BUG #12664: numeric_recv does not accept large values

The following bug has been logged on the website:

Bug reference: 12664
Logged by: Emil Lenngren
Email address: emil(dot)lenngren(at)gmail(dot)com
PostgreSQL version: 9.4.0
Operating system: N/A
Description:

According to the documentation for data type "numeric" at
http://www.postgresql.org/docs/9.4/static/datatype-numeric.html:
"up to 131072 digits before the decimal point; up to 16383 digits after the
decimal point"
"Note: The maximum allowed precision when explicitly specified in the type
declaration is 1000; NUMERIC without a specified precision is subject to the
limits described in Table 8-2."

In the binary representation, digits are grouped into groups of 4. The
number of such groups is checked in numeric_recv:

len = (uint16) pq_getmsgint(buf, sizeof(uint16));
if (len < 0 || len > NUMERIC_MAX_PRECISION + NUMERIC_MAX_RESULT_SCALE)
ereport(ERROR,
(errcode(ERRCODE_INVALID_BINARY_REPRESENTATION),
errmsg("invalid length in external \"numeric\" value")));

but NUMERIC_MAX_PRECISION + NUMERIC_MAX_RESULT_SCALE is 3000 which means a
limit of only 12000 digits. numeric_in has no such limit.

The guard should check something like (len > ceil((131072+16383)/4))
instead, or not at all, since the function make_result also checks the
length. Checking for len < 0 also seems a bit needless since len already
contains an unsigned 16-bit value.