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Change type "char"'s I/O format for non-ASCII characters.

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Previously, a byte with the high bit set was just transmitted
as-is by charin() and charout().  This is problematic if the
database encoding is multibyte, because the result of charout()
won't be validly encoded, which breaks various stuff that
expects all text strings to be validly encoded.  We've
previously decided to enforce encoding validity rather than try
to individually harden each place that might have a problem with
such strings, so it's time to do something about "char".

To fix, represent high-bit-set characters as \ooo (backslash
and three octal digits), following the ancient "escape" format
for bytea.  charin() will continue to accept the old way as well,
though that is only reachable in single-byte encodings.

Add some test cases just so there is coverage for this code.
We'll otherwise leave this question undocumented as it was before,
because we don't really want to encourage end-user use of "char".

For the moment, back-patch into v15 so that this change appears
in 15beta3.  If there's not great pushback we should consider
absorbing this change into the older branches.

Discussion: https://postgr.es/m/2318797.1638558730@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2022-08-02 10:29:35 -04:00
parent 5b94d3ccb7
commit c034b629cc
6 changed files with 263 additions and 28 deletions
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10
doc/src/sgml/datatype.sgml
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@ -1338,9 +1338,10 @@ SELECT b, char_length(b) FROM test2;
<para>
There are two other fixed-length character types in
<productname>PostgreSQL</productname>, shown in <xref
linkend="datatype-character-special-table"/>. The <type>name</type>
type exists <emphasis>only</emphasis> for the storage of identifiers
in the internal system catalogs and is not intended for use by the general user. Its
linkend="datatype-character-special-table"/>.
These are not intended for general-purpose use, only for use
in the internal system catalogs.
The <type>name</type> type is used to store identifiers. Its
length is currently defined as 64 bytes (63 usable characters plus
terminator) but should be referenced using the constant
<symbol>NAMEDATALEN</symbol> in <literal>C</literal> source code.
@ -1348,7 +1349,8 @@ SELECT b, char_length(b) FROM test2;
is therefore adjustable for special uses); the default maximum
length might change in a future release. The type <type>"char"</type>
(note the quotes) is different from <type>char(1)</type> in that it
only uses one byte of storage. It is internally used in the system
only uses one byte of storage, and therefore can store only a single
ASCII character. It is used in the system
catalogs as a simplistic enumeration type.
</para>