[Bug locale/28255] New: A locale with zero collation rules cause fnmatch, regexec, and regcomp failures.

["carlos at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28255

            Bug ID: 28255
           Summary: A locale with zero collation rules cause fnmatch,
                    regexec, and regcomp failures.
           Product: glibc
           Version: 2.35
            Status: NEW
          Severity: normal
          Priority: P2
         Component: locale
          Assignee: unassigned at sourceware dot org
          Reporter: carlos at redhat dot com
  Target Milestone: ---

It should be possible to build a locale with zero collation rules. Such a
locale would fall back to using strcmp and wcscmp during collation.

Such a collation, one with zero collation rules, would also lack any data for
_NL_COLLATE_COLLSEQMB, and _NL_COLLATE_COLLSEQWC. Any code using these tables
must be gated on nrules != 0.

However, as-of today, the only such locale in glibc was the builtin C locale,
and even though it has 0 collation rules, it has identity tables for
_NL_COLLATE_COLLSEQMB, and _NL_COLLATE_COLLSEQWC.

These identity tables are used unconditionally by fnmatch, regexec, and
regcomp, which means that when you don't generate them, the code does not
properly handle regular expressions.

The fix for this is quite extensive, and I didn't want to include it in glibc
2.34, but the fixes are present here in v4 of my C.UTF-8 patch series:
https://patchwork.sourceware.org/project/glibc/patch/20210729063515.1541388-2-carlos@redhat.com/

The fix is also insufficient because statically linked applications would not
have the fixes, so we must still keep the identity tables to allow static
applications to continue working with the new binary locales (until we manage
to make C.UTF-8 builtin).

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