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ircd::db: Additional comments for fixes; minor reorg.

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This commit is contained in:
Jason Volk 2020-05-03 16:34:07 -07:00
parent 77dc612735
commit 05638f9e77
1 changed files with 39 additions and 11 deletions
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50
ircd/db_fixes.cc
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@ -10,17 +10,9 @@
/////////////////////////////////////////////////////////////////////////////////
//
// This unit exists to mitigate instances of bugs in RocksDB documented
// by https://github.com/facebook/rocksdb/issues/4654. In summary, some RocksDB
// code makes direct use of std::mutex and std::condition_variable unlike the
// rest of RocksDB code which uses the rocksdb::port and rocksdb::Env wrapper
// interfaces. We have adapted the latter to work with ircd::ctx userspace
// threading (see: db_port.cc and db_env.cc), but the former is a direct
// interface to kernel pthreads which are incompatible in this context.
//
// Our mitigation is made possible by dynamic linking. It is a legitimate use
// of runtime interposition as stated in official documentation for this exact
// purpose: overriding buggy functions in library dependencies.
// This unit exists to mitigate instances of bugs in RocksDB and its builds.
// It requires a complete copy of the rocksdb sourcecode to operate; though
// said source does not have to be built.
//
#define ROCKSDB_PLATFORM_POSIX
@ -41,6 +33,18 @@ __has_include("table/block_fetcher.h")
#warning "RocksDB source is not available. Cannot interpose bugfixes."
#endif
///////////////////////////////////////////////////////////////////////////////
//
// https://github.com/facebook/rocksdb/issues/4654. In summary, some RocksDB
// code makes direct use of std::mutex and std::condition_variable unlike the
// rest of RocksDB code which uses the rocksdb::port and rocksdb::Env wrapper
// interfaces. We have adapted the latter to work with ircd::ctx userspace
// threading (see: db_port.cc and db_env.cc), but the former is a direct
// interface to kernel pthreads which are incompatible in this context.
//
// Our mitigation is made possible by dynamic linking. It is a legitimate use
// of runtime interposition as stated in official documentation for this exact
// purpose: overriding buggy functions in library dependencies.
// This section overrides a class member function in rocksdb::WriteThread which
// originally made use of pthread primitives to handle two threads contending
// for write access in RocksDB's single-writer design. This function is entered
@ -50,6 +54,7 @@ __has_include("table/block_fetcher.h")
// which tells the kernel to stop the thread until satisfied. Since we are not
// using kernel-driven threads, this is a deadlock.
//
#if defined(IRCD_DB_FIXES_ROCKSDB)
uint8_t
rocksdb::WriteThread::BlockingAwaitState(Writer *const w,
@ -82,6 +87,13 @@ rocksdb::WriteThread::BlockingAwaitState(Writer *const w,
}
#endif
///////////////////////////////////////////////////////////////////////////////
//
// DeleteScheduler unconditionally starts an std::thread (pthread_create)
// rather than using the rocksdb::Env system. We override this function to
// simply not start that thread.
//
#if defined(IRCD_DB_FIXES_ROCKSDB)
rocksdb::DeleteScheduler::DeleteScheduler(Env* env,
int64_t rate_bytes_per_sec,
@ -113,6 +125,12 @@ rocksdb::DeleteScheduler::~DeleteScheduler()
}
#endif
//
// To effectively employ the DeleteScheduler bypass we also interpose the
// function which dispatches deletions to the scheduler to remove the branch
// and directly conduct the deletion.
//
#if defined(IRCD_DB_FIXES_ROCKSDB)
rocksdb::Status
rocksdb::DeleteSSTFile(const ImmutableDBOptions *db_options,
@ -125,6 +143,16 @@ rocksdb::DeleteSSTFile(const ImmutableDBOptions *db_options,
}
#endif
///////////////////////////////////////////////////////////////////////////////
//
// On platforms where hardware crc32 acceleration is not available and for
// use with valgrind, the crc32 checks over the data can be cumbersome. While
// rocksdb offers options in several places to disable checksum checking, these
// options are not honored in several places internally within rocksdb. Thus
// in case a developer wants to manually bypass the checksumming this stub is
// available.
//
#if defined(IRCD_DB_FIXES_ROCKSDB) && defined(IRCD_DB_BYPASS_CHECKSUM)
void
rocksdb::BlockFetcher::CheckBlockChecksum()