gh-143241: Fix infinite loop in zoneinfo._common.load_data (#143243)

Correctly reject truncated TZif files in `ZoneInfo.from_file`.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Doc/library/zoneinfo.rst

@@ -206,6 +206,9 @@ The ``ZoneInfo`` class has two alternate constructors:
 
     Objects created via this constructor cannot be pickled (see `pickling`_).
 
+    :exc:`ValueError` is raised if the data read from *file_obj* is not a valid
+    TZif file.
+
 .. classmethod:: ZoneInfo.no_cache(key)
 
     An alternate constructor that bypasses the constructor's cache. It is

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -252,6 +252,8 @@ class ZoneInfoTest(TzPathUserMixin, ZoneInfoTestBase):
         bad_zones = [
             b"",  # Empty file
             b"AAAA3" + b" " * 15,  # Bad magic
+            # Truncated V2 file (should not loop indefinitely)
+            b"TZif2" + (b"\x00" * 39) + b"TZif2" + (b"\x00" * 39) + b"\n" + b"Part",
         ]
 
         for bad_zone in bad_zones:

Lib/zoneinfo/_common.py

@@ -118,11 +118,10 @@ def load_data(fobj):
         c = fobj.read(1)  # Should be \n
         assert c == b"\n", c
 
-        tz_bytes = b""
-        while (c := fobj.read(1)) != b"\n":
-            tz_bytes += c
-
-        tz_str = tz_bytes
+        line = fobj.readline()
+        if not line.endswith(b"\n"):
+            raise ValueError("Invalid TZif file: unexpected end of file")
+        tz_str = line.rstrip(b"\n")
     else:
         tz_str = None
 

Misc/NEWS.d/next/Library/2025-12-28-13-49-06.gh-issue-143241.5H4b8d.rst

@@ -0,0 +1,2 @@
+:mod:`zoneinfo`: fix infinite loop in :meth:`ZoneInfo.from_file
+<zoneinfo.ZoneInfo.from_file>` when parsing a malformed TZif file. Patch by Fatih Celik.