From bb985870f0cc8dab220d8b6b992ee48d2fc546a0 Mon Sep 17 00:00:00 2001
From: Martchus <martchus@gmx.net>
Date: Fri, 15 Apr 2022 19:18:35 +0200
Subject: [PATCH] Avoid overflow when running out of IDs

* Throw an exception instead
* Add function that allows re-using lower IDs instead
* Move functions to query IDs to read-only operations
---
 lmdb-typed.cc | 11 ---------
 lmdb-typed.hh | 67 ++++++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 58 insertions(+), 20 deletions(-)

diff --git a/lmdb-typed.cc b/lmdb-typed.cc
index fea6674..c2d8ff6 100644
--- a/lmdb-typed.cc
+++ b/lmdb-typed.cc
@@ -2,15 +2,4 @@
 
 namespace LMDBSafe {
 
-IDType MDBGetMaxID(MDBRWTransaction &txn, MDBDbi &dbi)
-{
-    auto cursor = txn->getRWCursor(dbi);
-    MDBOutVal maxidval, maxcontent;
-    auto maxid = IDType(0);
-    if (!cursor.get(maxidval, maxcontent, MDB_LAST)) {
-        maxid = maxidval.get<IDType>();
-    }
-    return maxid;
-}
-
 } // namespace LMDBSafe
diff --git a/lmdb-typed.hh b/lmdb-typed.hh
index b096b6e..a4a5bfe 100644
--- a/lmdb-typed.hh
+++ b/lmdb-typed.hh
@@ -13,14 +13,6 @@ namespace LMDBSafe {
  */
 using IDType = std::uint32_t;
 
-/*!
- * \brief Returns the highest ID used in a database. Returns 0 for an empty DB.
- * \remarks
- * This makes us start everything at ID "1", which might make it possible to
- * treat id 0 as special.
- */
-LMDB_SAFE_EXPORT IDType MDBGetMaxID(MDBRWTransaction &txn, MDBDbi &dbi);
-
 /*!
  * \brief Converts \a t to an std::string.
  *
@@ -298,6 +290,63 @@ public:
             return stat.ms_entries;
         }
 
+        /*!
+         * \brief Returns the highest ID or 0 if the database is empty.
+         */
+        IDType maxID()
+        {
+            auto cursor = (*d_parent.d_txn)->getCursor(d_parent.d_parent->d_main);
+            MDBOutVal idval, maxcontent;
+            auto id = IDType(0);
+            if (!cursor.get(idval, maxcontent, MDB_LAST)) {
+                id = idval.get<IDType>();
+            }
+            return id;
+        }
+
+        /*!
+         * \brief Returns the next highest ID in the database.
+         * \remarks Never returns 0 so it can be used as special "no such ID" value.
+         * \throws Throws LMDBError when running out of IDs.
+         */
+        IDType nextID()
+        {
+            const auto id = maxID();
+            if (id < std::numeric_limits<IDType>::max()) {
+                return id + 1;
+            }
+            throw LMDBError("Running out of IDs");
+        }
+
+        /*!
+         * \brief Returns an ID not used in the database so far.
+         * \remarks
+         * - Lower IDs are reused but an extensive search for "gabs" is avoided.
+         * - Never returns 0 so it can be used as special "no such ID" value.
+         * \throws Throws LMDBError when running out of IDs.
+         */
+        IDType newID()
+        {
+            auto cursor = (*d_parent.d_txn)->getCursor(d_parent.d_parent->d_main);
+            MDBOutVal idval, maxcontent;
+            auto id = IDType(1);
+            if (!cursor.get(idval, maxcontent, MDB_FIRST)) {
+                id = idval.get<IDType>();
+            }
+            if (id > 1) {
+                return id - 1;
+            }
+            if (!cursor.get(idval, maxcontent, MDB_LAST)) {
+                id = idval.get<IDType>();
+            } else {
+                id = 0;
+            }
+            if (id < std::numeric_limits<IDType>::max()) {
+                return id + 1;
+            }
+            throw LMDBError("Running out of IDs");
+        }
+
         //! Get item with id, from main table directly
         bool get(IDType id, T &t)
         {
@@ -798,7 +847,7 @@ public:
         {
             unsigned int flags = 0;
             if (!id) {
-                id = MDBGetMaxID(*d_txn, d_parent->d_main) + 1;
+                id = this->nextID();
                 flags = MDB_APPEND;
             }
             (*d_txn)->put(d_parent->d_main, id, serToString(t), flags);