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diff --git a/doc/developer-notes.md b/doc/developer-notes.md index f10ad8e877..f4513dca1f 100644 --- a/doc/developer-notes.md +++ b/doc/developer-notes.md @@ -543,7 +543,10 @@ its upstream repository. Current subtrees include: - src/leveldb - - Upstream at https://github.com/google/leveldb ; Maintained by Google, but open important PRs to Core to avoid delay + - Upstream at https://github.com/google/leveldb ; Maintained by Google, but + open important PRs to Core to avoid delay. + - **Note**: Follow the instructions in [Upgrading LevelDB](#upgrading-leveldb) when + merging upstream changes to the leveldb subtree. - src/libsecp256k1 - Upstream at https://github.com/bitcoin-core/secp256k1/ ; actively maintaned by Core contributors. @@ -554,6 +557,52 @@ Current subtrees include: - src/univalue - Upstream at https://github.com/jgarzik/univalue ; report important PRs to Core to avoid delay. +Upgrading LevelDB +--------------------- + +Extra care must be taken when upgrading LevelDB. This section explains issues +you must be aware of. + +### File Descriptor Counts + +In most configurations we use the default LevelDB value for `max_open_files`, +which is 1000 at the time of this writing. If LevelDB actually uses this many +file descriptors it will cause problems with Bitcoin's `select()` loop, because +it may cause new sockets to be created where the fd value is >= 1024. For this +reason, on 64-bit Unix systems we rely on an internal LevelDB optimization that +uses `mmap()` + `close()` to open table files without actually retaining +references to the table file descriptors. If you are upgrading LevelDB, you must +sanity check the changes to make sure that this assumption remains valid. + +In addition to reviewing the upstream changes in `env_posix.cc`, you can use `lsof` to +check this. For example, on Linux this command will show open `.ldb` file counts: + +```bash +$ lsof -p $(pidof bitcoind) |\ + awk 'BEGIN { fd=0; mem=0; } /ldb$/ { if ($4 == "mem") mem++; else fd++ } END { printf "mem = %s, fd = %s\n", mem, fd}' +mem = 119, fd = 0 +``` + +The `mem` value shows how many files are mmap'ed, and the `fd` value shows you +many file descriptors these files are using. You should check that `fd` is a +small number (usually 0 on 64-bit hosts). + +See the notes in the `SetMaxOpenFiles()` function in `dbwrapper.cc` for more +details. + +### Consensus Compatibility + +It is possible for LevelDB changes to inadvertently change consensus +compatibility between nodes. This happened in Bitcoin 0.8 (when LevelDB was +first introduced). When upgrading LevelDB you should review the upstream changes +to check for issues affecting consensus compatibility. + +For example, if LevelDB had a bug that accidentally prevented a key from being +returned in an edge case, and that bug was fixed upstream, the bug "fix" would +be an incompatible consensus change. In this situation the correct behavior +would be to revert the upstream fix before applying the updates to Bitcoin's +copy of LevelDB. In general you should be wary of any upstream changes affecting +what data is returned from LevelDB queries. Git and GitHub tips --------------------- |