github.com/bugraaydogar/snapd@v0.0.0-20210315170335-8c70bb858939/seed/seedwriter/writer.go

// -*- Mode: Go; indent-tabs-mode: t -*-

/*
 * Copyright (C) 2019-2020 Canonical Ltd
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 3 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

// Package seedwrite implements writing image seeds.
package seedwriter

import (
	"errors"
	"fmt"
	"strings"

	"github.com/snapcore/snapd/asserts"
	"github.com/snapcore/snapd/asserts/snapasserts"
	"github.com/snapcore/snapd/osutil"
	"github.com/snapcore/snapd/seed/internal"
	"github.com/snapcore/snapd/snap"
	"github.com/snapcore/snapd/snap/channel"
	"github.com/snapcore/snapd/snap/naming"
	"github.com/snapcore/snapd/strutil"
)

// Options holds the options for a Writer.
type Options struct {
	SeedDir string

	DefaultChannel string

	// The label for the recovery system for Core20 models
	Label string

	// TestSkipCopyUnverifiedModel is set to support naive tests
	// using an unverified model, the resulting image is broken
	TestSkipCopyUnverifiedModel bool
}

// OptionsSnap represents an options-referred snap with its option values.
// E.g. a snap passed to ubuntu-image via --snap.
// If Name is set the snap is from the store. If Path is set the snap
// is local at Path location.
type OptionsSnap struct {
	Name    string
	SnapID  string
	Path    string
	Channel string
}

func (s *OptionsSnap) SnapName() string {
	return s.Name
}

func (s *OptionsSnap) ID() string {
	return s.SnapID
}

var _ naming.SnapRef = (*OptionsSnap)(nil)

// SeedSnap holds details of a snap being added to a seed.
type SeedSnap struct {
	naming.SnapRef
	Channel string
	Path    string

	// Info is the *snap.Info for the seed snap, filling this is
	// delegated to the Writer using code, via Writer.SetInfo.
	Info *snap.Info
	// ARefs are references to the snap assertions if applicable,
	// filling these is delegated to the Writer using code, the
	// assumption is that the corresponding assertions can be
	// found in the database passed to Writer.Start.
	ARefs []*asserts.Ref

	local      bool
	modelSnap  *asserts.ModelSnap
	optionSnap *OptionsSnap
}

func (sn *SeedSnap) modes() []string {
	if sn.modelSnap == nil {
		// run is the assumed mode for extra snaps not listed
		// in the model
		return []string{"run"}
	}
	return sn.modelSnap.Modes
}

var _ naming.SnapRef = (*SeedSnap)(nil)

/* Writer writes Core 16/18 and Core 20 seeds.

Its methods need to be called in sequences that match prescribed
flows.

Some methods can be skipped given some conditions.

SnapsToDownload and Downloaded needs to be called in a loop where the
SeedSnaps returned by SnapsToDownload get SetInfo called with
*snap.Info retrieved from the store and then the snaps can be
downloaded at SeedSnap.Path, after which Downloaded must be invoked
and the flow breaks out of the loop only when it returns complete =
true. In the loop as well assertions for the snaps can be fetched and
SeedSnap.ARefs set.

Optionally a similar but simpler mechanism covers local snaps, where
LocalSnaps returns SeedSnaps that can be filled with information
derived from the snap at SeedSnap.Path, then InfoDerived is called.

                      V-------->\
                      |         |
               SetOptionsSnaps  |
                      |         v
                      | ________/
                      v
         /          Start       \
         |            |         |
         |            v         |
   no    |   /    LocalSnaps    | no option
   local |   |        |         | snaps
   snaps |   |        v         |
         |   |    InfoDerived   |
         |   |        |         |
         |   \        |         /
          >   > SnapsToDownload<
                      |     ^
                      v     |
                   SetInfo  |
                      |     | complete = false
                      v     /
                  Downloaded
                      |
                      | complete = true
                      |
                      v
                  SeedSnaps (copy files)
                      |
                      v
                  WriteMeta

*/
type Writer struct {
	model  *asserts.Model
	opts   *Options
	policy policy
	tree   tree

	// warnings keep a list of warnings produced during the
	// process, no more warnings should be produced after
	// Downloaded signaled complete
	warnings []string

	db asserts.RODatabase

	expectedStep writerStep

	modelRefs []*asserts.Ref

	optionsSnaps []*OptionsSnap
	// consumedOptSnapNum counts which options snaps have been consumed
	// by either cross matching or matching with a model snap
	consumedOptSnapNum int
	// extraSnapsGuessNum is essentially #(optionsSnaps) -
	// consumedOptSnapNum
	extraSnapsGuessNum int

	byNameOptSnaps *naming.SnapSet

	localSnaps      map[*OptionsSnap]*SeedSnap
	byRefLocalSnaps *naming.SnapSet

	availableSnaps  *naming.SnapSet
	availableByMode map[string]*naming.SnapSet

	// toDownload tracks which set of snaps SnapsToDownload should compute
	// next
	toDownload              snapsToDownloadSet
	toDownloadConsideredNum int

	snapsFromModel []*SeedSnap
	extraSnaps     []*SeedSnap
}

type policy interface {
	allowsDangerousFeatures() error

	checkDefaultChannel(channel.Channel) error
	checkSnapChannel(ch channel.Channel, whichSnap string) error

	systemSnap() *asserts.ModelSnap

	modelSnapDefaultChannel() string
	extraSnapDefaultChannel() string

	checkBase(s *snap.Info, modes []string, availableByMode map[string]*naming.SnapSet) error

	checkAvailable(snpRef naming.SnapRef, modes []string, availableByMode map[string]*naming.SnapSet) bool

	needsImplicitSnaps(availableByMode map[string]*naming.SnapSet) (bool, error)
	implicitSnaps(availableByMode map[string]*naming.SnapSet) []*asserts.ModelSnap
	implicitExtraSnaps(availableByMode map[string]*naming.SnapSet) []*OptionsSnap
}

type tree interface {
	mkFixedDirs() error

	snapPath(*SeedSnap) (string, error)

	localSnapPath(*SeedSnap) (string, error)

	writeAssertions(db asserts.RODatabase, modelRefs []*asserts.Ref, snapsFromModel []*SeedSnap, extraSnaps []*SeedSnap) error

	writeMeta(snapsFromModel []*SeedSnap, extraSnaps []*SeedSnap) error
}

// New returns a Writer to write a seed for the given model and using
// the given Options.
func New(model *asserts.Model, opts *Options) (*Writer, error) {
	if opts == nil {
		return nil, fmt.Errorf("internal error: Writer *Options is nil")
	}
	w := &Writer{
		model: model,
		opts:  opts,

		expectedStep: setOptionsSnapsStep,

		byNameOptSnaps:  naming.NewSnapSet(nil),
		byRefLocalSnaps: naming.NewSnapSet(nil),
	}

	var treeImpl tree
	var pol policy
	if model.Grade() != asserts.ModelGradeUnset {
		// Core 20
		if opts.Label == "" {
			return nil, fmt.Errorf("internal error: cannot write Core 20 seed without Options.Label set")
		}
		if err := internal.ValidateUC20SeedSystemLabel(opts.Label); err != nil {
			return nil, err
		}
		pol = &policy20{model: model, opts: opts, warningf: w.warningf}
		treeImpl = &tree20{grade: model.Grade(), opts: opts}
	} else {
		pol = &policy16{model: model, opts: opts, warningf: w.warningf}
		treeImpl = &tree16{opts: opts}
	}

	if opts.DefaultChannel != "" {
		deflCh, err := channel.ParseVerbatim(opts.DefaultChannel, "_")
		if err != nil {
			return nil, fmt.Errorf("cannot use global default option channel: %v", err)
		}
		if err := pol.checkDefaultChannel(deflCh); err != nil {
			return nil, err
		}
	}

	w.tree = treeImpl
	w.policy = pol
	return w, nil
}

type writerStep int

const (
	setOptionsSnapsStep = iota
	startStep
	localSnapsStep
	infoDerivedStep
	snapsToDownloadStep
	downloadedStep
	seedSnapsStep
	writeMetaStep
)

var writerStepNames = map[writerStep]string{
	startStep:           "Start",
	setOptionsSnapsStep: "SetOptionsSnaps",
	localSnapsStep:      "LocalSnaps",
	infoDerivedStep:     "InfoDerived",
	snapsToDownloadStep: "SnapsToDownload",
	downloadedStep:      "Downloaded",
	seedSnapsStep:       "SeedSnaps",
	writeMetaStep:       "WriteMeta",
}

func (ws writerStep) String() string {
	name := writerStepNames[ws]
	if name == "" {
		panic(fmt.Sprintf("unknown writerStep: %d", ws))
	}
	return name
}

func (w *Writer) checkStep(thisStep writerStep) error {
	if thisStep != w.expectedStep {
		// exceptions
		alright := false
		switch thisStep {
		case startStep:
			if w.expectedStep == setOptionsSnapsStep {
				alright = true
			}
		case snapsToDownloadStep:
			if w.expectedStep == localSnapsStep || w.expectedStep == infoDerivedStep {
				if len(w.localSnaps) != 0 {
					break
				}
				alright = true
			}
		}
		if !alright {
			expected := w.expectedStep.String()
			switch w.expectedStep {
			case setOptionsSnapsStep:
				expected = "Start|SetOptionsSnaps"
			case localSnapsStep:
				if len(w.localSnaps) == 0 {
					expected = "SnapsToDownload|LocalSnaps"
				}
			}
			return fmt.Errorf("internal error: seedwriter.Writer expected %s to be invoked on it at this point, not %v", expected, thisStep)
		}
	}
	w.expectedStep = thisStep + 1
	return nil
}

// warningf adds a warning that can be later retrieved via Warnings.
func (w *Writer) warningf(format string, a ...interface{}) {
	w.warnings = append(w.warnings, fmt.Sprintf(format, a...))
}

// SetOptionsSnaps accepts options-referred snaps represented as OptionsSnap.
func (w *Writer) SetOptionsSnaps(optSnaps []*OptionsSnap) error {
	if err := w.checkStep(setOptionsSnapsStep); err != nil {
		return err
	}

	if len(optSnaps) == 0 {
		return nil
	}

	for _, sn := range optSnaps {
		var whichSnap string
		local := false
		if sn.Name != "" {
			if sn.Path != "" {
				return fmt.Errorf("cannot specify both name and path for option snap %q", sn.Name)
			}
			snapName := sn.Name
			whichSnap = snapName
			if _, instanceKey := snap.SplitInstanceName(snapName); instanceKey != "" {
				// be specific about this error
				return fmt.Errorf("cannot use snap %q, parallel snap instances are unsupported", snapName)
			}
			if err := naming.ValidateSnap(snapName); err != nil {
				return err
			}

			if w.byNameOptSnaps.Contains(sn) {
				return fmt.Errorf("snap %q is repeated in options", snapName)
			}
			w.byNameOptSnaps.Add(sn)
		} else {
			if !strings.HasSuffix(sn.Path, ".snap") {
				return fmt.Errorf("local option snap %q does not end in .snap", sn.Path)
			}
			if !osutil.FileExists(sn.Path) {
				return fmt.Errorf("local option snap %q does not exist", sn.Path)
			}

			whichSnap = sn.Path
			local = true
		}
		if sn.Channel != "" {
			ch, err := channel.ParseVerbatim(sn.Channel, "_")
			if err != nil {
				return fmt.Errorf("cannot use option channel for snap %q: %v", whichSnap, err)
			}
			if err := w.policy.checkSnapChannel(ch, whichSnap); err != nil {
				return err
			}
		}
		if local {
			if w.localSnaps == nil {
				w.localSnaps = make(map[*OptionsSnap]*SeedSnap)
			}
			w.localSnaps[sn] = &SeedSnap{
				SnapRef: nil,
				Path:    sn.Path,

				local:      true,
				optionSnap: sn,
			}
		}
	}

	// used later to determine extra snaps
	w.optionsSnaps = optSnaps

	return nil
}

// Start starts the seed writing. It creates a RefAssertsFetcher using
// newFetcher and uses it to fetch model related assertions. For
// convenience it returns the fetcher possibly for use to fetch seed
// snap assertions, a task that the writer delegates as well as snap
// downloading. The writer assumes that the snap assertions will end up
// in the given db (writing assertion database).
func (w *Writer) Start(db asserts.RODatabase, newFetcher NewFetcherFunc) (RefAssertsFetcher, error) {
	if err := w.checkStep(startStep); err != nil {
		return nil, err
	}
	if db == nil {
		return nil, fmt.Errorf("internal error: Writer *asserts.RODatabase is nil")
	}
	if newFetcher == nil {
		return nil, fmt.Errorf("internal error: Writer newFetcherFunc is nil")
	}
	w.db = db

	f := MakeRefAssertsFetcher(newFetcher)

	if err := f.Save(w.model); err != nil {
		const msg = "cannot fetch and check prerequisites for the model assertion: %v"
		if !w.opts.TestSkipCopyUnverifiedModel {
			return nil, fmt.Errorf(msg, err)
		}
		// Some naive tests including ubuntu-image ones use
		// unverified models
		w.warningf(msg, err)
		f.ResetRefs()
	}

	// fetch device store assertion (and prereqs) if available
	if w.model.Store() != "" {
		err := snapasserts.FetchStore(f, w.model.Store())
		if err != nil {
			if nfe, ok := err.(*asserts.NotFoundError); !ok || nfe.Type != asserts.StoreType {
				return nil, err
			}
		}
	}

	w.modelRefs = f.Refs()

	if err := w.tree.mkFixedDirs(); err != nil {
		return nil, err
	}

	return f, nil
}

// LocalSnaps returns a list of seed snaps that are local.  The writer
// delegates to produce *snap.Info for them to then be set via
// SetInfo. If matching snap assertions can be found as well they can
// be passed into SeedSnap ARefs, assuming they were added to the
// writing assertion database.
func (w *Writer) LocalSnaps() ([]*SeedSnap, error) {
	if err := w.checkStep(localSnapsStep); err != nil {
		return nil, err
	}

	if len(w.localSnaps) == 0 {
		return nil, nil
	}

	lsnaps := make([]*SeedSnap, 0, len(w.localSnaps))
	for _, optSnap := range w.optionsSnaps {
		if sn := w.localSnaps[optSnap]; sn != nil {
			lsnaps = append(lsnaps, sn)
		}
	}
	return lsnaps, nil
}

// InfoDerived checks the local snaps metadata provided via setting it
// into the SeedSnaps returned by the previous LocalSnaps.
func (w *Writer) InfoDerived() error {
	if err := w.checkStep(infoDerivedStep); err != nil {
		return err
	}

	// loop this way to process for consistency in the same order
	// as LocalSnaps result
	for _, optSnap := range w.optionsSnaps {
		sn := w.localSnaps[optSnap]
		if sn == nil {
			continue
		}
		if sn.Info == nil {
			return fmt.Errorf("internal error: before seedwriter.Writer.InfoDerived snap %q Info should have been set", sn.Path)
		}

		if sn.Info.ID() == "" {
			// this check is here in case we relax the checks in
			// SetOptionsSnaps
			if err := w.policy.allowsDangerousFeatures(); err != nil {
				return err
			}
		}

		sn.SnapRef = sn.Info

		// local snap gets local revision
		if sn.Info.Revision.Unset() {
			sn.Info.Revision = snap.R(-1)
		}

		if w.byRefLocalSnaps.Contains(sn) {
			return fmt.Errorf("local snap %q is repeated in options", sn.SnapName())
		}

		// in case, merge channel given by name separately
		optSnap, _ := w.byNameOptSnaps.Lookup(sn).(*OptionsSnap)
		if optSnap != nil {
			w.consumedOptSnapNum++
		}
		if optSnap != nil && optSnap.Channel != "" {
			if sn.optionSnap.Channel != "" {
				if sn.optionSnap.Channel != optSnap.Channel {
					return fmt.Errorf("option snap has different channels specified: %q=%q vs %q=%q", sn.Path, sn.optionSnap.Channel, optSnap.Name, optSnap.Channel)
				}
			} else {
				sn.optionSnap.Channel = optSnap.Channel
			}
		}

		w.byRefLocalSnaps.Add(sn)
	}

	return nil
}

// SetInfo sets Info of the SeedSnap and possibly computes its
// destination Path.
func (w *Writer) SetInfo(sn *SeedSnap, info *snap.Info) error {
	if info.Confinement == snap.DevModeConfinement {
		if err := w.policy.allowsDangerousFeatures(); err != nil {
			return err
		}
	}
	sn.Info = info

	if sn.local {
		// nothing more to do
		return nil
	}

	p, err := w.tree.snapPath(sn)
	if err != nil {
		return err
	}
	sn.Path = p
	return nil
}

// SetRedirectChannel sets the redirect channel for the SeedSnap
// for the in case there is a default track for it.
func (w *Writer) SetRedirectChannel(sn *SeedSnap, redirectChannel string) error {
	if sn.local {
		return fmt.Errorf("internal error: cannot set redirect channel for local snap %q", sn.Path)
	}
	if sn.Info == nil {
		return fmt.Errorf("internal error: before using seedwriter.Writer.SetRedirectChannel snap %q Info should have been set", sn.SnapName())
	}
	if redirectChannel == "" {
		// nothing to do
		return nil
	}
	_, err := channel.ParseVerbatim(redirectChannel, "-")
	if err != nil {
		return fmt.Errorf("invalid redirect channel for snap %q: %v", sn.SnapName(), err)
	}
	sn.Channel = redirectChannel
	return nil

}

// snapsToDownloadSet indicates which set of snaps SnapsToDownload should compute
type snapsToDownloadSet int

const (
	toDownloadModel snapsToDownloadSet = iota
	toDownloadImplicit
	toDownloadExtra
	toDownloadExtraImplicit
)

var errSkipOptional = errors.New("skip")

func (w *Writer) modelSnapToSeed(modSnap *asserts.ModelSnap) (*SeedSnap, error) {
	sn, _ := w.byRefLocalSnaps.Lookup(modSnap).(*SeedSnap)
	var optSnap *OptionsSnap
	if sn == nil {
		// not local, to download
		optSnap, _ = w.byNameOptSnaps.Lookup(modSnap).(*OptionsSnap)
		if modSnap.Presence == "optional" && optSnap == nil {
			// an optional snap that is not confirmed
			// by an OptionsSnap entry is skipped
			return nil, errSkipOptional
		}
		sn = &SeedSnap{
			SnapRef: modSnap,

			local:      false,
			optionSnap: optSnap,
		}
	} else {
		optSnap = sn.optionSnap
	}

	channel, err := w.resolveChannel(modSnap.SnapName(), modSnap, optSnap)
	if err != nil {
		return nil, err
	}
	sn.modelSnap = modSnap
	sn.Channel = channel
	return sn, nil
}

func (w *Writer) modelSnapsToDownload(modSnaps []*asserts.ModelSnap) (toDownload []*SeedSnap, err error) {
	if w.snapsFromModel == nil {
		w.snapsFromModel = make([]*SeedSnap, 0, len(modSnaps))
	}
	toDownload = make([]*SeedSnap, 0, len(modSnaps))

	alreadyConsidered := len(w.snapsFromModel)
	for _, modSnap := range modSnaps {
		sn, err := w.modelSnapToSeed(modSnap)
		if err == errSkipOptional {
			continue
		}
		if err != nil {
			return nil, err
		}
		if !sn.local {
			toDownload = append(toDownload, sn)
		}
		if sn.optionSnap != nil {
			w.consumedOptSnapNum++
		}
		w.snapsFromModel = append(w.snapsFromModel, sn)
	}
	w.toDownloadConsideredNum = len(w.snapsFromModel) - alreadyConsidered
	w.extraSnapsGuessNum = len(w.optionsSnaps) - w.consumedOptSnapNum

	return toDownload, nil
}

func (w *Writer) modSnaps() ([]*asserts.ModelSnap, error) {
	// model snaps are accumulated/processed in the order
	//  * system snap if implicit
	//  * essential snaps (in Model.EssentialSnaps order)
	//  * not essential snaps
	modSnaps := append([]*asserts.ModelSnap{}, w.model.EssentialSnaps()...)
	if systemSnap := w.policy.systemSnap(); systemSnap != nil {
		prepend := true
		for _, modSnap := range modSnaps {
			if naming.SameSnap(modSnap, systemSnap) {
				prepend = false
				modes := modSnap.Modes
				expectedModes := systemSnap.Modes
				if len(modes) != len(expectedModes) {
					return nil, fmt.Errorf("internal error: system snap %q explicitly listed in model carries wrong modes: %q", systemSnap.SnapName(), modes)
				}
				for _, mod := range expectedModes {
					if !strutil.ListContains(modes, mod) {
						return nil, fmt.Errorf("internal error: system snap %q explicitly listed in model carries wrong modes: %q", systemSnap.SnapName(), modes)
					}
				}
				break
			}
		}
		if prepend {
			modSnaps = append([]*asserts.ModelSnap{systemSnap}, modSnaps...)
		}
	}
	modSnaps = append(modSnaps, w.model.SnapsWithoutEssential()...)
	return modSnaps, nil
}

func (w *Writer) optExtraSnaps() []*OptionsSnap {
	extra := make([]*OptionsSnap, 0, w.extraSnapsGuessNum)
	for _, optSnap := range w.optionsSnaps {
		var snapRef naming.SnapRef = optSnap
		if sn := w.localSnaps[optSnap]; sn != nil {
			snapRef = sn
		}
		if w.availableSnaps.Contains(snapRef) {
			continue
		}
		extra = append(extra, optSnap)
	}
	return extra
}

func (w *Writer) extraSnapToSeed(optSnap *OptionsSnap) (*SeedSnap, error) {
	sn := w.localSnaps[optSnap]
	if sn == nil {
		// not local, to download
		sn = &SeedSnap{
			SnapRef: optSnap,

			local:      false,
			optionSnap: optSnap,
		}
	}
	if sn.SnapName() == "" {
		return nil, fmt.Errorf("internal error: option extra snap has no associated name: %#v %#v", optSnap, sn)
	}

	channel, err := w.resolveChannel(sn.SnapName(), nil, optSnap)
	if err != nil {
		return nil, err
	}
	sn.Channel = channel
	return sn, nil
}

func (w *Writer) extraSnapsToDownload(extraSnaps []*OptionsSnap) (toDownload []*SeedSnap, err error) {
	if w.extraSnaps == nil {
		w.extraSnaps = make([]*SeedSnap, 0, len(extraSnaps))
	}
	toDownload = make([]*SeedSnap, 0, len(extraSnaps))

	alreadyConsidered := len(w.extraSnaps)
	for _, optSnap := range extraSnaps {
		sn, err := w.extraSnapToSeed(optSnap)
		if err != nil {
			return nil, err
		}
		if !sn.local {
			toDownload = append(toDownload, sn)
		}
		w.extraSnaps = append(w.extraSnaps, sn)
	}
	w.toDownloadConsideredNum = len(w.extraSnaps) - alreadyConsidered

	return toDownload, nil
}

// SnapsToDownload returns a list of seed snaps to download. Once that
// is done and their SeedSnaps Info with SetInfo and ARefs fields are
// set, Downloaded should be called next.
func (w *Writer) SnapsToDownload() (snaps []*SeedSnap, err error) {
	if err := w.checkStep(snapsToDownloadStep); err != nil {
		return nil, err
	}

	switch w.toDownload {
	case toDownloadModel:
		modSnaps, err := w.modSnaps()
		if err != nil {
			return nil, err
		}
		toDownload, err := w.modelSnapsToDownload(modSnaps)
		if err != nil {
			return nil, err
		}
		if w.extraSnapsGuessNum > 0 {
			// this check is here in case we relax the checks in
			// SetOptionsSnaps
			if err := w.policy.allowsDangerousFeatures(); err != nil {
				return nil, err
			}
		}
		return toDownload, nil
	case toDownloadImplicit:
		return w.modelSnapsToDownload(w.policy.implicitSnaps(w.availableByMode))
	case toDownloadExtra:
		return w.extraSnapsToDownload(w.optExtraSnaps())
	case toDownloadExtraImplicit:
		return w.extraSnapsToDownload(w.policy.implicitExtraSnaps(w.availableByMode))
	default:
		panic(fmt.Sprintf("unknown to-download set: %d", w.toDownload))
	}
}

func (w *Writer) resolveChannel(whichSnap string, modSnap *asserts.ModelSnap, optSnap *OptionsSnap) (string, error) {
	var optChannel string
	if optSnap != nil {
		optChannel = optSnap.Channel
	}
	if optChannel == "" {
		optChannel = w.opts.DefaultChannel
	}

	if modSnap != nil && modSnap.PinnedTrack != "" {
		resChannel, err := channel.ResolvePinned(modSnap.PinnedTrack, optChannel)
		if err == channel.ErrPinnedTrackSwitch {
			return "", fmt.Errorf("option channel %q for %s has a track incompatible with the pinned track from model assertion: %s", optChannel, whichModelSnap(modSnap, w.model), modSnap.PinnedTrack)
		}
		if err != nil {
			// shouldn't happen given that we check that
			// the inputs parse before
			return "", fmt.Errorf("internal error: cannot resolve pinned track %q and option channel %q for snap %q", modSnap.PinnedTrack, optChannel, whichSnap)
		}
		return resChannel, nil
	}

	var defaultChannel string
	if modSnap != nil {
		defaultChannel = modSnap.DefaultChannel
		if defaultChannel == "" {
			defaultChannel = w.policy.modelSnapDefaultChannel()
		}
	} else {
		defaultChannel = w.policy.extraSnapDefaultChannel()
	}

	resChannel, err := channel.Resolve(defaultChannel, optChannel)
	if err != nil {
		// shouldn't happen given that we check that
		// the inputs parse before
		return "", fmt.Errorf("internal error: cannot resolve model default channel %q and option channel %q for snap %q", defaultChannel, optChannel, whichSnap)
	}
	return resChannel, nil
}

func (w *Writer) checkBase(info *snap.Info, modes []string) error {
	// Sanity check, note that we could support this case
	// if we have a use-case but it requires changes in the
	// devicestate/firstboot.go ordering code.
	if info.Type() == snap.TypeGadget && !w.model.Classic() && info.Base != w.model.Base() {
		return fmt.Errorf("cannot use gadget snap because its base %q is different from model base %q", info.Base, w.model.Base())
	}

	// snap explicitly listed as not needing a base snap (e.g. a content-only snap)
	if info.Base == "none" {
		return nil
	}

	return w.policy.checkBase(info, modes, w.availableByMode)
}

func (w *Writer) downloaded(seedSnaps []*SeedSnap) error {
	if w.availableSnaps == nil {
		w.availableSnaps = naming.NewSnapSet(nil)
		w.availableByMode = make(map[string]*naming.SnapSet)
		w.availableByMode["run"] = naming.NewSnapSet(nil)
	}

	for _, sn := range seedSnaps {
		if sn.Info == nil {
			return fmt.Errorf("internal error: before seedwriter.Writer.Downloaded snap %q Info should have been set", sn.SnapName())
		}
		w.availableSnaps.Add(sn)
		for _, mode := range sn.modes() {
			byMode := w.availableByMode[mode]
			if byMode == nil {
				byMode = naming.NewSnapSet(nil)
				w.availableByMode[mode] = byMode
			}
			byMode.Add(sn)
		}
	}

	for _, sn := range seedSnaps {
		info := sn.Info
		if !sn.local {
			if info.ID() == "" {
				return fmt.Errorf("internal error: before seedwriter.Writer.Downloaded snap %q snap-id should have been set", sn.SnapName())
			}
		}
		if info.ID() != "" {
			if sn.ARefs == nil {
				return fmt.Errorf("internal error: before seedwriter.Writer.Downloaded snap %q ARefs should have been set", sn.SnapName())
			}
		}

		// TODO: optionally check that model snap name and
		// info snap name match

		if err := checkType(sn, w.model); err != nil {
			return err
		}

		needsClassic := info.NeedsClassic()
		if needsClassic && !w.model.Classic() {
			return fmt.Errorf("cannot use classic snap %q in a core system", info.SnapName())
		}

		modes := sn.modes()

		if err := w.checkBase(info, modes); err != nil {
			return err
		}
		// error about missing default providers
		for dp := range snap.NeededDefaultProviders(info) {
			if !w.policy.checkAvailable(naming.Snap(dp), modes, w.availableByMode) {
				// TODO: have a way to ignore this issue on a snap by snap basis?
				return fmt.Errorf("cannot use snap %q without its default content provider %q being added explicitly%s", info.SnapName(), dp, errorMsgForModesSuffix(modes))
			}
		}

		if err := w.checkPublisher(sn); err != nil {
			return err
		}
	}

	return nil
}

// Downloaded checks the downloaded snaps metadata provided via
// setting it into the SeedSnaps returned by the previous
// SnapsToDownload. It also returns whether the seed snap set is
// complete or SnapsToDownload should be called again.
func (w *Writer) Downloaded() (complete bool, err error) {
	if err := w.checkStep(downloadedStep); err != nil {
		return false, err
	}

	var considered []*SeedSnap
	switch w.toDownload {
	default:
		panic(fmt.Sprintf("unknown to-download set: %d", w.toDownload))
	case toDownloadImplicit:
		fallthrough
	case toDownloadModel:
		considered = w.snapsFromModel
	case toDownloadExtraImplicit:
		fallthrough
	case toDownloadExtra:
		considered = w.extraSnaps
	}

	considered = considered[len(considered)-w.toDownloadConsideredNum:]
	err = w.downloaded(considered)
	if err != nil {
		return false, err
	}

	switch w.toDownload {
	case toDownloadModel:
		implicitNeeded, err := w.policy.needsImplicitSnaps(w.availableByMode)
		if err != nil {
			return false, err
		}
		if implicitNeeded {
			w.toDownload = toDownloadImplicit
			w.expectedStep = snapsToDownloadStep
			return false, nil
		}
		fallthrough
	case toDownloadImplicit:
		if w.extraSnapsGuessNum > 0 {
			w.toDownload = toDownloadExtra
			w.expectedStep = snapsToDownloadStep
			return false, nil
		}
	case toDownloadExtra:
		implicitNeeded, err := w.policy.needsImplicitSnaps(w.availableByMode)
		if err != nil {
			return false, err
		}
		if implicitNeeded {
			w.toDownload = toDownloadExtraImplicit
			w.expectedStep = snapsToDownloadStep
			return false, nil
		}
	case toDownloadExtraImplicit:
		// nothing to do
		// TODO: consider generalizing the logic and optionally asking
		// the policy again
	default:
		panic(fmt.Sprintf("unknown to-download set: %d", w.toDownload))
	}

	return true, nil
}

func (w *Writer) checkPublisher(sn *SeedSnap) error {
	if sn.local && sn.ARefs == nil {
		// nothing to do
		return nil
	}
	info := sn.Info
	var kind string
	switch info.Type() {
	case snap.TypeKernel:
		kind = "kernel"
	case snap.TypeGadget:
		kind = "gadget"
	default:
		return nil
	}
	// TODO: share helpers with devicestate if the policy becomes much more complicated
	snapDecl, err := w.snapDecl(sn)
	if err != nil {
		return err
	}
	publisher := snapDecl.PublisherID()
	if publisher != w.model.BrandID() && publisher != "canonical" {
		return fmt.Errorf("cannot use %s %q published by %q for model by %q", kind, info.SnapName(), publisher, w.model.BrandID())
	}
	return nil
}

func (w *Writer) snapDecl(sn *SeedSnap) (*asserts.SnapDeclaration, error) {
	for _, ref := range sn.ARefs {
		if ref.Type == asserts.SnapDeclarationType {
			a, err := ref.Resolve(w.db.Find)
			if err != nil {
				return nil, fmt.Errorf("internal error: lost saved assertion")
			}
			return a.(*asserts.SnapDeclaration), nil
		}
	}
	return nil, fmt.Errorf("internal error: snap %q has no snap-declaration set", sn.SnapName())
}

// Warnings returns the warning messages produced so far. No warnings
// should be generated after Downloaded signaled complete.
func (w *Writer) Warnings() []string {
	return w.warnings
}

// SeedSnaps checks seed snaps and copies local snaps into the seed using copySnap.
func (w *Writer) SeedSnaps(copySnap func(name, src, dst string) error) error {
	if err := w.checkStep(seedSnapsStep); err != nil {
		return err
	}

	seedSnaps := func(snaps []*SeedSnap) error {
		for _, sn := range snaps {
			info := sn.Info
			if !sn.local {
				expectedPath, err := w.tree.snapPath(sn)
				if err != nil {
					return err
				}
				if sn.Path != expectedPath {
					return fmt.Errorf("internal error: before seedwriter.Writer.SeedSnaps snap %q Path should have been set to %q", sn.SnapName(), expectedPath)
				}
				if !osutil.FileExists(expectedPath) {
					return fmt.Errorf("internal error: before seedwriter.Writer.SeedSnaps snap file %q should exist", expectedPath)
				}
			} else {
				var snapPath func(*SeedSnap) (string, error)
				if sn.Info.ID() != "" {
					// actually asserted
					snapPath = w.tree.snapPath
				} else {
					// purely local
					snapPath = w.tree.localSnapPath
				}
				dst, err := snapPath(sn)
				if err != nil {
					return err
				}
				if err := copySnap(info.SnapName(), sn.Path, dst); err != nil {
					return err
				}
				// record final destination path
				sn.Path = dst
			}
		}
		return nil
	}

	if err := seedSnaps(w.snapsFromModel); err != nil {
		return err
	}
	if err := seedSnaps(w.extraSnaps); err != nil {
		return err
	}

	return nil
}

// WriteMeta writes seed metadata and assertions into the seed.
func (w *Writer) WriteMeta() error {
	if err := w.checkStep(writeMetaStep); err != nil {
		return err
	}

	snapsFromModel := w.snapsFromModel
	extraSnaps := w.extraSnaps

	if err := w.tree.writeAssertions(w.db, w.modelRefs, snapsFromModel, extraSnaps); err != nil {
		return err
	}

	return w.tree.writeMeta(snapsFromModel, extraSnaps)
}

// query accessors

func (w *Writer) checkSnapsAccessor() error {
	if w.expectedStep < seedSnapsStep {
		return fmt.Errorf("internal error: seedwriter.Writer cannot query seed snaps before Downloaded signaled complete")
	}
	return nil
}

// BootSnaps returns the seed snaps involved in the boot process.
// It can be invoked only after Downloaded returns complete ==
// true. It returns an error for classic models as for those no snaps
// participate in boot before user space.
func (w *Writer) BootSnaps() ([]*SeedSnap, error) {
	if err := w.checkSnapsAccessor(); err != nil {
		return nil, err
	}
	if w.model.Classic() {
		return nil, fmt.Errorf("no snaps participating in boot on classic")
	}
	var bootSnaps []*SeedSnap
	for _, sn := range w.snapsFromModel {
		bootSnaps = append(bootSnaps, sn)
		if sn.Info.Type() == snap.TypeGadget {
			break

		}
	}
	return bootSnaps, nil
}

// UnassertedSnaps returns references for all unasserted snaps in the seed.
// It can be invoked only after Downloaded returns complete ==
// true.
func (w *Writer) UnassertedSnaps() ([]naming.SnapRef, error) {
	if err := w.checkSnapsAccessor(); err != nil {
		return nil, err
	}
	var res []naming.SnapRef
	for _, sn := range w.snapsFromModel {
		if sn.Info.ID() != "" {
			continue
		}
		res = append(res, sn.SnapRef)
	}

	for _, sn := range w.extraSnaps {
		if sn.Info.ID() != "" {
			continue
		}
		res = append(res, sn.SnapRef)
	}
	return res, nil
}