ITS#8505 Clarify fork() caveat, mdb_env_get_fd(), flock->fcntl.

libraries/liblmdb/lmdb.h

@@ -96,11 +96,12 @@
  *	  transactions.  Each transaction belongs to one thread.  See below.
  *	  The #MDB_NOTLS flag changes this for read-only transactions.
  *
- *	- Use an MDB_env* in the process which opened it, without fork()ing.
+ *	- Use an MDB_env* in the process which opened it, not after fork().
  *
  *	- Do not have open an LMDB database twice in the same process at
  *	  the same time.  Not even from a plain open() call - close()ing it
- *	  breaks flock() advisory locking.
+ *	  breaks fcntl() advisory locking.  (It is OK to reopen it after
+ *	  fork() - exec*(), since the lockfile has FD_CLOEXEC set.)
  *
  *	- Avoid long-lived transactions.  Read transactions prevent
  *	  reuse of pages freed by newer write transactions, thus the
@@ -794,6 +795,10 @@ int  mdb_env_get_flags(MDB_env *env, unsigned int *flags);
 int  mdb_env_get_path(MDB_env *env, const char **path);
 
 	/** @brief Return the filedescriptor for the given environment.
+	 *
+	 * This function may be called after fork(), so the descriptor can be
+	 * closed before exec*().  Other LMDB file descriptors have FD_CLOEXEC.
+	 * (Until LMDB 0.9.18, only the lockfile had that.)
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[out] fd Address of a mdb_filehandle_t to contain the descriptor.