Add ALLOW_LMDB_NOLOCK build option

This option lets you open LMDB files with the MDB_NOLOCK flag. You
should not set this flag if you will be reading LMDBs with any
possibility of simultaneous read and write.

CMakeLists.txt

@@ -26,6 +26,7 @@ caffe_option(BUILD_python_layer "Build the Caffe Python layer" ON)
 caffe_option(USE_OPENCV "Build with OpenCV support" ON)
 caffe_option(USE_LEVELDB "Build with levelDB" ON)
 caffe_option(USE_LMDB "Build with lmdb" ON)
+caffe_option(ALLOW_LMDB_NOLOCK "Allow MDB_NOLOCK when reading LMDB files (only if necessary)" OFF)
 
 # ---[ Dependencies
 include(cmake/Dependencies.cmake)

Makefile

@@ -313,6 +313,9 @@ ifeq ($(USE_LEVELDB), 1)
 endif
 ifeq ($(USE_LMDB), 1)
 	COMMON_FLAGS += -DUSE_LMDB
+ifeq ($(ALLOW_LMDB_NOLOCK), 1)
+	COMMON_FLAGS += -DALLOW_LMDB_NOLOCK
+endif
 endif
 
 # CPU-only configuration

Makefile.config.example

@@ -12,6 +12,11 @@
 # USE_LEVELDB := 0
 # USE_LMDB := 0
 
+# uncomment to allow MDB_NOLOCK when reading LMDB files (only if necessary)
+#	You should not set this flag if you will be reading LMDBs with any
+#	possibility of simultaneous read and write
+# ALLOW_LMDB_NOLOCK := 1
+
 # To customize your choice of compiler, uncomment and set the following.
 # N.B. the default for Linux is g++ and the default for OSX is clang++
 # CUSTOM_CXX := g++

cmake/ConfigGen.cmake

@@ -62,6 +62,9 @@ function(caffe_generate_export_configs)
 
   if(USE_LMDB)
     list(APPEND Caffe_DEFINITIONS -DUSE_LMDB)
+    if (ALLOW_LMDB_NOLOCK)
+        list(APPEND Caffe_DEFINITIONS -DALLOW_LMDB_NOLOCK)
+    endif()
   endif()
 
   if(USE_LEVELDB)

cmake/Dependencies.cmake

@@ -34,6 +34,9 @@ if(USE_LMDB)
   include_directories(SYSTEM ${LMDB_INCLUDE_DIR})
   list(APPEND Caffe_LINKER_LIBS ${LMDB_LIBRARIES})
   add_definitions(-DUSE_LMDB)
+  if(ALLOW_LMDB_NOLOCK)
+    add_definitions(-DALLOW_LMDB_NOLOCK)
+  endif()
 endif()
 
 # ---[ LevelDB

cmake/Summary.cmake

@@ -117,6 +117,7 @@ function(caffe_print_configuration_summary)
   caffe_status("  USE_OPENCV        :   ${USE_OPENCV}")
   caffe_status("  USE_LEVELDB       :   ${USE_LEVELDB}")
   caffe_status("  USE_LMDB          :   ${USE_LMDB}")
+  caffe_status("  ALLOW_LMDB_NOLOCK :   ${ALLOW_LMDB_NOLOCK}")
   caffe_status("")
   caffe_status("Dependencies:")
   caffe_status("  BLAS              : " APPLE THEN "Yes (vecLib)" ELSE "Yes (${BLAS})")

cmake/Templates/caffe_config.h.in

@@ -35,3 +35,4 @@
 #cmakedefine USE_OPENCV
 #cmakedefine USE_LEVELDB
 #cmakedefine USE_LMDB
+#cmakedefine ALLOW_LMDB_NOLOCK

src/caffe/util/db_lmdb.cpp

@@ -19,7 +19,22 @@ void LMDB::Open(const string& source, Mode mode) {
   if (mode == READ) {
     flags = MDB_RDONLY | MDB_NOTLS;
   }
-  MDB_CHECK(mdb_env_open(mdb_env_, source.c_str(), flags, 0664));
+  int rc = mdb_env_open(mdb_env_, source.c_str(), flags, 0664);
+#ifndef ALLOW_LMDB_NOLOCK
+  MDB_CHECK(rc);
+#else
+  if (rc == EACCES) {
+    LOG(WARNING) << "Permission denied. Trying with MDB_NOLOCK ...";
+    // Close and re-open environment handle
+    mdb_env_close(mdb_env_);
+    MDB_CHECK(mdb_env_create(&mdb_env_));
+    // Try again with MDB_NOLOCK
+    flags |= MDB_NOLOCK;
+    MDB_CHECK(mdb_env_open(mdb_env_, source.c_str(), flags, 0664));
+  } else {
+    MDB_CHECK(rc);
+  }
+#endif
   LOG(INFO) << "Opened lmdb " << source;
 }