src/main/java/com/android/apkzlib/sign/ManifestGenerationExtension.java

/*
 * Copyright (C) 2016 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.apkzlib.sign;

import com.android.apkzlib.utils.CachedSupplier;
import com.android.apkzlib.utils.IOExceptionRunnable;
import com.android.apkzlib.zfile.ManifestAttributes;
import com.android.apkzlib.zip.StoredEntry;
import com.android.apkzlib.zip.ZFile;
import com.android.apkzlib.zip.ZFileExtension;
import com.google.common.base.Preconditions;
import com.google.common.base.Verify;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.util.jar.Attributes;
import java.util.jar.Manifest;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;

/**
 * Extension to {@link ZFile} that will generate a manifest. The extension will register
 * automatically with the {@link ZFile}.
 *
 * <p>Creating this extension will ensure a manifest for the zip exists.
 * This extension will generate a manifest if one does not exist and will update an existing
 * manifest, if one does exist. The extension will also provide access to the manifest so that
 * others may update the manifest.
 *
 * <p>Apart from standard manifest elements, this extension does not handle any particular manifest
 * features such as signing or adding custom attributes. It simply generates a plain manifest and
 * provides infrastructure so that other extensions can add data in the manifest.
 *
 * <p>The manifest itself will only be written when the {@link ZFileExtension#beforeUpdate()}
 * notification is received, meaning all manifest manipulation is done in-memory.
 */
public class ManifestGenerationExtension {

    /**
     * Name of META-INF directory.
     */
    private static final String META_INF_DIR = "META-INF";

    /**
     * Name of the manifest file.
     */
    static final String MANIFEST_NAME = META_INF_DIR + "/MANIFEST.MF";

    /**
     * Who should be reported as the manifest builder.
     */
    @Nonnull
    private final String builtBy;

    /**
     * Who should be reported as the manifest creator.
     */
    @Nonnull
    private final String createdBy;

    /**
     * The file this extension is attached to. {@code null} if not yet registered.
     */
    @Nullable
    private ZFile zFile;

    /**
     * The zip file's manifest.
     */
    @Nonnull
    private final Manifest manifest;

    /**
     * Byte representation of the manifest. There is no guarantee that two writes of the java's
     * {@code Manifest} object will yield the same byte array (there is no guaranteed order
     * of entries in the manifest).
     *
     * <p>Because we need the byte representation of the manifest to be stable if there are
     * no changes to the manifest, we cannot rely on {@code Manifest} to generate the byte
     * representation every time we need the byte representation.
     *
     * <p>This cache will ensure that we will request one byte generation from the {@code Manifest}
     * and will cache it. All further requests of the manifest's byte representation will
     * receive the same byte array.
     */
    @Nonnull
    private CachedSupplier<byte[]> manifestBytes;

    /**
     * Has the current manifest been changed and not yet flushed? If {@link #dirty} is
     * {@code true}, then {@link #manifestBytes} should not be valid. This means that
     * marking the manifest as dirty should also invalidate {@link #manifestBytes}. To avoid
     * breaking the invariant, instead of setting {@link #dirty}, {@link #markDirty()} should
     * be called.
     */
    private boolean dirty;

    /**
     * The extension to register with the {@link ZFile}. {@code null} if not registered.
     */
    @Nullable
    private ZFileExtension extension;

    /**
     * Creates a new extension. This will not register the extension with the provided
     * {@link ZFile}. Until {@link #register(ZFile)} is invoked, this extension is not used.
     *
     * @param builtBy who built the manifest?
     * @param createdBy who created the manifest?
     */
    public ManifestGenerationExtension(@Nonnull String builtBy, @Nonnull String createdBy) {
        this.builtBy = builtBy;
        this.createdBy = createdBy;
        manifest = new Manifest();
        dirty = false;
        manifestBytes = new CachedSupplier<>(() -> {
            ByteArrayOutputStream outBytes = new ByteArrayOutputStream();
            try {
                manifest.write(outBytes);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }

            return outBytes.toByteArray();
        });
    }

    /**
     * Marks the manifest as being dirty, <i>i.e.</i>, its data has changed since it was last
     * read and/or written.
     */
    private void markDirty() {
        dirty = true;
        manifestBytes.reset();
    }

    /**
     * Registers the extension with the {@link ZFile} provided in the constructor.
     *
     * @param zFile the zip file to add the extension to
     * @throws IOException failed to analyze the zip
     */
    public void register(@Nonnull ZFile zFile) throws IOException {
        Preconditions.checkState(extension == null, "register() has already been invoked.");
        this.zFile = zFile;

        rebuildManifest();

        extension = new ZFileExtension() {
            @Nullable
            @Override
            public IOExceptionRunnable beforeUpdate() {
                return ManifestGenerationExtension.this::updateManifest;
            }
        };

        this.zFile.addZFileExtension(extension);
    }

    /**
     * Rebuilds the zip file's manifest, if it needs changes.
     */
    private void rebuildManifest() throws IOException {
        Verify.verifyNotNull(zFile, "zFile == null");

        StoredEntry manifestEntry = zFile.get(MANIFEST_NAME);

        if (manifestEntry != null) {
            /*
             * Read the manifest entry in the zip file. Make sure we store these byte sequence
             * because writing the manifest may not generate the same byte sequence, which may
             * trigger an unnecessary re-sign of the jar.
             */
            manifest.clear();
            byte[] manifestBytes = manifestEntry.read();
            manifest.read(new ByteArrayInputStream(manifestBytes));
            this.manifestBytes.precomputed(manifestBytes);
        }

        Attributes mainAttributes = manifest.getMainAttributes();
        String currentVersion = mainAttributes.getValue(ManifestAttributes.MANIFEST_VERSION);
        if (currentVersion == null) {
            setMainAttribute(
                    ManifestAttributes.MANIFEST_VERSION,
                    ManifestAttributes.CURRENT_MANIFEST_VERSION);
        } else {
            if (!currentVersion.equals(ManifestAttributes.CURRENT_MANIFEST_VERSION)) {
                throw new IOException("Unsupported manifest version: " + currentVersion + ".");
            }
        }

        /*
         * We "blindly" override all other main attributes.
         */
        setMainAttribute(ManifestAttributes.BUILT_BY, builtBy);
        setMainAttribute(ManifestAttributes.CREATED_BY, createdBy);
    }

    /**
     * Sets the value of a main attribute.
     *
     * @param attribute the attribute
     * @param value the value
     */
    private void setMainAttribute(@Nonnull String attribute, @Nonnull String value) {
        Attributes mainAttributes = manifest.getMainAttributes();
        String current = mainAttributes.getValue(attribute);
        if (!value.equals(current)) {
            mainAttributes.putValue(attribute, value);
            markDirty();
        }
    }

    /**
     * Updates the manifest in the zip file, if it has been changed.
     *
     * @throws IOException failed to update the manifest
     */
    private void updateManifest() throws IOException {
        Verify.verifyNotNull(zFile, "zFile == null");

        if (!dirty) {
            return;
        }

        zFile.add(MANIFEST_NAME, new ByteArrayInputStream(manifestBytes.get()));
        dirty = false;
    }
}