No-op VMRuntime.setThreadNiceness via ASM transform

paparazzi-gradle-plugin/src/main/java/app/cash/paparazzi/gradle/PaparazziPlugin.kt

@@ -16,6 +16,7 @@
 package app.cash.paparazzi.gradle
 
 import app.cash.paparazzi.gradle.instrumentation.ResourcesCompatVisitorFactory
+import app.cash.paparazzi.gradle.instrumentation.ThreadNicenessVisitorFactory
 import app.cash.paparazzi.gradle.reporting.DiffImage
 import app.cash.paparazzi.gradle.reporting.PaparazziTestReporter
 import app.cash.paparazzi.gradle.utils.artifactViewFor
@@ -177,6 +178,10 @@ public class PaparazziPlugin @Inject constructor(
           ResourcesCompatVisitorFactory::class.java,
           InstrumentationScope.ALL
         ) { }
+        testInstrumentation.transformClassesWith(
+          ThreadNicenessVisitorFactory::class.java,
+          InstrumentationScope.ALL
+        ) { }
         testInstrumentation.setAsmFramesComputationMode(
           FramesComputationMode.COMPUTE_FRAMES_FOR_INSTRUMENTED_METHODS
         )

paparazzi-gradle-plugin/src/main/java/app/cash/paparazzi/gradle/instrumentation/ThreadNicenessVisitorFactory.kt

@@ -0,0 +1,143 @@
+/*
+ * Copyright (C) 2026 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package app.cash.paparazzi.gradle.instrumentation
+
+import com.android.build.api.instrumentation.AsmClassVisitorFactory
+import com.android.build.api.instrumentation.ClassContext
+import com.android.build.api.instrumentation.ClassData
+import com.android.build.api.instrumentation.InstrumentationParameters
+import org.objectweb.asm.ClassVisitor
+import org.objectweb.asm.MethodVisitor
+import org.objectweb.asm.Opcodes
+
+/**
+ * Transforms [dalvik.system.VMRuntime] to no-op [setThreadNiceness], which calls
+ * [java.lang.Thread.setPosixNicenessInternal] — a method that does not exist on
+ * the JVM and causes [NoSuchMethodError] at runtime.
+ */
+internal abstract class ThreadNicenessVisitorFactory : AsmClassVisitorFactory<InstrumentationParameters.None> {
+  override fun createClassVisitor(classContext: ClassContext, nextClassVisitor: ClassVisitor): ClassVisitor {
+    return ThreadNicenessTransform(nextClassVisitor)
+  }
+
+  override fun isInstrumentable(classData: ClassData): Boolean = classData.className == VMRUNTIME_CLASS_NAME
+
+  internal class ThreadNicenessTransform(delegate: ClassVisitor) : ClassVisitor(Opcodes.ASM9, delegate) {
+    override fun visitMethod(
+      access: Int,
+      name: String,
+      descriptor: String,
+      signature: String?,
+      exceptions: Array<String>?
+    ): MethodVisitor {
+      val methodVisitor = super.visitMethod(access, name, descriptor, signature, exceptions)
+      if (name == SET_THREAD_NICENESS_METHOD_NAME) {
+        // Replace the method body with a no-op return.
+        return NoOpMethodVisitor(api, methodVisitor, descriptor)
+      }
+      return methodVisitor
+    }
+
+    /**
+     * Replaces the entire method body with a return of the appropriate default value.
+     */
+    private class NoOpMethodVisitor(
+      api: Int,
+      delegate: MethodVisitor,
+      private val descriptor: String
+    ) : MethodVisitor(api, delegate) {
+      override fun visitCode() {
+        super.visitCode()
+        // Emit a return matching the method's return type.
+        val returnType = descriptor.substringAfterLast(')')
+        when (returnType) {
+          "V" -> super.visitInsn(Opcodes.RETURN)
+          "I", "Z", "B", "C", "S" -> {
+            super.visitInsn(Opcodes.ICONST_0)
+            super.visitInsn(Opcodes.IRETURN)
+          }
+          "J" -> {
+            super.visitInsn(Opcodes.LCONST_0)
+            super.visitInsn(Opcodes.LRETURN)
+          }
+          "F" -> {
+            super.visitInsn(Opcodes.FCONST_0)
+            super.visitInsn(Opcodes.FRETURN)
+          }
+          "D" -> {
+            super.visitInsn(Opcodes.DCONST_0)
+            super.visitInsn(Opcodes.DRETURN)
+          }
+          else -> {
+            super.visitInsn(Opcodes.ACONST_NULL)
+            super.visitInsn(Opcodes.ARETURN)
+          }
+        }
+        super.visitMaxs(2, 2)
+        super.visitEnd()
+      }
+
+      // Suppress original bytecode instructions.
+      override fun visitInsn(opcode: Int) = Unit
+      override fun visitIntInsn(opcode: Int, operand: Int) = Unit
+      override fun visitVarInsn(opcode: Int, varIndex: Int) = Unit
+      override fun visitTypeInsn(opcode: Int, type: String) = Unit
+      override fun visitFieldInsn(opcode: Int, owner: String, name: String, descriptor: String) = Unit
+      override fun visitMethodInsn(opcode: Int, owner: String, name: String, descriptor: String, isInterface: Boolean) =
+        Unit
+      override fun visitJumpInsn(opcode: Int, label: org.objectweb.asm.Label) = Unit
+      override fun visitLabel(label: org.objectweb.asm.Label) = Unit
+      override fun visitLdcInsn(value: Any) = Unit
+      override fun visitIincInsn(varIndex: Int, increment: Int) = Unit
+      override fun visitTableSwitchInsn(
+        min: Int,
+        max: Int,
+        dflt: org.objectweb.asm.Label,
+        vararg labels: org.objectweb.asm.Label
+      ) = Unit
+      override fun visitLookupSwitchInsn(
+        dflt: org.objectweb.asm.Label,
+        keys: IntArray,
+        labels: Array<org.objectweb.asm.Label>
+      ) = Unit
+      override fun visitMultiANewArrayInsn(descriptor: String, numDimensions: Int) = Unit
+      override fun visitTryCatchBlock(
+        start: org.objectweb.asm.Label,
+        end: org.objectweb.asm.Label,
+        handler: org.objectweb.asm.Label,
+        type: String?
+      ) = Unit
+      override fun visitMaxs(maxStack: Int, maxLocals: Int) = Unit
+      override fun visitEnd() = Unit
+      override fun visitLineNumber(line: Int, start: org.objectweb.asm.Label) = Unit
+      override fun visitFrame(type: Int, numLocal: Int, local: Array<out Any>?, numStack: Int, stack: Array<out Any>?) =
+        Unit
+      override fun visitLocalVariable(
+        name: String,
+        descriptor: String,
+        signature: String?,
+        start: org.objectweb.asm.Label,
+        end: org.objectweb.asm.Label,
+        index: Int
+      ) = Unit
+    }
+  }
+
+  internal companion object {
+    const val VMRUNTIME_CLASS_NAME = "dalvik.system.VMRuntime"
+    const val SET_THREAD_NICENESS_METHOD_NAME = "setThreadNiceness"
+  }
+}

paparazzi-gradle-plugin/src/test/java/app/cash/paparazzi/gradle/PaparazziPluginTest.kt

@@ -1368,6 +1368,17 @@ class PaparazziPluginTest {
       .runFixture(fixtureRoot) { build() }
   }
 
+  @Test
+  fun handlerThreadNiceness() {
+    val fixtureRoot = File("src/test/projects/handler-thread-niceness")
+
+    val result = gradleRunner
+      .withArguments("testDebugUnitTest", "--stacktrace")
+      .runFixture(fixtureRoot) { build() }
+
+    assertThat(result.task(":testDebugUnitTest")).isNotNull()
+  }
+
   @Test
   fun verifySnapshot() {
     val fixtureRoot = File("src/test/projects/verify-snapshot")

paparazzi-gradle-plugin/src/test/projects/handler-thread-niceness/build.gradle

@@ -0,0 +1,20 @@
+plugins {
+  id 'com.android.library'
+  id 'kotlin-android'
+  id 'app.cash.paparazzi'
+}
+
+android {
+  namespace = 'app.cash.paparazzi.plugin.test'
+  compileSdk = libs.versions.compileSdk.get() as int
+  defaultConfig {
+    minSdk = libs.versions.minSdk.get() as int
+  }
+  compileOptions {
+    sourceCompatibility = libs.versions.javaTarget.get()
+    targetCompatibility = libs.versions.javaTarget.get()
+  }
+  kotlinOptions {
+    jvmTarget = libs.versions.javaTarget.get()
+  }
+}

paparazzi-gradle-plugin/src/test/projects/handler-thread-niceness/src/test/java/app/cash/paparazzi/plugin/test/HandlerThreadNicenessTest.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2026 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package app.cash.paparazzi.plugin.test
+
+import dalvik.system.VMRuntime
+import org.junit.Test
+
+class HandlerThreadNicenessTest {
+  @Test
+  fun setThreadNicenessDoesNotCrash() {
+    // VMRuntime.setThreadNiceness calls Thread.setPosixNicenessInternal,
+    // a Dalvik-specific method that doesn't exist on the JVM.
+    // Without the ASM transform, this throws NoSuchMethodError.
+    VMRuntime.getRuntime().setThreadNiceness(Thread.currentThread(), 10)
+  }
+}