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diff --git a/comm/mailnews/db/msgdb/src/nsMsgDatabaseEnumerators.h b/comm/mailnews/db/msgdb/src/nsMsgDatabaseEnumerators.h
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+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef _nsMsgDatabaseEnumerators_H_
+#define _nsMsgDatabaseEnumerators_H_
+
+/*
+ * This file provides some enumerator classes, private to nsMsgDatabase.
+ * The outside world would only ever see these as nsIMsgEnumerator or
+ * nsIMsgThreadEnumerator.
+ *
+ * These enumerators automatically register themselves with the nsMsgDatabase
+ * during construction/destruction. This lets the database track all
+ * outstanding enumerators, so they can be invalidated if the database is
+ * destroyed or ForceClosed().
+ * Due to this lifecycle coupling, we try to avoid using refcounted pointers
+ * here, as we don't want outstanding enumerators to lock an otherwise unused
+ * database in existence.
+ */
+
+#include "nsMsgEnumerator.h"
+#include "nsCOMPtr.h"
+#include "nsTArray.h"
+#include "mdb.h"
+#include "nsIDBChangeListener.h"
+
+#include "nsIMsgSearchTerm.h"
+#include "nsIMsgSearchSession.h"
+
+class nsMsgDatabase;
+class nsIMdbTable;
+class nsIMdbTableRowCursor;
+class nsIMsgFolder;
+
+/**
+ * Enumerates over messages, forwards or backward, with an optional filter fn.
+ */
+class nsMsgDBEnumerator : public nsBaseMsgEnumerator {
+ public:
+  // nsIMsgEnumerator support.
+  NS_IMETHOD GetNext(nsIMsgDBHdr** aItem) override;
+  NS_IMETHOD HasMoreElements(bool* aResult) override;
+
+  // Function type for filtering which messages are enumerated.
+  typedef nsresult (*nsMsgDBEnumeratorFilter)(nsIMsgDBHdr* hdr, void* closure);
+
+  nsMsgDBEnumerator(nsMsgDatabase* db, nsIMdbTable* table,
+                    nsMsgDBEnumeratorFilter filter, void* closure,
+                    bool iterateForwards = true);
+  // Called by db when no longer valid (db is being destroyed or ForcedClosed).
+  void Invalidate();
+
+ protected:
+  // internals
+  nsresult GetRowCursor();
+
+  // Returns next message or nullptr if none more.
+  virtual nsresult InternalGetNext(nsIMsgDBHdr** nextHdr);
+
+  // Our source DB. Not refcounted, because we don't want to lock the DB
+  // in existence. The enumerator is registered with the DB, and the DB will
+  // call Invalidate() if it is destroyed or ForceClosed().
+  nsMsgDatabase* mDB;
+  nsCOMPtr<nsIMdbTableRowCursor> mRowCursor;
+  mdb_pos mRowPos;
+  nsCOMPtr<nsIMsgDBHdr> mResultHdr;
+  bool mDone;
+  bool mIterateForwards;
+  nsMsgDBEnumeratorFilter mFilter;
+  nsIMdbTable* mTable;
+  void* mClosure;
+
+  virtual ~nsMsgDBEnumerator() override;
+};
+
+/**
+ * Enumerate over messages which match the given search terms.
+ */
+class nsMsgFilteredDBEnumerator : public nsMsgDBEnumerator {
+ public:
+  nsMsgFilteredDBEnumerator(nsMsgDatabase* db, nsIMdbTable* table,
+                            bool reverse);
+  virtual ~nsMsgFilteredDBEnumerator() override;
+  nsresult InitSearchSession(
+      const nsTArray<RefPtr<nsIMsgSearchTerm>>& searchTerms,
+      nsIMsgFolder* folder);
+
+ protected:
+  virtual nsresult InternalGetNext(nsIMsgDBHdr** nextHdr) override;
+
+  nsCOMPtr<nsIMsgSearchSession> m_searchSession;
+};
+
+/**
+ * Helper class for fetching message threads from a database.
+ * This enumerator automatically registers itself with the nsMsgDatabase.
+ * If the DB is destroyed or ForceClosed() it will call the enumerators
+ * Invalidate() method.
+ */
+class nsMsgDBThreadEnumerator : public nsBaseMsgThreadEnumerator {
+ public:
+  // nsIMsgThreadEnumerator support.
+  NS_IMETHOD GetNext(nsIMsgThread** aItem) override;
+  NS_IMETHOD HasMoreElements(bool* aResult) override;
+
+  // Function type for filtering threads that appear in the enumeration.
+  typedef nsresult (*nsMsgDBThreadEnumeratorFilter)(nsIMsgThread* thread);
+
+  nsMsgDBThreadEnumerator(nsMsgDatabase* db,
+                          nsMsgDBThreadEnumeratorFilter filter);
+
+  // Called by DB when being destroyed or ForcedClosed.
+  void Invalidate();
+
+ protected:
+  virtual ~nsMsgDBThreadEnumerator();
+  nsresult GetTableCursor(void);
+  nsresult PrefetchNext();
+
+  // Our source DB. Not refcounted, because we don't want to lock the DB
+  // in existence. The enumerator is registered with the DB, and the DB will
+  // call Invalidate() if it is destroyed or ForceClosed().
+  nsMsgDatabase* mDB;
+  nsCOMPtr<nsIMdbPortTableCursor> mTableCursor;
+  RefPtr<nsIMsgThread> mResultThread;
+  bool mDone;
+  bool mNextPrefetched;
+  nsMsgDBThreadEnumeratorFilter mFilter;
+};
+
+#endif  // _nsMsgDatabaseEnumerators_H_