gh-158181: tarfile: raise ReadError instead of IndexError on a truncated GNU sparse header

[not-ekalabya]

• Issue: tarfile: IndexError instead of ReadError on a truncated GNU sparse member's extended header #158181

Description

Fixes #158181.

_proc_sparse() reads the extended header block for a GNU
GNUTYPE_SPARSE member with a plain tarfile.fileobj.read(BLOCKSIZE),
without checking for a short or empty result the way most other truncated-
data reads in this module do (see copyfileobj's len(buf) < bufsize
check for the same pattern). A truncated archive whose sparse member has
isextended set makes the parse loop call nti() on a short/empty buffer,
and nti() raises IndexError (s[0] on empty input) rather than the
documented tarfile.ReadError, escaping every except/try in
next()/TarFile.__init__ that's written against the documented contract.

This raises ReadError("unexpected end of data") directly when the read is
short, matching the exact pattern already used at
copy_or_raise/copyfileobj and several other call sites in this module.

Found via

Found via an automated code-review pass using GLM-5.3-flash paired with
scopegrep, a semantic code-retrieval tool; independently re-verified and
fixed by hand.

Testing

I don't have a build of this checkout's CPython (this environment's
installed interpreter is 3.12, and main's stdlib already uses syntax/
builtins 3.12 can't parse -- e.g. frozendict), so I could not run
Lib/test/test_tarfile.py directly. I verified the fix with a standalone
harness that loads Lib/tarfile.py via importlib against the system's
already-imported re/shutil (to sidestep the _sre version mismatch),
with a minimal shim for the one new builtin the module needs at import
time:

• Before the fix, a hand-built truncated GNU sparse ('S' typeflag,
  isextended=1, no extended header block following) raised
  IndexError: index out of range from nti(), reproducing the report.
• After the fix, the same input raises
  tarfile.ReadError: unexpected end of data.

Please run the real test suite (particularly Lib/test/test_tarfile.py's
sparse-file tests) as part of review, since I could not do so myself here.

Drafted with Claude Sonnet 5 (Anthropic); reviewed by @not-ekalabya before merge.

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