vitessio/vitess · error

unreachable

Error message

unreachable

What it means

weightStringUnicode pads each codepoint's weight in the unicode (uci) layout and handles padding widths 0-2; any other width is impossible by construction. Hitting panic("unreachable") means an internal invariant broke — the padding width derived from the collation's weight table exceeded the expected range.

Source

Thrown at go/mysql/collations/colldata/unicode.go:263

				break
			}

			src = src[width:]
			dst = append(dst, byte((r>>16)&0xFF), byte((r>>8)&0xFF), byte(r&0xFF))
		}

		if numCodepoints == PadToMax {
			for len(dst)+2 < cap(dst) {
				dst = append(dst, 0x00, 0x00, 0x20)
			}
			switch cap(dst) - len(dst) {
			case 0:
			case 1:
				dst = append(dst, 0x00)
			case 2:
				dst = append(dst, 0x00, 0x00)
			default:
				panic("unreachable")
			}
		}
	} else {
		for numCodepoints > 0 {
			r, width, ok := cs.DecodeRune(src)
			if !ok {
				break
			}

			src = src[width:]
			dst = append(dst, byte((r>>16)&0xFF), byte((r>>8)&0xFF), byte(r&0xFF))
			numCodepoints--
		}
		for numCodepoints > 0 {
			dst = append(dst, 0x00, 0x00, 0x20)
			numCodepoints--
		}
	}

View on GitHub (pinned to 01a25a7d17)

Solutions

1. Rebuild from a clean checkout — regenerated collation tables are likely out of sync with the code
2. If a custom collation was added, fix its weight widths so each codepoint's weight list yields at most 2 padding bytes
3. Report as a bug with the collation name and input string if stock collations trigger it

defer func() {
	if r := recover(); r != nil {
		log.Warn("unicode WeightString invariant failure", slog.Any("panic", r))
		// fall back to a safe hash or propagate an error
	}
}()

When it happens

Trigger: Calling WeightString on a unicode-layout collation whose generated weight data yields a padding case other than 0, 1, or 2 — i.e. corrupted or hand-edited collation tables, or a custom collation built with wrong weight widths.

Common situations: Running a custom-built Vitess binary with regenerated collation tables that don't match the generated code; modifying uca/unicode table generation scripts incorrectly.

Related errors

• WeightStringLen called with non-MOD4 length
• len(p.Patch)%3
• This should never happen.
• bad unsigned integer type
• bad type aggregation for signed/unsigned types

AI-assisted analysis of vitessio/vitess@01a25a7d17 (2026-09-01). Data as JSON: /api/errors/ee63cb7cb693a80d. Report an issue: GitHub.