slapos.grid.promise: prevent json file corruption while updating data

history and statistics files are sometimes corrupted because of invalid json
We now use json.dump to write the entire file at once. Probably this will consume
more resources.

slapos/grid/promise/__init__.py

@@ -41,6 +41,7 @@ import traceback
 import psutil
 import inspect
 import hashlib
+import errno
 from datetime import datetime
 from multiprocessing import Process, Queue as MQueue
 from six.moves import queue, reload_module
@@ -458,12 +459,20 @@ class PromiseLauncher(object):
     state_dict.pop('path', '')
     state_dict.pop('type', '')
     state_dict["status"] = "ERROR" if result.item.hasFailed() else "OK"
+    history_dict = {
+      "date": time.time(),
+      "data": [state_dict]
+    }
 
-    if not os.path.exists(history_file) or not os.stat(history_file).st_size:
-      with open(history_file, 'w') as f:
-        f.write("""{
-          "date": %s,
-          "data": %s}""" % (time.time(), json.dumps([state_dict])))
+    try:
+      with open(history_file) as f:
+        history_dict = json.load(f)
+    except OSError as e:
+      if e.errno != errno.ENOENT:
+        raise
+    except ValueError:
+      # Broken json, use default history_dict
+      pass
     else:
       previous_state_list = previous_state_dict.get(result.name, None)
       if previous_state_list is not None:
@@ -473,36 +482,38 @@ class PromiseLauncher(object):
             current_sum == checksum:
           # Only save the changes and not the same info
           return
-  
+
       state_dict.pop('title', '')
       state_dict.pop('name', '')
-      with open (history_file, mode="r+") as f:
-        f.seek(0,2)
-        f.seek(f.tell() -2)
-        f.write('%s}' % ',{}]'.format(json.dumps(state_dict)))
+      history_dict['data'].append(state_dict)
+
+    with open (history_file, mode="w") as f:
+      json.dump(history_dict, f)
 
   def _saveStatisticsData(self, stat_file_path, date, success, error):
     # csv-like document for success/error statictics
-    if not os.path.exists(stat_file_path) or os.stat(stat_file_path).st_size == 0:
-      with open(stat_file_path, 'w') as fstat:
-        fstat.write("""{
-          "date": %s,
-          "data": ["Date, Success, Error, Warning"]}""" % time.time())
-  
+    stat_dict = {
+      "date": time.time(),
+      "data": ["Date, Success, Error, Warning"]
+    }
+    try:
+      with open(stat_file_path) as f:
+        stat_dict = json.load(f)
+    except OSError as e:
+      if e.errno != errno.ENOENT:
+        raise
+    except ValueError:
+      # Broken json, we use default
+      pass
+
     current_state = '%s, %s, %s, %s' % (
         date,
         success,
         error,
         '')
-  
-    # append to file
-    # XXX this is bad, it is getting everywhere.
-    if current_state:
-      with open (stat_file_path, mode="r+") as fstat:
-        fstat.seek(0,2)
-        position = fstat.tell() -2
-        fstat.seek(position)
-        fstat.write('%s}' % ',"{}"]'.format(current_state))
+    stat_dict['data'].append(current_state)
+    with open (stat_file_path, mode="w") as f:
+      json.dump(stat_dict, f)
 
   def _loadPromiseResult(self, promise_title):
     promise_output_file = os.path.join(