BUG #19634: Hash partition with large MODULUS causes "invalid memory alloc request size"

The following bug has been logged on the website:

Bug reference:      19634
Logged by:          Zheng Hacker
Email address:      hackerzheng666@gmail.com
PostgreSQL version: 19beta3
Operating system:   Linux x86_64
Description:        

Creating a hash partition with MODULUS >= 268435457 causes an internal error
  "invalid memory alloc request size" on any subsequent query against the
  partitioned table. The table becomes permanently unusable — SELECT,
INSERT,
  and all other operations fail with the same error. Only DROP TABLE works.

  Minimal reproducer (tested on PG 20devel commit 609f969, 2026-08-21):

    CREATE TABLE t (id int) PARTITION BY HASH (id);
    CREATE TABLE t_p0 PARTITION OF t FOR VALUES WITH (MODULUS 268435457,
REMAINDER 0);
    SELECT * FROM t;  -- ERROR: invalid memory alloc request size 1073741828

  Root cause:
  In src/backend/partitioning/partbounds.c, create_hash_bounds() (line 390)
  allocates an array indexed by greatest_modulus:

    boundinfo->nindexes = greatest_modulus;
    boundinfo->indexes = palloc_array(int, greatest_modulus);

  When greatest_modulus >= 268435457, this requests 268435457 * 4 =
1073741828
  bytes, exceeding MaxAllocSize (1073741823 = 1GB - 1). No bounds check
exists
  on the modulus value before this allocation.

  The validation in check_new_partition_bound() (line ~2927) runs AFTER
  create_hash_bounds() is called during partition descriptor loading, so it
  never gets a chance to reject the invalid modulus.

  Impact:
  - Affects all versions since hash partitioning was introduced (PG 11+)
  - The partition is created successfully (CREATE TABLE succeeds)
  - But any access to the parent table fails permanently
  - Only DROP TABLE recovers the table
  - Any unprivileged user with CREATE TABLE permission can trigger this

  Suggested fix:
  Add a bounds check in create_hash_bounds() before the allocation, or
  validate modulus against MaxAllocSize in check_new_partition_bound()
  before partition descriptor loading.

  PostgreSQL version: 20devel (commit 609f969)
  OS: Ubuntu 22.04 x86_64