Describe the bug

Decoding certain valid PostgreSQL NUMERIC values in binary format can hang or do excessive work (and infinite loop). Inputs with very large weight and small dscale trigger:

• int16 overflow in an intermediate calculation
• huge loops rescaling by powers of 10
• a possible infinite loop when the accumulated integer becomes zero

Root cause: the decoder attempts to rescale to the wire dscale and uses int16 math. PostgreSQL’s numeric_recv does not rescale to dscale (it’s display-only).

To Reproduce

Steps:

1. Use pgtype to decode the following binary NUMERIC payload.
2. It represents ndigits=2, weight=11134, sign=0, dscale=1, digits=[6,7000].

Runnable example (no DB needed):

package main

import (
    "encoding/base64"
    "fmt"
    "time"

    "github.com/jackc/pgx/v5"
    "github.com/jackc/pgx/v5/pgtype"
)

func main() {
    const b64 = "AAIrfgAAAAEABhtY" // ndigits=2, weight=11134, sign=0, dscale=1, digits=[6,7000]
    payload, _ := base64.StdEncoding.DecodeString(b64)

    m := pgtype.NewMap()
    var n pgtype.Numeric

    done := make(chan error, 1)
    go func() { done <- m.Scan(pgtype.NumericOID, pgx.BinaryFormatCode, payload, &n) }()

    select {
    case err := <-done:
        fmt.Println("done", err, n.Valid)
    case <-time.After(5000 * time.Millisecond):
        fmt.Println("hang / excessive work")
    }
}

Please run with the race detector: go run -race main.go.

Expected behavior

Numeric decodes immediately to a valid value without rescaling to dscale, matching PostgreSQL numeric_recv.

Actual behavior

• Decoder performs ~21k sequential big.Int div/mul operations, often making the value 0.
• Trailing-zero loop can spin on zero (infinite loop).
• CPU spikes; program appears to hang.

Version

• Go: go version -> e.g. go1.25.x darwin/arm64
• PostgreSQL: psql -c 'select version()'
• pgx: grep 'github.com/jackc/pgx/v[0-9]' go.mod -> e.g. v5.7.6

Additional context

• PostgreSQL reference: src/backend/utils/adt/numeric.c (numeric_recv) — dscale is display-only; no rescale during decode.
• Minimal fix in pgx decoder:
  • Do not rescale by dscale in binary decode.
  • Guard trailing-zero removal with accum != 0.
  • If rescaling is needed elsewhere, use big.Int.Exp instead of per-digit loops.