Run background GC in Schema 144 and update GCs done in Schema 146

The initial preparatory GC in schema 146 can be skipped for two reasons:
1) Several updates were done to keep All-Users in good state in
   preceding schemas by not writing loose objects.
2) Schema 144 now triggers a GC in the background. Schema 144 belongs to
   the same Gerrit release as 146, so users will never run an older
   version of 144(w/o the background GC) and a newer version of 146
   (w/o the preparatory in-line GC).

The inline GC in schema 146 which runs every 50 packs has been converted
to a background GC. The intent of this inline GC was to keep All-Users
from getting worse when there are a large number of user refs that need
to be rewritten. Running a background GC also achieves that goal and has
the added advantage of not holding up a data migration thread to run GC,
thereby not slowing down the schema and allowing other schemas to
proceed without waiting for the GC to complete.

Change-Id: I16b048cf75671c40edc38e54d339735a93c1ebc9

3 files changed, 18 insertions, 44 deletions

diff --git a/java/com/google/gerrit/server/schema/SchemaVersion.java b/java/com/google/gerrit/server/schema/SchemaVersion.java
index da87c50691..b78b5a6233 100644
--- a/java/com/google/gerrit/server/schema/SchemaVersion.java
+++ b/java/com/google/gerrit/server/schema/SchemaVersion.java
@@ -345,7 +345,22 @@ public abstract class SchemaVersion {
     backgroundGcThread.execute(new GcTask(project, repoManager, ui));
   }
 
-  protected void gc(Repository repo, UpdateUI ui, ProgressMonitor pm, Stopwatch sw)
+  protected static ObjectId emptyTree(ObjectInserter oi) throws IOException {
+    return oi.insert(Constants.OBJ_TREE, new byte[] {});
+  }
+
+  protected static ObjectInserter getPackInserterFirst(Repository repo) {
+    if (repo instanceof FileRepository) {
+      return ((FileRepository) repo).getObjectDatabase().newPackInserter();
+    }
+    return repo.getObjectDatabase().newInserter();
+  }
+
+  private static long countDone(Collection<Future> futures) {
+    return futures.stream().filter(Future::isDone).count();
+  }
+
+  private void gc(Repository repo, UpdateUI ui, ProgressMonitor pm, Stopwatch sw)
       throws IOException, ParseException {
     FileRepository r = (FileRepository) repo;
     GC gc = new GC(r);
@@ -362,21 +377,6 @@ public abstract class SchemaVersion {
     gc.gc();
   }
 
-  protected static ObjectId emptyTree(ObjectInserter oi) throws IOException {
-    return oi.insert(Constants.OBJ_TREE, new byte[] {});
-  }
-
-  protected static ObjectInserter getPackInserterFirst(Repository repo) {
-    if (repo instanceof FileRepository) {
-      return ((FileRepository) repo).getObjectDatabase().newPackInserter();
-    }
-    return repo.getObjectDatabase().newInserter();
-  }
-
-  private static long countDone(Collection<Future> futures) {
-    return futures.stream().filter(Future::isDone).count();
-  }
-
   private class GcTask implements Runnable {
     final GitRepositoryManager repoManager;
     final Project.NameKey project;
diff --git a/java/com/google/gerrit/server/schema/Schema_144.java b/java/com/google/gerrit/server/schema/Schema_144.java
index 88f24c2a7c..adddc2e8d1 100644
--- a/java/com/google/gerrit/server/schema/Schema_144.java
+++ b/java/com/google/gerrit/server/schema/Schema_144.java
@@ -101,6 +101,7 @@ public class Schema_144 extends SchemaVersion {
         }
         inserter.flush();
         bru.execute(rw, NullProgressMonitor.INSTANCE);
+        runGcInBackground(repoManager, allUsersName, ui);
       }
     } catch (IOException | ConfigInvalidException e) {
       throw new OrmException("Failed to migrate external IDs to NoteDb", e);
diff --git a/java/com/google/gerrit/server/schema/Schema_146.java b/java/com/google/gerrit/server/schema/Schema_146.java
index bc90c6d73d..e955bada74 100644
--- a/java/com/google/gerrit/server/schema/Schema_146.java
+++ b/java/com/google/gerrit/server/schema/Schema_146.java
@@ -32,7 +32,6 @@ import java.sql.ResultSet;
 import java.sql.SQLException;
 import java.sql.Statement;
 import java.sql.Timestamp;
-import java.text.ParseException;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -43,7 +42,6 @@ import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.ReentrantLock;
-import org.eclipse.jgit.internal.storage.file.FileRepository;
 import org.eclipse.jgit.internal.storage.file.PackInserter;
 import org.eclipse.jgit.lib.BatchRefUpdate;
 import org.eclipse.jgit.lib.CommitBuilder;
@@ -52,10 +50,8 @@ import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectInserter;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.PersonIdent;
-import org.eclipse.jgit.lib.ProgressMonitor;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Repository;
-import org.eclipse.jgit.lib.TextProgressMonitor;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevSort;
 import org.eclipse.jgit.revwalk.RevWalk;
@@ -100,9 +96,6 @@ public class Schema_146 extends SchemaVersion {
   protected void migrateData(ReviewDb db, UpdateUI ui) throws OrmException, SQLException {
     ui.message("Migrating accounts");
     Set<Entry<Account.Id, Timestamp>> accounts = scanAccounts(db, ui).entrySet();
-    ui.message("Run full gc as preparation for the migration");
-    gc(ui);
-    ui.message(String.format("... (%.3f s) full gc completed", elapsed()));
     Set<List<Entry<Account.Id, Timestamp>>> batches =
         Sets.newHashSet(Iterables.partition(accounts, 500));
     ExecutorService pool = createExecutor(ui);
@@ -159,9 +152,7 @@ public class Schema_146 extends SchemaVersion {
         inserter.flush();
         bru.execute(rw, NullProgressMonitor.INSTANCE);
         if (inserter instanceof PackInserter && packs.incrementAndGet() % 50 == 0) {
-          ui.message("Run full gc");
-          gc(ui);
-          ui.message(String.format("... (%.3f s) full gc completed", elapsed()));
+          runGcInBackground(repoManager, allUsersName, ui);
         }
       }
       showProgress(ui, migratedAccounts.addAndGet(batch.size()));
@@ -183,24 +174,6 @@ public class Schema_146 extends SchemaVersion {
     }
   }
 
-  private void gc(UpdateUI ui) {
-    try (Repository repo = repoManager.openRepository(allUsersName)) {
-      if (repo instanceof FileRepository && gcLock.tryLock()) {
-        ProgressMonitor pm = new TextProgressMonitor();
-        try {
-          gc(repo, ui, pm, sw);
-        } catch (IOException | ParseException e) {
-          throw new RuntimeException(e);
-        } finally {
-          gcLock.unlock();
-          pm.endTask();
-        }
-      }
-    } catch (IOException e) {
-      throw new UncheckedIOException(e);
-    }
-  }
-
   private void rewriteUserBranch(
       RevWalk rw,
       ObjectInserter oi,