Error in calculating length of encoded base64 string

Greetings, everyone!

While working on an extension I've found an error in how length of 
encoded base64 string is calulated;

This error is present in 3 files across all supported versions:

/src/common/base64.c, function pg_b64_enc_len;
/src/backend/utils/adt/encode.c, function pg_base64_enc_len;
/contrib/pgcrypto/pgp-armor.c, function pg_base64_enc_len (copied from 
encode.c).

In all three cases the length is calculated as follows:

(srclen + 2) * 4 / 3; (plus linefeed in latter two cases)

There's also a comment /* 3 bytes will be converted to 4 */

This formula is wrong. Let's calculate encoded length for different 
starting lengths:

starting length 2: (2 + 2) * 4 / 3 = 5,
starting length 3: (3 + 2) * 4 / 3 = 6,
starting length 4: (4 + 2) * 4 / 3 = 8,
starting length 6: (6 + 2) * 4 / 3 = 10,
starting length 10: (10 + 2) * 4 / 3 = 16,

when it should be 4, 4, 8, 8, 16.

So the suggestion is to change the formula to a right one: (srclen + 2) 
/ 3 * 4;

The patch is attached.

Oleg Tselebrovskiy, Postgres Pro