resources: Remove baked in resources (#36042)

This allows removing a LazyLock around the resources.
We override the baked in resources unconditionally in servoshell
upon initialization anyway
([desktop](https://github.com/servo/servo/blob/9f93ccd9427265f39f1a2de38389fec0e80f7bea/ports/servoshell/desktop/cli.rs#L15),
[android](https://github.com/servo/servo/blob/9f93ccd9427265f39f1a2de38389fec0e80f7bea/ports/servoshell/egl/android/simpleservo.rs#L49),
[ohos](https://github.com/servo/servo/blob/9f93ccd9427265f39f1a2de38389fec0e80f7bea/ports/servoshell/egl/ohos/simpleservo.rs#L43)
), meaning that the baked in resources
are unused in servoshell.

For 3rd-party embedders, we probably also want to the let them know
early that they should initialize the resources, instead of
restricting the panics to production mode.

Rippy is the only resource which was required. Since it is only
253 bytes large, we just bake that resource in as a fallback.

We do want to make using the resources easy from tests, so we add
some logic to keep the baked in resources for tests only and initialize
the resource reader on first access.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by
`[X]` when the step is complete, and replace `___` with appropriate
data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because ___

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

4 files changed, 53 insertions, 28 deletions

diff --git a/components/net/image_cache.rs b/components/net/image_cache.rs
index 68d14f4c10c..078380bb466 100644
--- a/components/net/image_cache.rs
+++ b/components/net/image_cache.rs
@@ -31,6 +31,11 @@ use webrender_api::{ImageDescriptor, ImageDescriptorFlags, ImageFormat};
 
 use crate::resource_thread::CoreResourceThreadPool;
 
+// We bake in rippy.png as a fallback, in case the embedder does not provide
+// a rippy resource. this version is 253 bytes large, don't exchange it against
+// something in higher resolution.
+const FALLBACK_RIPPY: &[u8] = include_bytes!("../../resources/rippy.png");
+
 //
 // TODO(gw): Remaining work on image cache:
 //     * Make use of the prefetch support in various parts of the code.
@@ -51,7 +56,9 @@ fn decode_bytes_sync(key: LoadKey, bytes: &[u8], cors: CorsStatus) -> DecoderMsg
 }
 
 fn get_placeholder_image(compositor_api: &CrossProcessCompositorApi, data: &[u8]) -> Arc<Image> {
-    let mut image = load_from_memory(data, CorsStatus::Unsafe).unwrap();
+    let mut image = load_from_memory(data, CorsStatus::Unsafe)
+        .or_else(|| load_from_memory(FALLBACK_RIPPY, CorsStatus::Unsafe))
+        .expect("load fallback image failed");
     set_webrender_image_key(compositor_api, &mut image);
     Arc::new(image)
 }
diff --git a/components/shared/embedder/Cargo.toml b/components/shared/embedder/Cargo.toml
index 1dfe363a93b..d938169d37a 100644
--- a/components/shared/embedder/Cargo.toml
+++ b/components/shared/embedder/Cargo.toml
@@ -11,6 +11,11 @@ rust-version.workspace = true
 name = "embedder_traits"
 path = "lib.rs"
 
+[features]
+# bakes default resources into the library.
+# This feature is mainly intended for testing purposes.
+baked-default-resources = []
+
 [dependencies]
 base = { workspace = true }
 cfg-if = { workspace = true }
diff --git a/components/shared/embedder/resources.rs b/components/shared/embedder/resources.rs
index a5c34a33f30..29e64bf9629 100644
--- a/components/shared/embedder/resources.rs
+++ b/components/shared/embedder/resources.rs
@@ -3,30 +3,42 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use std::path::PathBuf;
-use std::sync::{LazyLock, RwLock};
+use std::sync::RwLock;
 
-use cfg_if::cfg_if;
+static RES: RwLock<Option<Box<dyn ResourceReaderMethods + Sync + Send>>> = RwLock::new(None);
 
-static RES: LazyLock<RwLock<Option<Box<dyn ResourceReaderMethods + Sync + Send>>>> =
-    LazyLock::new(|| {
-        cfg_if! {
-            if #[cfg(any(servo_production, target_env = "ohos"))] {
-                RwLock::new(None)
-            } else {
-                // Static assert that this is really a non-production build, rather
-                // than a failure of the build script’s production check.
-                const _: () = assert!(cfg!(servo_do_not_use_in_production));
+#[cfg(feature = "baked-default-resources")]
+static INIT_TEST_RESOURCES: std::sync::Once = std::sync::Once::new();
 
-                RwLock::new(Some(resources_for_tests()))
-            }
-        }
-    });
+#[cfg(all(feature = "baked-default-resources", servo_production))]
+const _: () = assert!(
+    false,
+    "baked-default-resources should not be used in production"
+);
 
+/// The Embedder should initialize the ResourceReader early.
 pub fn set(reader: Box<dyn ResourceReaderMethods + Sync + Send>) {
     *RES.write().unwrap() = Some(reader);
 }
 
+#[cfg(not(feature = "baked-default-resources"))]
 pub fn read_bytes(res: Resource) -> Vec<u8> {
+    if let Some(reader) = RES.read().unwrap().as_ref() {
+        reader.read(res)
+    } else {
+        log::error!("Resource reader not set.");
+        vec![]
+    }
+}
+
+#[cfg(feature = "baked-default-resources")]
+pub fn read_bytes(res: Resource) -> Vec<u8> {
+    INIT_TEST_RESOURCES.call_once(|| {
+        let mut reader = RES.write().unwrap();
+        if reader.is_none() {
+            *reader = Some(resources_for_tests())
+        }
+    });
     RES.read()
         .unwrap()
         .as_ref()
@@ -42,16 +54,16 @@ pub fn sandbox_access_files() -> Vec<PathBuf> {
     RES.read()
         .unwrap()
         .as_ref()
-        .expect("Resource reader not set.")
-        .sandbox_access_files()
+        .map(|reader| reader.sandbox_access_files())
+        .unwrap_or_default()
 }
 
 pub fn sandbox_access_files_dirs() -> Vec<PathBuf> {
     RES.read()
         .unwrap()
         .as_ref()
-        .expect("Resource reader not set.")
-        .sandbox_access_files_dirs()
+        .map(|reader| reader.sandbox_access_files_dirs())
+        .unwrap_or_default()
 }
 
 pub enum Resource {
@@ -93,10 +105,10 @@ pub enum Resource {
     QuirksModeCSS,
     /// A placeholder image to display if we couldn't get the requested image.
     ///
-    /// ## Safety
+    /// ## Panic
     ///
-    /// Servo will crash if this is an invalid image. Check `resources/rippy.png` in Servo codebase to see what
-    /// a default rippy png should look like.
+    /// If the resource is not provided, servo will fallback to a baked in default (See resources/rippy.png).
+    /// However, if the image is provided but invalid, Servo will crash.
     RippyPNG,
     /// A CSS file to style the media controls.
     /// It can be empty but then media controls will not be styled.
@@ -145,12 +157,10 @@ pub trait ResourceReaderMethods {
     fn sandbox_access_files_dirs(&self) -> Vec<PathBuf>;
 }
 
-/// Bake all of our resources into this crate for tests, unless we are `cfg!(servo_production)`.
+/// Provides baked in resources for tests.
 ///
-/// Local non-production embedder builds (e.g. servoshell) can still override these with [`set`].
-/// On OpenHarmony we never want to include files, since we ship all the files in the application
-/// bundle anyway.
-#[cfg(not(any(servo_production, target_env = "ohos")))]
+/// Embedder builds (e.g. servoshell) should use [`set`] and ship the resources themselves.
+#[cfg(feature = "baked-default-resources")]
 fn resources_for_tests() -> Box<dyn ResourceReaderMethods + Sync + Send> {
     struct ResourceReader;
     impl ResourceReaderMethods for ResourceReader {
diff --git a/components/shared/net/Cargo.toml b/components/shared/net/Cargo.toml
index 2c0a4995ec4..79ea936a688 100644
--- a/components/shared/net/Cargo.toml
+++ b/components/shared/net/Cargo.toml
@@ -40,3 +40,6 @@ servo_rand = { path = "../../rand" }
 servo_url = { path = "../../url" }
 url = { workspace = true }
 uuid = { workspace = true }
+
+[dev-dependencies]
+embedder_traits = { workspace = true, features = ["baked-default-resources"] }