slapgrid: Fix promise logging with instance python

See merge request nexedi/slapos.core!373

slapos/grid/slapgrid.py

@@ -726,16 +726,21 @@ stderr_logfile_backups=1
         else:
           command.append('--' + option)
           command.append(str(value))
+      # Compute timeout as the sum of all promise timeouts + empirical margin.
       promises = plugins + len(listifdir(legacy_promise_dir))
-      # Add a timeout margin to let the process kill the promises and cleanup
       timeout = promises * self.promise_timeout + 10
       # The runpromise script uses stderr exclusively to propagate exception
       # messages. It otherwise redirects stderr to stdout so that all outputs
       # from the promises go to stdout.
+      def preexec_fn():
+        err = os.dup(2)
+        os.dup2(1, 2)
+        dropPrivileges(uid, gid, logger=self.logger)
+        os.dup2(err, 2)
       try:
         process = SlapPopen(
           command,
-          preexec_fn=lambda: dropPrivileges(uid, gid, logger=self.logger),
+          preexec_fn=preexec_fn,
           cwd=instance_path,
           universal_newlines=True,
           stdout=subprocess.PIPE,
@@ -749,10 +754,10 @@ stderr_logfile_backups=1
         elif process.returncode:
           raise Exception(stderr)
         elif stderr:
-          self.logger.warn('Promise runner unexpected output:\n%s', stderr)
+          self.logger.warn('Unexpected promise runner output:\n%s', stderr)
       except subprocess.TimeoutExpired:
         killProcessTree(process.pid, self.logger)
-        # The timeout margin was exceeded but this should be infrequent
+        # If this happens, it might be that the timeout margin is too small.
         raise Exception('Promise runner timed out')
 
     else:

slapos/grid/utils.py

@@ -299,7 +299,7 @@ def dropPrivileges(uid, gid, logger):
   else:
     raise ValueError('%s it was possible to go back to uid = %r and gid = '
                      '%r which is fatal.' % (message_pre, current_uid, current_gid))
-  logger.debug('Succesfully dropped privileges to uid=%r gid=%r' % (uid, gid))
+  logger.debug('Successfully dropped privileges to uid=%r gid=%r' % (uid, gid))
 
 
 def bootstrapBuildout(path, logger, buildout=None,