Read an xz release in Go, so the build seat needs no xz tool (hq issue 345)

Betterbird's pinned .tar.xz failed on the build seat with "this build seat has
no xz to read it": the seat's image is the catalogue's build-agent recipe,
which installs git and docker-cli alone, and xz had been added only to
cmd/mesh-builder/Dockerfile, which the seat does not build from. The stream
is now read with github.com/ulikunitz/xz (vendored), its dictionary capped
at xz's own largest preset, and what it unpacks to bounded as before; the
xz line in the unused Dockerfile is taken back out.

vendor/github.com/ulikunitz/xz/lzma/distcodec.go

@@ -0,0 +1,140 @@
+// Copyright 2014-2022 Ulrich Kunitz. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package lzma
+
+// Constants used by the distance codec.
+const (
+	// minimum supported distance
+	minDistance = 1
+	// maximum supported distance, value is used for the eos marker.
+	maxDistance = 1 << 32
+	// number of the supported len states
+	lenStates = 4
+	// start for the position models
+	startPosModel = 4
+	// first index with align bits support
+	endPosModel = 14
+	// bits for the position slots
+	posSlotBits = 6
+	// number of align bits
+	alignBits = 4
+)
+
+// distCodec provides encoding and decoding of distance values.
+type distCodec struct {
+	posSlotCodecs [lenStates]treeCodec
+	posModel      [endPosModel - startPosModel]treeReverseCodec
+	alignCodec    treeReverseCodec
+}
+
+// deepcopy initializes dc as deep copy of the source.
+func (dc *distCodec) deepcopy(src *distCodec) {
+	if dc == src {
+		return
+	}
+	for i := range dc.posSlotCodecs {
+		dc.posSlotCodecs[i].deepcopy(&src.posSlotCodecs[i])
+	}
+	for i := range dc.posModel {
+		dc.posModel[i].deepcopy(&src.posModel[i])
+	}
+	dc.alignCodec.deepcopy(&src.alignCodec)
+}
+
+// newDistCodec creates a new distance codec.
+func (dc *distCodec) init() {
+	for i := range dc.posSlotCodecs {
+		dc.posSlotCodecs[i] = makeTreeCodec(posSlotBits)
+	}
+	for i := range dc.posModel {
+		posSlot := startPosModel + i
+		bits := (posSlot >> 1) - 1
+		dc.posModel[i] = makeTreeReverseCodec(bits)
+	}
+	dc.alignCodec = makeTreeReverseCodec(alignBits)
+}
+
+// lenState converts the value l to a supported lenState value.
+func lenState(l uint32) uint32 {
+	if l >= lenStates {
+		l = lenStates - 1
+	}
+	return l
+}
+
+// Encode encodes the distance using the parameter l. Dist can have values from
+// the full range of uint32 values. To get the distance offset the actual match
+// distance has to be decreased by 1. A distance offset of 0xffffffff (eos)
+// indicates the end of the stream.
+func (dc *distCodec) Encode(e *rangeEncoder, dist uint32, l uint32) (err error) {
+	// Compute the posSlot using nlz32
+	var posSlot uint32
+	var bits uint32
+	if dist < startPosModel {
+		posSlot = dist
+	} else {
+		bits = uint32(30 - nlz32(dist))
+		posSlot = startPosModel - 2 + (bits << 1)
+		posSlot += (dist >> uint(bits)) & 1
+	}
+
+	if err = dc.posSlotCodecs[lenState(l)].Encode(e, posSlot); err != nil {
+		return
+	}
+
+	switch {
+	case posSlot < startPosModel:
+		return nil
+	case posSlot < endPosModel:
+		tc := &dc.posModel[posSlot-startPosModel]
+		return tc.Encode(dist, e)
+	}
+	dic := directCodec(bits - alignBits)
+	if err = dic.Encode(e, dist>>alignBits); err != nil {
+		return
+	}
+	return dc.alignCodec.Encode(dist, e)
+}
+
+// Decode decodes the distance offset using the parameter l. The dist value
+// 0xffffffff (eos) indicates the end of the stream. Add one to the distance
+// offset to get the actual match distance.
+func (dc *distCodec) Decode(d *rangeDecoder, l uint32) (dist uint32, err error) {
+	posSlot, err := dc.posSlotCodecs[lenState(l)].Decode(d)
+	if err != nil {
+		return
+	}
+
+	// posSlot equals distance
+	if posSlot < startPosModel {
+		return posSlot, nil
+	}
+
+	// posSlot uses the individual models
+	bits := (posSlot >> 1) - 1
+	dist = (2 | (posSlot & 1)) << bits
+	var u uint32
+	if posSlot < endPosModel {
+		tc := &dc.posModel[posSlot-startPosModel]
+		if u, err = tc.Decode(d); err != nil {
+			return 0, err
+		}
+		dist += u
+		return dist, nil
+	}
+
+	// posSlots use direct encoding and a single model for the four align
+	// bits.
+	dic := directCodec(bits - alignBits)
+	if u, err = dic.Decode(d); err != nil {
+		return 0, err
+	}
+	dist += u << alignBits
+	if u, err = dc.alignCodec.Decode(d); err != nil {
+		return 0, err
+	}
+	dist += u
+	return dist, nil
+}