fix test suite on openbsd, document openbsd limitations

fixes drycpp#6

README.md

@@ -329,6 +329,19 @@ possible LMDB error conditions:
 * `MDB_KEYEXIST` and `MDB_NOTFOUND` are handled specially by some functions.
 
 
+
+## OpenBSD
+
+OpenBSD is only partially supported by LMDB. The issue is that OpenBSD does not have a unified buffer cache. This means that modifications made to a file through `write()` will not be visible to processes that have memory mapped the file. This is something that [may be fixed some day](http://openbsd-archive.7691.n7.nabble.com/Will-mmap-and-the-read-buffer-cache-be-unified-anyone-working-with-it-td271270.html).
+
+In the mean-time, on OpenBSD you should always open environments with the `MDB_WRITEMAP` flag:
+
+    env.open("/path/to/db/", MDB_WRITEMAP);
+
+Because nested transactions are incompatible with `MDB_WRITEMAP`, they cannot be used on OpenBSD. The test suite disables the nested transaction tests on OpenBSD. 
+
+
+
 ## Support
 
 To report a bug or submit a patch for lmdb++, please file an issue in the [issue tracker on GitHub](https://github.com/hoytech/lmdbxx/issues).

check.cc

@@ -7,12 +7,19 @@
 
 
 int main() {
+  unsigned int envFlags = 0;
+
+#ifdef __OpenBSD__
+  envFlags = MDB_WRITEMAP;
+#endif
+
+
   std::string longLivedValue;
 
   try {
     auto env = lmdb::env::create();
     env.set_max_dbs(64);
-    env.open("testdb/");
+    env.open("testdb/", envFlags);
     lmdb::dbi mydb;
     lmdb::dbi mydbdups;
 
@@ -250,6 +257,8 @@ int main() {
 
     // Nested transactions
 
+    // These tests will fail when WRITEMAP enabled, which must be done on OpenBSD
+#ifndef __OpenBSD__
     {
         auto txn = lmdb::txn::begin(env);
 
@@ -332,6 +341,7 @@ int main() {
 
         if (!caught) throw std::runtime_error("bad nested tx 5");
     }
+#endif
 
 
 
@@ -374,7 +384,7 @@ int main() {
       auto env = lmdb::env::create();
       env.set_max_dbs(64);
       env.set_mapsize(30000);
-      env.open("testdb/");
+      env.open("testdb/", envFlags);
 
       lmdb::dbi mydb;