Fix mushrooms not generating in swamps

During feature generation, light data is not initialised and
will always return 15 in Starlight. Vanilla can possibly return
0 if partially initialised, which allows some mushroom blocks to
generate.

In general, the brightness value from the light engine should not
be used until the chunk is ready. To emulate Vanilla behavior better,
we return 0 as the brightness during world gen unless the target
chunk is finished lighting.

The regular light retrieval outside of WorldGenRegion remains
the same, as behaviorally chunks not lit should be at max
skylight and zero block light.

Fixes https://github.com/PaperMC/Starlight/issues/193

src/main/java/ca/spottedleaf/moonrise/mixin/starlight/world/WorldGenRegionMixin.java

@@ -0,0 +1,46 @@
+package ca.spottedleaf.moonrise.mixin.starlight.world;
+
+import net.minecraft.core.BlockPos;
+import net.minecraft.server.level.WorldGenRegion;
+import net.minecraft.world.level.LightLayer;
+import net.minecraft.world.level.WorldGenLevel;
+import net.minecraft.world.level.chunk.ChunkAccess;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+
+@Mixin(WorldGenRegion.class)
+public abstract class WorldGenRegionMixin implements WorldGenLevel {
+
+    @Shadow
+    public abstract ChunkAccess getChunk(int i, int j);
+
+    /**
+     * @reason During feature generation, light data is not initialised and will always return 15 in Starlight. Vanilla
+     * can possibly return 0 if partially initialised, which allows some mushroom blocks to generate.
+     * In general, the brightness value from the light engine should not be used until the chunk is ready. To emulate
+     * Vanilla behavior better, we return 0 as the brightness during world gen unless the target chunk is finished
+     * lighting.
+     * @author Spottedleaf
+     */
+    @Override
+    public int getBrightness(final LightLayer lightLayer, final BlockPos blockPos) {
+        final ChunkAccess chunk = this.getChunk(blockPos.getX() >> 4, blockPos.getZ() >> 4);
+        if (!chunk.isLightCorrect()) {
+            return 0;
+        }
+        return this.getLightEngine().getLayerListener(lightLayer).getLightValue(blockPos);
+    }
+
+    /**
+     * @reason See above
+     * @author Spottedleaf
+     */
+    @Override
+    public int getRawBrightness(final BlockPos blockPos, final int subtract) {
+        final ChunkAccess chunk = this.getChunk(blockPos.getX() >> 4, blockPos.getZ() >> 4);
+        if (!chunk.isLightCorrect()) {
+            return 0;
+        }
+        return this.getLightEngine().getRawBrightness(blockPos, subtract);
+    }
+}

src/main/resources/moonrise.mixins.json

@@ -58,6 +58,7 @@
     "starlight.world.ChunkSerializerMixin",
     "starlight.world.LevelMixin",
     "starlight.world.ServerWorldMixin",
+    "starlight.world.WorldGenRegionMixin",
     "util_thread_counts.UtilMixin",
     "util_threading_detector.ThreadingDetectorMixin",
     "util_time_source.UtilMixin"