Re: Proposal: tighten validation for legacy EUC encodings or document that accepted byte sequences may be unconvertible to UTF8

On 02.05.26 04:31, Zhongpu Chen wrote:
> See the related bug report https://www.postgresql.org/message-id/
> CA%2B1gyqL7uiQhfLcYWpHNUKQgHjQc7sOPthSTiaxLDZzcrGFYSg%40mail.gmail.com
> <https://www.postgresql.org/message-id/
> CA%2B1gyqL7uiQhfLcYWpHNUKQgHjQc7sOPthSTiaxLDZzcrGFYSg%40mail.gmail.com>
>
> Currently PostgreSQL accepts structurally well-formed EUC_CN byte
> sequences such as 0xA2A3 into text columns. The value round-trips when
> client_encoding is EUC_CN, but fails when client_encoding is UTF8
> because euc_cn_to_utf8 has no mapping.
>
> If this behavior is intentional for compatibility, the documentation
> should explicitly say that validation for some legacy encodings is byte-
> structure validation, not mapping-table validation.
> If it is not intentional, stricter validation could reject unassigned
> byte positions at input time.

It is in general not necessarily required that all text in all non-UTF8
encodings must be convertible to UTF8.

(This is also a result of history: These encodings were implemented in
PostgreSQL before Unicode.)

That said, I can see how different behaviors might be desirable.

My first question would be, are these non-convertible byte sequences
just characters that don't map to Unicode, or are they invalid within
the definition of the EUC-* encodings themselves? If the latter, then
we should just reject them (modulo some backward compatibility), similar
to how we reject certain Unicode code points that exist "structurally"
but are not valid for one reason or another.

Alternatively, if these byte sequences are valid characters but they
just didn't end up in Unicode for some reason, then rejecting them might
break valid uses.

(I don't know much about EUC-* to be able to answer these.)