db_lmdb: test for mmap support at init time

It'll make it clearer when a DB init failure is due to being
on a filesystem which does not support mmap

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -25,6 +25,13 @@
 // STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 // THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+#ifndef _WIN32
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+#endif
+
 #include "db_lmdb.h"
 
 #include <boost/filesystem.hpp>
@@ -1287,6 +1294,26 @@ BlockchainLMDB::BlockchainLMDB(bool batch_transactions): BlockchainDB()
   m_hardfork = nullptr;
 }
 
+void BlockchainLMDB::check_mmap_support()
+{
+#ifndef _WIN32
+  const boost::filesystem::path mmap_test_file = m_folder / boost::filesystem::unique_path();
+  int mmap_test_fd = ::open(mmap_test_file.string().c_str(), O_RDWR | O_CREAT, 0600);
+  if (mmap_test_fd < 0)
+    throw0(DB_ERROR((std::string("Failed to check for mmap support: open failed: ") + strerror(errno)).c_str()));
+  epee::misc_utils::auto_scope_leave_caller scope_exit_handler = epee::misc_utils::create_scope_leave_handler([mmap_test_fd, &mmap_test_file]() {
+    ::close(mmap_test_fd);
+    boost::filesystem::remove(mmap_test_file.string());
+  });
+  if (write(mmap_test_fd, "mmaptest", 8) != 8)
+    throw0(DB_ERROR((std::string("Failed to check for mmap support: write failed: ") + strerror(errno)).c_str()));
+  void *mmap_res = mmap(NULL, 8, PROT_READ, MAP_SHARED, mmap_test_fd, 0);
+  if (mmap_res == MAP_FAILED)
+    throw0(DB_ERROR("This filesystem does not support mmap: use --data-dir to place the blockchain on a filesystem which does"));
+  munmap(mmap_res, 8);
+#endif
+}
+
 void BlockchainLMDB::open(const std::string& filename, const int db_flags)
 {
   int result;
@@ -1328,6 +1355,8 @@ void BlockchainLMDB::open(const std::string& filename, const int db_flags)
 
   m_folder = filename;
 
+  check_mmap_support();
+
 #ifdef __OpenBSD__
   if ((mdb_flags & MDB_WRITEMAP) == 0) {
     MCLOG_RED(el::Level::Info, "global", "Running on OpenBSD: forcing WRITEMAP");

src/blockchain_db/lmdb/db_lmdb.h

@@ -356,6 +356,7 @@ public:
   static int compare_string(const MDB_val *a, const MDB_val *b);
 
 private:
+  void check_mmap_support();
   void do_resize(uint64_t size_increase=0);
 
   bool need_resize(uint64_t threshold_size=0) const;