Make FileLogWriter, which writes json-formatted log lines to a specif…

…ied file.

paasta_tools/utils.py

@@ -16,9 +16,11 @@
 import contextlib
 import datetime
 import errno
+import fcntl
 import glob
 import hashlib
 import importlib
+import io
 import json
 import logging
 import os
@@ -578,6 +580,50 @@ def log(self, service, line, component, level=DEFAULT_LOGLEVEL, cluster=ANY_CLUS
         pass
 
 
+@register_log_writer('file')
+class FileLogWriter(LogWriter):
+    def __init__(self, path_format, mode='a+', line_delimeter='\n', flock=False):
+        self.path_format = path_format
+        self.mode = mode
+        self.flock = flock
+        self.line_delimeter = line_delimeter
+
+    @contextlib.contextmanager
+    def maybe_flock(self, fd):
+        if self.flock:
+            try:
+                fcntl.flock(fd, fcntl.LOCK_EX)
+                yield
+            finally:
+                fcntl.flock(fd, fcntl.LOCK_UN)
+        else:
+            yield
+
+    def format_path(self, service, component, level, cluster, instance):
+        return self.path_format.format(
+            service=service,
+            component=component,
+            level=level,
+            cluster=cluster,
+            instance=instance,
+        )
+
+    def log(self, service, line, component, level=DEFAULT_LOGLEVEL, cluster=ANY_CLUSTER, instance=ANY_INSTANCE):
+        path = self.format_path(service, component, level, cluster, instance)
+
+        # We use io.FileIO here because it guarantees that write() is implemented with a single write syscall,
+        # and on Linux, writes to O_APPEND files with a single write syscall are atomic.
+        #
+        # https://docs.python.org/2/library/io.html#io.FileIO
+        # http://article.gmane.org/gmane.linux.kernel/43445
+
+        to_write = "%s%s" % (format_log_line(level, cluster, service, instance, component, line), self.line_delimeter)
+
+        with io.FileIO(path, mode=self.mode, closefd=True) as f:
+            with self.maybe_flock(f):
+                f.write(to_write)
+
+
 def _timeout(process):
     """Helper function for _run. It terminates the process.
     Doesn't raise OSError, if we try to terminate a non-existing

tests/test_utils.py

@@ -1229,3 +1229,58 @@ def test_null_log_writer():
     """Basic smoke test for NullLogWriter"""
     lw = utils.NullLogWriter(driver='null')
     lw.log('fake_service', 'fake_line', 'build', 'BOGUS_LEVEL')
+
+
+class TestFileLogWriter:
+    def test_smoke(self):
+        """Smoke test for FileLogWriter"""
+        fw = utils.FileLogWriter('/dev/null')
+        fw.log('fake_service', 'fake_line', 'build', 'BOGUS_LEVEL')
+
+    def test_format_path(self):
+        """Test the path formatting for FileLogWriter"""
+        fw = utils.FileLogWriter("/logs/{service}/{component}/{level}/{cluster}/{instance}")
+        expected = "/logs/a/b/c/d/e"
+        assert expected == fw.format_path("a", "b", "c", "d", "e")
+
+    def test_maybe_flock(self):
+        """Make sure we flock and unflock when flock=True"""
+        with mock.patch("paasta_tools.utils.fcntl") as mock_fcntl:
+            fw = utils.FileLogWriter("/dev/null", flock=True)
+            mock_file = mock.Mock()
+            with fw.maybe_flock(mock_file):
+                mock_fcntl.flock.assert_called_once_with(mock_file, mock_fcntl.LOCK_EX)
+                mock_fcntl.flock.reset_mock()
+
+            mock_fcntl.flock.assert_called_once_with(mock_file, mock_fcntl.LOCK_UN)
+
+    def test_maybe_flock_flock_false(self):
+        """Make sure we don't flock/unflock when flock=False"""
+        with mock.patch("paasta_tools.utils.fcntl") as mock_fcntl:
+            fw = utils.FileLogWriter("/dev/null", flock=False)
+            mock_file = mock.Mock()
+            with fw.maybe_flock(mock_file):
+                assert mock_fcntl.flock.call_count == 0
+
+            assert mock_fcntl.flock.call_count == 0
+
+    def test_log_makes_exactly_one_write_call(self):
+        """We want to make sure that log() makes exactly one call to write, since that's how we ensure atomicity."""
+        fake_file = mock.Mock()
+        fake_contextmgr = mock.Mock(
+            __enter__=lambda _self: fake_file,
+            __exit__=lambda _self, t, v, tb: None
+        )
+
+        fake_line = "text" * 1000000
+
+        with mock.patch("paasta_tools.utils.io.FileIO", return_value=fake_contextmgr, autospec=True) as mock_FileIO:
+            fw = utils.FileLogWriter("/dev/null", flock=False)
+
+            with mock.patch("paasta_tools.utils.format_log_line", return_value=fake_line, autospec=True) as fake_fll:
+                fw.log("service", "line", "component", level="level", cluster="cluster", instance="instance")
+
+            fake_fll.assert_called_once_with("level", "cluster", "service", "instance", "component", "line")
+
+            mock_FileIO.assert_called_once_with("/dev/null", mode=fw.mode, closefd=True)
+            fake_file.write.assert_called_once_with("%s\n" % fake_line)