From f7ed2c69f3840f54bd3e54973821b308a998b063 Mon Sep 17 00:00:00 2001
From: Lars Wirzenius <liw@liw.fi>
Date: Mon, 25 Mar 2019 09:19:01 +0200
Subject: Add: initial commit

---
 debuglog.py | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 debuglog.py

(limited to 'debuglog.py')

diff --git a/debuglog.py b/debuglog.py
new file mode 100644
index 0000000..7b34270
--- /dev/null
+++ b/debuglog.py
@@ -0,0 +1,77 @@
+import collections
+import json
+
+
+class DebugLog(object):
+
+    '''Debug logging class.
+
+    The Python standard library contains the library called logging,
+    which is fine for general purpose logging. For logs meant to help
+    developers analyse problems, they are less than ideal.
+
+    * Log files are essentially free-form text, with only minimal
+      structure (timestamps), and that is unreliable to parse when
+      log messages may contain newlines.
+    * It is not very efficient.
+    * Typically, a log grows quite large, making it use a lot of disk
+      space. This is wasteful when most developers would only need
+      the end of the log, at the point of a crash, to solve most problems.
+      (But sometimes the whole log from the whole runtime would be good.)
+
+    This class provides a different kind of logging. The log file is only
+    written when requested, not continously, and is stored in a ring buffer,
+    meaning only the last messages are written, with earlier ones forgotten.
+    This is fine for crash logs. The logs are written in a format that is
+    easily machine parseable (JSON).
+
+    '''
+
+    DEFAULT_RING_BUFFER_SIZE = 1000
+
+    def __init__(self):
+        self._max_buffer_size = self.DEFAULT_RING_BUFFER_SIZE
+        self.clear()
+
+    def set_ring_buffer_size(self, num_entries):
+        self._max_buffer_size = num_entries
+        while len(self._ring_buffer) > self._max_buffer_size:
+            self._ring_buffer.popleft()
+
+    def add_to_preamble(self, **kwargs):
+        self._preamble.append(kwargs)
+
+    def log(self, **kwargs):
+        self._ring_buffer.append(kwargs)
+
+    def write_ring_buffer(self, filename_or_handle):
+
+        def write(f):
+            f.write('[\n')
+            first = True
+            for q in [self._preamble, self._ring_buffer]:
+                for entry in q:
+                    if first:
+                        first = False
+                    else:
+                        f.write(',\n')
+                    json.dump(entry, f, indent=4)
+            f.write('\n]\n')
+
+        if hasattr(filename_or_handle, 'write'):
+            write(filename_or_handle)
+        else:
+            with open(filename_or_handle, 'a') as f:
+                write(f)
+
+    def clear(self):
+        self._preamble = collections.deque([], self._max_buffer_size)
+        self._ring_buffer = collections.deque([], self._max_buffer_size)
+
+
+if __name__ == '__main__':
+    x = DebugLog()
+    x.add_to_preamble(version='1.2')
+    x.log(msg='yoyoyo', url='foobar')
+    x.log(msg='blahblah', data=123)
+    x.write_ring_buffer('/dev/stdout')
-- 
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