleservo.rs

@@ -107,6 +107,7 @@ pub fn init(
     waker: Box<dyn EventLoopWaker>,
     callbacks: Box<dyn HostTrait>,
 ) -> Result<(), &'static str> {
+    crate::init_tracing();
     resources::set(Box::new(ResourceReaderInstance::new()));
 
     if let Some(prefs) = init_opts.prefs {

ports/servoshell/egl/ohos/simpleservo.rs

@@ -40,6 +40,7 @@ pub fn init(
     callbacks: Box<dyn HostTrait>,
 ) -> Result<ServoGlue, &'static str> {
     info!("Entered simpleservo init function");
+    crate::init_tracing();
     resources::set(Box::new(ResourceReaderInstance::new()));
 
     gl.clear_color(1.0, 1.0, 1.0, 1.0);

ports/servoshell/lib.rs

@@ -39,17 +39,28 @@ pub fn main() {
     desktop::cli::main()
 }
 
-#[cfg(target_os = "android")]
-pub fn main() {
-    println!(
-        "Cannot start /ports/servoshell/ on Android. \
-                Use /support/android/apk/ + `libservoshell.so` instead"
-    );
-}
+pub fn init_tracing() {
+    #[cfg(feature = "tracing")]
+    {
+        let subscriber = tracing_subscriber::registry();
 
-#[cfg(target_env = "ohos")]
-pub fn main() {
-    println!("You shouldn't start /ports/servoshell/ on OpenHarmony.");
+        #[cfg(feature = "tracing-perfetto")]
+        let subscriber = {
+            use tracing_subscriber::layer::SubscriberExt;
+            // Set up a PerfettoLayer for performance tracing.
+            // The servo.pftrace file can be uploaded to https://ui.perfetto.dev for analysis.
+            let file = std::fs::File::create("servo.pftrace").unwrap();
+            let perfetto_layer = tracing_perfetto::PerfettoLayer::new(std::sync::Mutex::new(file));
+            subscriber.with(perfetto_layer)
+        };
+
+        // Same as SubscriberInitExt::init, but avoids initialising the tracing-log compat layer,
+        // since it would break Servo’s FromScriptLogger and FromCompositorLogger.
+        // <https://docs.rs/tracing-subscriber/0.3.18/tracing_subscriber/util/trait.SubscriberInitExt.html#method.init>
+        // <https://docs.rs/tracing/0.1.40/tracing/#consuming-log-records>
+        tracing::subscriber::set_global_default(subscriber)
+            .expect("Failed to set tracing subscriber");
+    }
 }
 
 pub fn servo_version() -> String {

ports/servoshell/main.rs

@@ -25,6 +25,8 @@ fn main() {
         if #[cfg(not(any(target_os = "android", target_env = "ohos")))] {
             servoshell::main()
         } else {
+            // Android: see ports/servoshell/egl/android/simpleservo.rs.
+            // OpenHarmony: see ports/servoshell/egl/ohos/simpleservo.rs.
             println!(
                 "Cannot run the servoshell `bin` executable on platforms such as \
                  Android or OpenHarmony. On these platforms you need to compile \