From c61bb637193a8dc3e31f33ce889a8562bcb2b12f Mon Sep 17 00:00:00 2001
From: PerryLink <255665900+PerryLink@users.noreply.github.com>
Date: Sat, 17 Oct 2026 16:34:20 +0000
Subject: [PATCH] fix: bind max_fds before using it in the process collector

Signed-off-by: PerryLink <255665900+PerryLink@users.noreply.github.com>
---
 prometheus_client/process_collector.py |  5 ++++-
 tests/test_process_collector.py        | 22 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/prometheus_client/process_collector.py b/prometheus_client/process_collector.py
index 2894e874..268634d2 100644
--- a/prometheus_client/process_collector.py
+++ b/prometheus_client/process_collector.py
@@ -79,6 +79,7 @@ def collect(self) -> Iterable[Metric]:
         except OSError:
             pass
 
+        max_fds: Optional[GaugeMetricFamily] = None
         try:
             with open(os.path.join(pid, 'limits'), 'rb') as limits:
                 for line in limits:
@@ -90,7 +91,9 @@ def collect(self) -> Iterable[Metric]:
             open_fds = GaugeMetricFamily(self._prefix + 'open_fds',
                                          'Number of open file descriptors.',
                                          len(os.listdir(os.path.join(pid, 'fd'))))
-            result.extend([open_fds, max_fds])
+            result.append(open_fds)
+            if max_fds is not None:
+                result.append(max_fds)
         except OSError:
             pass
 
diff --git a/tests/test_process_collector.py b/tests/test_process_collector.py
index 81be6a55..4f716a04 100644
--- a/tests/test_process_collector.py
+++ b/tests/test_process_collector.py
@@ -1,4 +1,5 @@
 import os
+import tempfile
 import unittest
 
 from prometheus_client import CollectorRegistry, ProcessCollector
@@ -47,6 +48,27 @@ def test_working_584(self):
         self.assertEqual(None, self.registry.get_sample_value('process_max_fds'))
         self.assertEqual(None, self.registry.get_sample_value('process_open_fds'))
 
+    def test_limits_without_max_open_files(self):
+        # A readable limits file is not guaranteed to contain the
+        # "Max open files" line, see
+        # https://github.com/home-assistant/core/issues/140045.  The collector
+        # used to raise UnboundLocalError (and with it break the import of
+        # prometheus_client), instead of just omitting process_max_fds.
+        with tempfile.TemporaryDirectory() as tmpdir:
+            proc = os.path.join(tmpdir, 'proc')
+            os.makedirs(os.path.join(proc, '123', 'fd'))
+            with open(os.path.join(proc, 'stat'), 'wb') as stat:
+                stat.write(b'btime 1234567890\n')
+            with open(os.path.join(proc, '123', 'limits'), 'wb') as limits:
+                limits.write(b'Limit                     Soft Limit           Hard Limit           Units\n'
+                             b'Max cpu time              unlimited            unlimited            seconds\n'
+                             b'Max open fides            2048                 4096                 files\n')
+            registry = CollectorRegistry(auto_describe=True)
+            ProcessCollector(proc=proc, pid=lambda: 123, registry=registry)
+
+            self.assertEqual(0.0, registry.get_sample_value('process_open_fds'))
+            self.assertEqual(None, registry.get_sample_value('process_max_fds'))
+
     def test_working_fake_pid(self):
         collector = ProcessCollector(proc=self.test_proc, pid=lambda: 123, registry=self.registry)
         collector._ticks = 100