slap/standalone: Increase slapos error log size

The output of slapos commands has been observed to be truncated earlier
than desired in some cases due to long promise messages in output of
slapos node instance and long paths included in the logs.

See merge request nexedi/slapos.core!551

slapos/slap/standalone.py

@@ -942,13 +942,15 @@ class StandaloneSlapOS(object):
             if process_info['exitstatus'] == 0:
               return
             if retry >= max_retry:
-              # get the last lines of output, at most `error_lines`. If
-              # these lines are long, the output may be truncated.
-              _, log_offset, _ = supervisor.tailProcessStdoutLog(command, 0, 0)
-              output, _, _ = supervisor.tailProcessStdoutLog(
-                  command, log_offset - (2 << 13), 2 << 13)
+              # get the last lines of output, at most `error_lines`.
+              getTail = supervisor.tailProcessStdoutLog
+              output, size, overflow = getTail(command, 0, 2 << 15)
+              output_lines = output.splitlines()
+              if overflow:
+                if error_lines == 0 or len(output_lines) < error_lines:
+                  output_lines = getTail(command, 0, size)[0].splitlines()
               raise SlapOSNodeCommandError({
-                  'output': '\n'.join(output.splitlines()[-error_lines:]),
+                  'output': '\n'.join(output_lines[-error_lines:]),
                   'exitstatus': process_info['exitstatus'],
               })
             break

slapos/tests/test_standalone.py

@@ -656,3 +656,35 @@ class TestSlapOSStandaloneInstance(SlapOSStandaloneTestCase):
     self.assertEqual(set([True]), set([p.is_running() for p in process_list]))
     self.stopStandalone()
     self.assertEqual(set([False]), set([p.is_running() for p in process_list]))
+
+
+class TestSlapOSStandaloneLongSoftwareError(SlapOSStandaloneTestCase):
+  def test(self):
+    with tempfile.NamedTemporaryFile(suffix="-%s.cfg" % self.id()) as f:
+      very_long_string = "a" * (2 << 16)
+      software_url = f.name
+      f.write(
+          textwrap.dedent(
+              '''
+              [buildout]
+              parts = fail
+              newest = false
+
+              [fail]
+              recipe = plone.recipe.command==1.1
+              stop-on-error = true
+              command = echo %r && false
+      ''' % very_long_string).encode())
+      f.flush()
+
+      self.standalone.supply(software_url)
+
+      # Not using assertRaises context manager for python2 compatibility
+      try:
+        self.standalone.waitForSoftware()
+      except SlapOSNodeSoftwareError as e:
+        self.assertIn(very_long_string, str(e))
+      except Exception as e:
+        self.fail("waitForSoftware raised unexpected exception %r" % e)
+      else:
+        self.fail("waitForSoftware should have raised")
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