Invalidate Linux page cache before mmap to prevent stale reads after ZFS rollback

After a ZFS/LVM snapshot rollback, the Linux page cache may retain
pre-rollback data for mmap'd files. When chainbase opens shared_memory.bin
via boost::interprocess::managed_mapped_file, it can read stale LIB values
from the page cache instead of the rolled-back data on disk. Meanwhile,
RocksDB uses pread() and reads correct post-rollback data, causing a
LIB mismatch assertion failure on startup.

Add posix_fadvise(POSIX_FADV_DONTNEED) before the mmap open to evict
any cached pages, ensuring fresh data is read from disk. The call has
negligible overhead (<0.01ms for a 7GB file).

Also improve the RocksDB LIB mismatch error message to explain the
likely cause and suggest remediation steps.

Author: dnotestein

libraries/chain/external_storage/rocksdb_storage_provider.cpp

@@ -312,7 +312,11 @@ void rocksdb_storage_provider::load_lib()
 
   uint32_t lib = lib_slice_t::unpackSlice( _value );
 
-  FC_ASSERT( lib == _cached_irreversible_block, "Inconsistency in last irreversible block - expected ${c}, stored ${s}",
+  FC_ASSERT( lib == _cached_irreversible_block,
+    "Inconsistency in last irreversible block - expected ${c} (from shared_memory), stored ${s} (from RocksDB). "
+    "If this occurs after a ZFS/LVM snapshot rollback, it may be caused by stale data in the Linux page cache. "
+    "Try: echo 3 > /proc/sys/vm/drop_caches, then restart. "
+    "Or use the rollback_zfs_datasets.sh script which handles cache invalidation automatically.",
     ( "c", static_cast< uint32_t >( _cached_irreversible_block ) )( "s", lib ) );
 
   ilog( "`${name}` RocksDB database LIB loaded with value ${l}.", ("name", name)( "l", lib ) );

libraries/chainbase/src/chainbase.cpp

@@ -7,6 +7,11 @@
 
 #include <filesystem>
 
+#ifdef __linux__
+#include <fcntl.h>
+#include <unistd.h>
+#endif
+
 namespace chainbase {
 
 size_t snapshot_base_serializer::worker_common_base::get_serialized_object_cache_max_size() const
@@ -225,6 +230,21 @@ size_t snapshot_base_serializer::worker_common_base::get_serialized_object_cache
         _file_size = shared_file_size;
       }
 
+#ifdef __linux__
+      // Evict any stale pages from the Linux page cache before mmap.
+      // After a ZFS/LVM snapshot rollback, the page cache may still contain
+      // pre-rollback data. Without this, mmap can return stale pages instead
+      // of reading the rolled-back data from disk.
+      {
+        int fd = ::open( abs_path.generic_string().c_str(), O_RDONLY );
+        if( fd >= 0 )
+        {
+          posix_fadvise( fd, 0, 0, POSIX_FADV_DONTNEED );
+          ::close( fd );
+        }
+      }
+#endif
+
       _segment.reset( new bip::managed_mapped_file( bip::open_only,
                                       abs_path.generic_string().c_str()
                                       ) );