KT-35472: Detect when content of annotation processor changes

Even if annotation processor classpath is the same, in case
individual entries change (e.g. bulding annotationo processor
from source), KATP should run non-incrementally.

Test: KaptIncrementalWithIsolatingApt.testUnchangedAnnotationProcessorClasspathButContentChanged

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/KaptIncrementalWithIsolatingApt.kt

@@ -95,6 +95,33 @@ class KaptIncrementalWithIsolatingApt : KaptIncrementalIT() {
         }
     }
 
+    @Test
+    fun testUnchangedAnnotationProcessorClasspathButContentChanged() {
+        val project = getProject()
+        val processorJar = project.projectDir.resolve("processor.jar").also { it.createNewFile() }
+        project.gradleBuildScript().appendText(
+            """
+            
+            dependencies {
+                kapt files("processor.jar")
+            }
+        """.trimIndent()
+        )
+
+        project.build("clean", "build") {
+            assertSuccessful()
+        }
+
+        ZipOutputStream(processorJar.outputStream()).use {
+            it.putNextEntry(ZipEntry("resource.txt"))
+            it.closeEntry()
+        }
+        project.build("build") {
+            assertSuccessful()
+            assertContains("Unable to use existing data, re-initializing classpath information for KAPT.")
+        }
+    }
+
     @Test
     fun testNonIncrementalWithUnrecognizedInputs() {
         val project = getProject()

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/kapt/incremental/ClasspathSnapshot.kt

@@ -69,6 +69,10 @@ open class ClasspathSnapshot protected constructor(
         if (!isCompatible(previousSnapshot)) {
             return KaptClasspathChanges.Unknown
         }
+        if (annotationProcessorClasspath.any { it in changedFiles }) {
+            // in case annotation processor classpath changes, we have to run non-incrementally
+            return KaptClasspathChanges.Unknown
+        }
 
         val unchangedBetweenCompilations = dataForFiles.keys.intersect(previousSnapshot.dataForFiles.keys).filter { it !in changedFiles }
         val currentToLoad = dataForFiles.keys.filter { it !in unchangedBetweenCompilations }.also { loadEntriesFor(it) }