Create basic reentrant ClasspathFetcher targetprep

This will take over the responsibilities of StrictJavaPackagesTest's
setup code, and by fetching and processing all classpath data, as
defined in go/stricter-java-packages-test .

Test: atest ClasspathFetcherTest
Bug: 254647172
Change-Id: I93d2651994f7602d60ae91ca2401b83ae8c57baf

java/com/android/modules/targetprep/ClasspathFetcher.java

@@ -0,0 +1,96 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * 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 com.android.modules.targetprep;
+
+import com.android.tradefed.config.Option;
+import com.android.tradefed.device.DeviceNotAvailableException;
+import com.android.tradefed.device.ITestDevice;
+import com.android.tradefed.invoker.TestInformation;
+import com.android.tradefed.log.LogUtil.CLog;
+import com.android.tradefed.targetprep.BaseTargetPreparer;
+import com.android.tradefed.targetprep.TargetSetupError;
+import com.android.tradefed.util.RunUtil;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+/*
+ * Target preparer that fetches classpath relevant artifacts for a test in a 'reentrant' manner.
+ *
+ * <p>The preparer will fetch all <b>BOOTCLASSPATH</b>, <b>SYSTEMSERVERCLASSPATH</b> and shared
+ * libraries from the device, parse their contents and dump the classnames into a temporary
+ * directory.</p>
+ *
+ * <p>Additionally, the preparer <i>can</i> fetch, parse and dump data for module specific artifacts
+ * (i.e. apk-in-apex) if specified.</p>
+ *
+ * <p>Nested runs of the artifact fetcher (i.e. in the case of a top-level test config xml and a
+ * child test xml) will only fetch non-common elements, and remove temporary class dumps fetched
+ * during that particular preparer run.</p>
+ *
+ * <p>Each module's conformance framework test config xml must run the preparer (with module
+ * specific parameters) before the entrypoint test jar, and the top-level conformance framework
+ * config xml must also run it before any other module xml.</p>
+ *
+ * <p>The upshot is that when running all conformance framework tests for all modules, the shared
+ * artifacts are fetched and processed only once.</p>
+ */
+public class ClasspathFetcher extends BaseTargetPreparer {
+
+    public static final String DEVICE_JAR_ARTIFACTS_TAG = "device-jar-artifacts";
+
+    private boolean mFetchedArtifacts = false;
+
+    @Override
+    public void setUp(TestInformation testInfo)
+            throws TargetSetupError, DeviceNotAvailableException {
+        // The artifacts have been fetched already, no need to do anything else.
+        if (testInfo.properties().containsKey(DEVICE_JAR_ARTIFACTS_TAG)) {
+            return;
+        }
+        try {
+            final Path tmpDir = Files.createTempDirectory("device_artifacts");
+            testInfo.properties().put(DEVICE_JAR_ARTIFACTS_TAG, tmpDir.toAbsolutePath().toString());
+            // TODO(b/254647172): Fetch data
+            mFetchedArtifacts = true;
+        } catch(IOException e) {
+            throw new RuntimeException("Could not create temp artifacts dir!", e);
+        }
+    }
+
+    @Override
+    public void tearDown(TestInformation testInfo, Throwable e) {
+        if (mFetchedArtifacts) {
+            try {
+                final String path = testInfo.properties().get(DEVICE_JAR_ARTIFACTS_TAG);
+                if (path == null) {
+                    throw new IllegalStateException("Target preparer has previously fetched"
+                            + " artifacts, but the DEVICE_JAR_ARTIFACTS_TAG property was removed");
+                }
+                final File jarArtifactsDir = new File(path);
+                if (!jarArtifactsDir.delete()) {
+                    throw new RuntimeException("Failed to remove jar artifacts dir!");
+                }
+            } finally {
+                testInfo.properties().remove(DEVICE_JAR_ARTIFACTS_TAG);
+            }
+        }
+    }
+
+}