mitogen: Fix unclosed file in first stage

docs/changelog.rst

@@ -24,6 +24,7 @@ In progress (unreleased)
 * :gh:issue:`1213` tests: Enable default Python warnings
 * :gh:issue:`1111` :mod:`mitogen`: Replace uses of deprecated
   :py:func:`pkgutil.find_loader`
+* :gh:issue:`1213` :mod:`mitogen`: Fix unclosed file in first stage
 
 
 v0.3.21 (2025-01-20)

mitogen/parent.py

@@ -1429,7 +1429,9 @@ class Connection(object):
             os.environ['ARGV0']=sys.executable
             os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
         os.write(1,'MITO000\n'.encode())
-        C=zlib.decompress(os.fdopen(0,'rb').read(PREAMBLE_COMPRESSED_LEN))
+        fp=os.fdopen(0,'rb')
+        C=zlib.decompress(fp.read(PREAMBLE_COMPRESSED_LEN))
+        fp.close()
         fp=os.fdopen(W,'wb',0)
         fp.write(C)
         fp.close()