Populate APK files using hard links (2/n)

This is an effort to improve the storage usage of RollbackManager.

When committing a rollback, the apk/apex files will be deleted after
the install session is committed successfully. We can use hard links
to effectively transfer the files to PackageInstaller to avoid copy.

Bug: 168562373
Test: atest RollbackTest StagedRollbackTest

Change-Id: I749a17d769d1845bb0f9b34c8757b69b40b516f9

core/java/android/content/pm/IPackageInstallerSession.aidl

@@ -33,6 +33,7 @@ interface IPackageInstallerSession {
     ParcelFileDescriptor openRead(String name);
 
     void write(String name, long offsetBytes, long lengthBytes, in ParcelFileDescriptor fd);
+    void stageViaHardLink(String target);
 
     void addChecksums(String name, in Checksum[] checksums);
 

core/java/android/content/pm/PackageInstaller.java

@@ -1048,6 +1048,31 @@ public class PackageInstaller {
             }
         }
 
+        /**
+         * Populate an APK file by creating a hard link to avoid the need to copy.
+         * <p>
+         * Note this API is used by RollbackManager only and can only be called from system_server.
+         * {@code target} will be relabeled if link is created successfully. RollbackManager has
+         * to delete {@code target} when the session is committed successfully to avoid SELinux
+         * label conflicts.
+         * <p>
+         * Note No more bytes should be written to the file once the link is created successfully.
+         *
+         * @param target the path of the link target
+         *
+         * @hide
+         */
+        public void stageViaHardLink(String target) throws IOException {
+            try {
+                mSession.stageViaHardLink(target);
+            } catch (RuntimeException e) {
+                ExceptionUtils.maybeUnwrapIOException(e);
+                throw e;
+            } catch (RemoteException e) {
+                throw e.rethrowFromSystemServer();
+            }
+        }
+
         /**
          * Ensure that any outstanding data for given stream has been committed
          * to disk. This is only valid for streams returned from

services/core/java/com/android/server/pm/PackageInstallerSession.java

@@ -110,6 +110,7 @@ import android.os.ParcelableException;
 import android.os.Process;
 import android.os.RemoteException;
 import android.os.RevocableFileDescriptor;
+import android.os.SELinux;
 import android.os.SystemProperties;
 import android.os.UserHandle;
 import android.os.incremental.IStorageHealthListener;
@@ -1092,6 +1093,31 @@ public class PackageInstallerSession extends IPackageInstallerSession.Stub {
         }
     }
 
+    @Override
+    public void stageViaHardLink(String path) {
+        final int callingUid = Binder.getCallingUid();
+        if (callingUid != Process.SYSTEM_UID) {
+            throw new SecurityException("link() can only be run by the system");
+        }
+
+        try {
+            final File target = new File(path);
+            final File source = new File(stageDir, target.getName());
+            try {
+                Os.link(path, source.getAbsolutePath());
+                // Grant READ access for APK to be read successfully
+                Os.chmod(source.getAbsolutePath(), 0644);
+            } catch (ErrnoException e) {
+                e.rethrowAsIOException();
+            }
+            if (!SELinux.restorecon(source)) {
+                throw new IOException("Can't relabel file: " + source);
+            }
+        } catch (IOException e) {
+            throw ExceptionUtils.wrap(e);
+        }
+    }
+
     private ParcelFileDescriptor openTargetInternal(String path, int flags, int mode)
             throws IOException, ErrnoException {
         // TODO: this should delegate to DCS so the system process avoids

services/core/java/com/android/server/rollback/Rollback.java

@@ -581,9 +581,18 @@ class Rollback {
                             ParcelFileDescriptor.MODE_READ_ONLY)) {
                         final long token = Binder.clearCallingIdentity();
                         try {
-                            session.write(packageCodePath.getName(), 0,
-                                    packageCodePath.length(),
-                                    fd);
+                            boolean fallbackToCopy = false;
+                            try {
+                                // Populate apk/apex files using hard links to avoid copy
+                                session.stageViaHardLink(packageCodePath.getAbsolutePath());
+                            } catch (Exception ignore) {
+                                fallbackToCopy = true;
+                            }
+                            if (fallbackToCopy) {
+                                session.write(packageCodePath.getName(), 0,
+                                        packageCodePath.length(),
+                                        fd);
+                            }
                         } finally {
                             Binder.restoreCallingIdentity(token);
                         }