util.datamanager: Write to a temporary file and atomically move it into place

diff --git a/util/datamanager.lua b/util/datamanager.lua
index 29a5b994cfc413f512c61cbee181d33e16546e3a..5d3074a322883774e6e66f24df7b6bf043975be2 100644 (file)
--- a/util/datamanager.lua
+++ b/util/datamanager.lua
@@ -15,11 +15,12 @@ local pcall = pcall;
 local log = require "util.logger".init("datamanager");
 local io_open = io.open;
 local os_remove = os.remove;
+local os_rename = os.rename;
 local tostring, tonumber = tostring, tonumber;
 local error = error;
 local next = next;
 local t_insert = table.insert;
-local append = require "util.serialization".append;
+local t_concat = table.concat;
 local envloadfile = require"util.envload".envloadfile;
 local serialize = require "util.serialization".serialize;
 local path_separator = assert ( package.config:match ( "^([^\n]+)" ) , "package.config not in standard form" ) -- Extract directory seperator from package.config (an undocumented string that comes with lua)
@@ -149,6 +150,28 @@ function load(username, host, datastore)
        return ret;
 end
 
+local function atomic_store(filename, data)
+       local scratch = filename.."~";
+       local f, ok, msg;
+       repeat
+               f, msg = io_open(scratch, "w");
+               if not f then break end
+
+               ok, msg = f:write(data);
+               if not ok then break end
+
+               ok, msg = f:close();
+               if not ok then break end
+
+               return os_rename(scratch, filename);
+       until false;
+
+       -- Cleanup
+       if f then f:close(); end
+       os_remove(scratch);
+       return nil, msg;
+end
+
 function store(username, host, datastore, data)
        if not data then
                data = {};
@@ -160,14 +183,12 @@ function store(username, host, datastore, data)
        end
 
        -- save the datastore
-       local f, msg = io_open(getpath(username, host, datastore, nil, true), "w+");
-       if not f then
+       local d = "return " .. serialize(data) .. ";\n";
+       local ok, msg = atomic_store(getpath(username, host, datastore, nil, true), d);
+       if not ok then
                log("error", "Unable to write to %s storage ('%s') for user: %s@%s", datastore, msg, username or "nil", host or "nil");
                return nil, "Error saving to storage";
        end
-       f:write("return ");
-       append(f, data);
-       f:close();
        if next(data) == nil then -- try to delete empty datastore
                log("debug", "Removing empty %s datastore for user %s@%s", datastore, username or "nil", host or "nil");
                os_remove(getpath(username, host, datastore));
@@ -206,17 +227,15 @@ function list_store(username, host, datastore, data)
        end
        if callback(username, host, datastore) == false then return true; end
        -- save the datastore
-       local f, msg = io_open(getpath(username, host, datastore, "list", true), "w+");
-       if not f then
+       local d = {};
+       for _, item in ipairs(data) do
+               d[#d+1] = "item(" .. serialize(item) .. ");\n";
+       end
+       local ok, msg = atomic_store(getpath(username, host, datastore, "list", true), t_concat(d));
+       if not ok then
                log("error", "Unable to write to %s storage ('%s') for user: %s@%s", datastore, msg, username or "nil", host or "nil");
                return;
        end
-       for _, d in ipairs(data) do
-               f:write("item(");
-               append(f, d);
-               f:write(");\n");
-       end
-       f:close();
        if next(data) == nil then -- try to delete empty datastore
                log("debug", "Removing empty %s datastore for user %s@%s", datastore, username or "nil", host or "nil");
                os_remove(getpath(username, host, datastore, "list"));