Commits on Mar 18, 2024

1. android: fix broken rendering in emulators …

   Android's OpenGL emulation layer (goldfish-opengl) has pre-processing
   logic that looks for samplers of the type `samplerExternalOES` and
   does a simple textual [replacement to change the type][1] to `sampler2D`
   before compilation. It also [marks the sampler][2] as 'replaced' so
   it can emulate the correct type at runtime.
   
   However, this logic can lead to false positives when the sampler is
   declared inside conditional macros. Hence, the sampler's type can be
   incorrectly marked as `samplerExternalOES` even though the #if, #ifdef
   conditional logic would have declared the type as `sampler2D`.
   This seems to be a [known limitation][3].
   
   WebRender (in particular the shared.glsl include) has such conditional
   declaration of the texture units used from the shaders.
   In particular, the sampler [sColor0 here][4] is declared within ifdefs
   to have different types depending on the flags enabled, to allow the
   shader to work with different image target kinds.
   
   WebRender also maintain two versions of the compiled shaders in its cache:
     1. An unoptimized version with all the conditional logic preserved
        in the source until the shader is compiled at runtime on Android,
        when the shader is actually used.
     2. Multiple optimized versions for combinations of features required
        These versions are produced during Servo [build][5]. Thus the
        optimized versions eliminate most of the conditional declarations
        at build time.
   
   The bug in Servo with current code is because, [by default][6], WebRender
   uses the *unoptimized* versions of the shaders. This means the conditional
   GLSL source is evaluated at runtime by the Android emulator and thus ends
   up with the incorrect type for the sColor0 sampler unit, which breaks the
   [texture sampling in the fragment shader][7], causing it to always return
   Vec4(0, 0, 0, 1) and rendering all elements on the page black.
   
   This change forces WebRender to use the *optimized* version as a
   workaround - the optimized versions have unconditional code for the
   sampler declarations so are not susceptible to the emulator issue.
   
   [1]: https://android.googlesource.com/device/generic/goldfish-opengl/+/refs/tags/android-platform-11.0.0_r40/system/GLESv2_enc/GL2Encoder.cpp#1644
   [2]: https://android.googlesource.com/device/generic/goldfish-opengl/+/refs/tags/android-platform-11.0.0_r40/system/GLESv2_enc/GL2Encoder.cpp#1673
   [3]: https://android.googlesource.com/device/generic/goldfish-opengl/+/refs/tags/android-platform-11.0.0_r40/system/GLESv2_enc/GL2Encoder.cpp#1571
   [4]: https://github.com/servo/webrender/blob/b36399019cadcadeeed53759c96870577d4da136/webrender/res/shared.glsl#L206
   [5]: https://github.com/servo/webrender/blob/b36399019cadcadeeed53759c96870577d4da136/webrender/build.rs#L289
   [6]: https://github.com/servo/webrender/blob/b36399019cadcadeeed53759c96870577d4da136/webrender/src/renderer/init.rs#L214
   [7]: https://github.com/servo/webrender/blob/b36399019cadcadeeed53759c96870577d4da136/webrender/res/composite.glsl#L189
   
   Fixes servo#31726.
   
   Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

   

   mukilan committed Mar 18, 2024
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