Remove corrupt package references in db recover

When aptly crashes it is possible to get a corrupt database with a dangling key reference.
This results in an error with 'key not found', eg:

    ERROR: unable to load package Pall example-package 1.2.3 778cf6f877bf6e2d: key not found

This change makes `db recover` fix this situation by removing the dangling references.

cmd/db_recover.go

@@ -1,6 +1,9 @@
 package cmd
 
 import (
+	"fmt"
+
+	"github.com/aptly-dev/aptly/deb"
 	"github.com/smira/commander"
 
 	"github.com/aptly-dev/aptly/database/goleveldb"
@@ -16,7 +19,12 @@ func aptlyDbRecover(cmd *commander.Command, args []string) error {
 	}
 
 	context.Progress().Printf("Recovering database...\n")
-	err = goleveldb.RecoverDB(context.DBPath())
+	if err = goleveldb.RecoverDB(context.DBPath()); err != nil {
+		return err
+	}
+
+	context.Progress().Printf("Checking database integrity...\n")
+	err = checkIntegrity()
 
 	return err
 }
@@ -38,3 +46,36 @@ Example:
 
 	return cmd
 }
+
+func checkIntegrity() error {
+	return context.NewCollectionFactory().LocalRepoCollection().ForEach(checkRepo)
+}
+
+func checkRepo(repo *deb.LocalRepo) error {
+	collectionFactory := context.NewCollectionFactory()
+	repos := collectionFactory.LocalRepoCollection()
+
+	err := repos.LoadComplete(repo)
+	if err != nil {
+		return fmt.Errorf("load complete repo %q: %s", repo.Name, err)
+	}
+
+	dangling, err := deb.FindDanglingReferences(repo.RefList(), collectionFactory.PackageCollection())
+	if err != nil {
+		return fmt.Errorf("find dangling references: %w", err)
+	}
+
+	if len(dangling.Refs) > 0 {
+		for _, ref := range dangling.Refs {
+			context.Progress().Printf("Removing dangling database reference %q\n", ref)
+		}
+
+		repo.UpdateRefList(repo.RefList().Subtract(dangling))
+
+		if err = repos.Update(repo); err != nil {
+			return fmt.Errorf("update repo: %w", err)
+		}
+	}
+
+	return nil
+}