coredump: fix file handle leak in _gen_mem_chunk

Use ExitStack to ensure the conditionally-opened file for
FILE_SHARED/FILE_PRIVATE VMAs is always closed, even if an
exception occurs during seek or page iteration.

Assisted-by: Claude Code (claude-opus-4-6)
Signed-off-by: Adrian Reber <areber@redhat.com>

coredump/criu_coredump/coredump.py

@@ -32,6 +32,7 @@ import io
 import sys
 import ctypes
 import platform
+from contextlib import ExitStack
 
 from pycriu import images
 from . import elf
@@ -872,6 +873,9 @@ class coredump_generator:
             # current process.
             return b"\0" * size
 
+        # ExitStack ensures the file opened below (if any) is closed
+        # when the block exits, even if an exception is raised.
+        with ExitStack() as stack:
             if vma["status"] & status["VMA_FILE_SHARED"] or \
                vma["status"] & status["VMA_FILE_PRIVATE"]:
                 # Open file before iterating vma pages
@@ -879,10 +883,11 @@ class coredump_generator:
                 off = vma["pgoff"]
 
                 files = self.reg_files
-            fname = next(filter(lambda x: x["id"] == shmid, files))["name"]
+                fname = next(
+                    filter(lambda x: x["id"] == shmid, files))["name"]
 
                 try:
-                f = open(fname, 'rb')
+                    f = stack.enter_context(open(fname, 'rb'))
                 except FileNotFoundError:
                     sys.exit('Required file %s not found.' % fname)
 
@@ -947,10 +952,6 @@ class coredump_generator:
 
                 buf += page[n_skip:n_skip + n_read]
 
-        # Don't forget to close file.
-        if f is not None:
-            f.close()
-
             return buf
 
     def _gen_cmdline(self, pid):