Index: subversion/include/svn_error_codes.h
===================================================================
--- subversion/include/svn_error_codes.h	(revision 7627)
+++ subversion/include/svn_error_codes.h	(working copy)
@@ -507,6 +507,10 @@
               SVN_ERR_REPOS_CATEGORY_START + 7,
               "Error running post-commit hook")
 
+  SVN_ERRDEF (SVN_ERR_REPOS_LOCK_FAILED,
+              SVN_ERR_REPOS_CATEGORY_START + 8,
+              "Error trying to lock repository")
+
   /* generic ra errors */
 
   SVN_ERRDEF (SVN_ERR_RA_ILLEGAL_URL,
Index: subversion/libsvn_repos/repos.c
===================================================================
--- subversion/libsvn_repos/repos.c	(revision 7627)
+++ subversion/libsvn_repos/repos.c	(working copy)
@@ -1035,7 +1035,6 @@
 }
 
 
-
 const char *
 svn_repos_find_root_path (const char *path,
                           apr_pool_t *pool)
@@ -1055,11 +1054,173 @@
 }
 
 
+/* True on success, False on failure  */
+static svn_boolean_t
+lock_file_cleanup (void * arg)
+{
+  return apr_file_remove ((char *)arg, NULL) ? TRUE : FALSE;
+}
+
+/* True on success, False on failure  */
+static svn_boolean_t
+check_if_lock_file_exists (const char *lock_file_name,
+                           apr_pool_t *pool)
+{
+  apr_file_t *lock_file = NULL;
+
+  apr_file_open (&lock_file,
+                lock_file_name,
+                APR_READ,
+                APR_OS_DEFAULT,
+                pool);
+
+  if (NULL != lock_file)
+  {
+    apr_file_close(lock_file);
+    return TRUE;
+  }
+
+  return FALSE;
+}
+
+/* True on success, False on failure  */
+static svn_boolean_t
+check_if_we_own_lock_file (const char *lock_file_name,
+                           const char *lock_file_contents,
+                           apr_pool_t *pool)
+{
+  apr_size_t lock_file_contents_length = strlen (lock_file_contents);
+
+  /* Try to create our lock file */
+  apr_file_t *lock_file = NULL;
+
+  apr_file_open (&lock_file,
+                 lock_file_name,
+                 APR_READ,
+                 APR_OS_DEFAULT,
+                 pool);
+
+  if (NULL != lock_file)
+  {
+    /* See if we are the person who created the lock file */
+    apr_finfo_t lock_file_info;
+    char *current_lockfile_contents;
+    apr_size_t lock_file_size;
+
+    apr_file_info_get (&lock_file_info, APR_FINFO_SIZE, lock_file);
+    lock_file_size = lock_file_info.size;
+
+    current_lockfile_contents = (char *)apr_pcalloc(pool, lock_file_size + 1);
+    apr_file_read (lock_file, current_lockfile_contents, &lock_file_size);
+
+    apr_file_close (lock_file);
+
+    if (strcmp (current_lockfile_contents, lock_file_contents) == 0)
+      return TRUE;
+  }
+
+  return FALSE;
+}
+
+
+/* True on success, False on failure  */
+static svn_boolean_t
+create_lock_file (const char *lock_file_name,
+                  const char *lock_file_contents,
+                  apr_pool_t *pool)
+{
+  apr_size_t lock_file_contents_length = strlen (lock_file_contents);
+  apr_file_t *lock_file = NULL;
+
+  apr_file_open (&lock_file,
+                 lock_file_name,
+                 APR_READ | APR_WRITE | APR_CREATE | APR_EXCL,
+                 APR_OS_DEFAULT,
+                 pool);
+
+  if (NULL != lock_file)
+  {
+    apr_file_write (lock_file, lock_file_contents, &lock_file_contents_length);
+    apr_file_close (lock_file);
+    return TRUE;
+  }
+
+  return FALSE;
+}
+
+
+static svn_error_t *
+perform_conditional_external_locking (const char *path,
+                                      apr_pool_t *pool)
+{
+  svn_error_t * err = SVN_NO_ERROR;
+  svn_config_t *cfgp;
+  const char * config_lock_file_name = apr_psprintf (pool, "%s/conf/lock.conf", path);
+  const char *valuep;
+
+  SVN_ERR (svn_config_read (&cfgp, config_lock_file_name, FALSE, pool));
+
+  svn_config_get (cfgp, &valuep, "section", "option", SVN_CONFIG_FALSE);
+
+  if (!strcmp (valuep, SVN_CONFIG_TRUE))
+  {
+    const static apr_time_t zero_time = 0;
+    static apr_time_t lock_time = 0;
+
+    char *lock_file_name;
+    char *lock_file_contents;
+    apr_uid_t userid;
+    apr_gid_t groupid;
+    char *username = NULL;
+
+    if (zero_time == lock_time)
+      lock_time = apr_time_now ();
+
+    lock_file_name = apr_psprintf (pool, "%s/svn.lck", path);
+
+    apr_uid_current (&userid, &groupid, pool);
+    apr_uid_name_get (&username, userid, pool);
+
+    /* Generate what should be in the lock file */
+    lock_file_contents = apr_psprintf (pool, "%s/%d", username, lock_time);
+
+    if (check_if_lock_file_exists (lock_file_name, pool))
+    {
+      if (!check_if_we_own_lock_file (lock_file_name, lock_file_contents, pool))
+        return svn_error_createf
+          (SVN_ERR_REPOS_LOCK_FAILED, err,
+           "Repository at '%s' is currently locked by another user", path);
+    }
+    else
+    {
+      if (!create_lock_file (lock_file_name, lock_file_contents, pool))
+        return svn_error_createf
+          (SVN_ERR_REPOS_LOCK_FAILED, err,
+           "Unable to create lock file for repository '%s'", path);
+
+      /* Sleep for two seconds, and then see if the lock file has been corrupted */
+      apr_sleep (2000000);
+
+      if (check_if_we_own_lock_file (lock_file_name, lock_file_contents, pool))
+        apr_pool_cleanup_register (pool, lock_file_name, lock_file_cleanup, NULL);
+      else
+        return svn_error_createf
+          (SVN_ERR_REPOS_LOCK_FAILED, err,
+           "Race condition at repository '%s'! Aborting", path);
+    }
+  }
+
+  return SVN_NO_ERROR;
+}
+
+
 svn_error_t *
 svn_repos_open (svn_repos_t **repos_p,
                 const char *path,
                 apr_pool_t *pool)
 {
+  SVN_ERR ( perform_conditional_external_locking(path, pool));
+
   /* Fetch a repository object initialized with a shared read/write
      lock on the database. */