info.go

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

/*
 * Copyright (C) 2014-2020 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 snap

import (
	"bytes"
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"reflect"
	"sort"
	"strings"
	"time"

	"github.com/snapcore/snapd/dirs"
	"github.com/snapcore/snapd/osutil"
	"github.com/snapcore/snapd/osutil/sys"
	"github.com/snapcore/snapd/snap/naming"
	"github.com/snapcore/snapd/strutil"
	"github.com/snapcore/snapd/timeout"
)

// PlaceInfo offers all the information about where a snap and its data are
// located and exposed in the filesystem.
type PlaceInfo interface {
	// InstanceName returns the name of the snap decorated with instance
	// key, if any.
	InstanceName() string

	// SnapName returns the name of the snap.
	SnapName() string

	// SnapRevision returns the revision of the snap.
	SnapRevision() Revision

	// Filename returns the name of the snap with the revision
	// number, as used on the filesystem.
	Filename() string

	// MountDir returns the base directory of the snap.
	MountDir() string

	// MountFile returns the path where the snap file that is mounted is
	// installed.
	MountFile() string

	// HooksDir returns the directory containing the snap's hooks.
	HooksDir() string

	// DataDir returns the data directory of the snap.
	DataDir() string

	// UserDataDir returns the per user data directory of the snap.
	UserDataDir(home string) string

	// CommonDataDir returns the data directory common across revisions of the
	// snap.
	CommonDataDir() string

	// UserCommonDataDir returns the per user data directory common across
	// revisions of the snap.
	UserCommonDataDir(home string) string

	// UserXdgRuntimeDir returns the per user XDG_RUNTIME_DIR directory
	UserXdgRuntimeDir(userID sys.UserID) string

	// DataHomeDir returns the a glob that matches all per user data directories
	// of a snap.
	DataHomeDir() string

	// CommonDataHomeDir returns a glob that matches all per user data
	// directories common across revisions of the snap.
	CommonDataHomeDir() string

	// XdgRuntimeDirs returns a glob that matches all XDG_RUNTIME_DIR
	// directories for all users of the snap.
	XdgRuntimeDirs() string
}

// MinimalPlaceInfo returns a PlaceInfo with just the location information for a
// snap of the given name and revision.
func MinimalPlaceInfo(name string, revision Revision) PlaceInfo {
	storeName, instanceKey := SplitInstanceName(name)
	return &Info{SideInfo: SideInfo{RealName: storeName, Revision: revision}, InstanceKey: instanceKey}
}

// ParsePlaceInfoFromSnapFileName returns a PlaceInfo with just the location
// information for a snap of file name, failing if the snap file name is invalid
// This explicitly does not support filenames with instance names in them
func ParsePlaceInfoFromSnapFileName(sn string) (PlaceInfo, error) {
	if sn == "" {
		return nil, fmt.Errorf("empty snap file name")
	}
	if strings.Count(sn, "_") > 1 {
		// too many "_", probably has an instance key in the filename like in
		// snap-name_key_23.snap
		return nil, fmt.Errorf("too many '_' in snap file name")
	}
	idx := strings.IndexByte(sn, '_')
	switch {
	case idx < 0:
		return nil, fmt.Errorf("snap file name %q has invalid format (missing '_')", sn)
	case idx == 0:
		return nil, fmt.Errorf("snap file name %q has invalid format (no snap name before '_')", sn)
	}
	// ensure that _ is not the last element
	name := sn[:idx]
	revnoNSuffix := sn[idx+1:]
	rev, err := ParseRevision(strings.TrimSuffix(revnoNSuffix, ".snap"))
	if err != nil {
		return nil, fmt.Errorf("cannot parse revision in snap file name %q: %v", sn, err)
	}
	return &Info{SideInfo: SideInfo{RealName: name, Revision: rev}}, nil
}

// BaseDir returns the system level directory of given snap.
func BaseDir(name string) string {
	return filepath.Join(dirs.SnapMountDir, name)
}

// MountDir returns the base directory where it gets mounted of the snap with
// the given name and revision.
func MountDir(name string, revision Revision) string {
	return filepath.Join(BaseDir(name), revision.String())
}

// MountFile returns the path where the snap file that is mounted is installed.
func MountFile(name string, revision Revision) string {
	return filepath.Join(dirs.SnapBlobDir, fmt.Sprintf("%s_%s.snap", name, revision))
}

// ScopedSecurityTag returns the snap-specific, scope specific, security tag.
func ScopedSecurityTag(snapName, scopeName, suffix string) string {
	return fmt.Sprintf("snap.%s.%s.%s", snapName, scopeName, suffix)
}

// SecurityTag returns the snap-specific security tag.
func SecurityTag(snapName string) string {
	return fmt.Sprintf("snap.%s", snapName)
}

// AppSecurityTag returns the application-specific security tag.
func AppSecurityTag(snapName, appName string) string {
	return fmt.Sprintf("%s.%s", SecurityTag(snapName), appName)
}

// HookSecurityTag returns the hook-specific security tag.
func HookSecurityTag(snapName, hookName string) string {
	return ScopedSecurityTag(snapName, "hook", hookName)
}

// NoneSecurityTag returns the security tag for interfaces that
// are not associated to an app or hook in the snap.
func NoneSecurityTag(snapName, uniqueName string) string {
	return ScopedSecurityTag(snapName, "none", uniqueName)
}

// BaseDataDir returns the base directory for snap data locations.
func BaseDataDir(name string) string {
	return filepath.Join(dirs.SnapDataDir, name)
}

// DataDir returns the data directory for given snap name and revision. The name
// can be
// either a snap name or snap instance name.
func DataDir(name string, revision Revision) string {
	return filepath.Join(BaseDataDir(name), revision.String())
}

// CommonDataDir returns the common data directory for given snap name. The name
// can be either a snap name or snap instance name.
func CommonDataDir(name string) string {
	return filepath.Join(dirs.SnapDataDir, name, "common")
}

// HooksDir returns the directory containing the snap's hooks for given snap
// name. The name can be either a snap name or snap instance name.
func HooksDir(name string, revision Revision) string {
	return filepath.Join(MountDir(name, revision), "meta", "hooks")
}

// UserDataDir returns the user-specific data directory for given snap name. The
// name can be either a snap name or snap instance name.
func UserDataDir(home string, name string, revision Revision) string {
	return filepath.Join(home, dirs.UserHomeSnapDir, name, revision.String())
}

// UserCommonDataDir returns the user-specific common data directory for given
// snap name. The name can be either a snap name or snap instance name.
func UserCommonDataDir(home string, name string) string {
	return filepath.Join(home, dirs.UserHomeSnapDir, name, "common")
}

// UserSnapDir returns the user-specific directory for given
// snap name. The name can be either a snap name or snap instance name.
func UserSnapDir(home string, name string) string {
	return filepath.Join(home, dirs.UserHomeSnapDir, name)
}

// UserXdgRuntimeDir returns the user-specific XDG_RUNTIME_DIR directory for
// given snap name. The name can be either a snap name or snap instance name.
func UserXdgRuntimeDir(euid sys.UserID, name string) string {
	return filepath.Join(dirs.XdgRuntimeDirBase, fmt.Sprintf("%d/snap.%s", euid, name))
}

// SideInfo holds snap metadata that is crucial for the tracking of
// snaps and for the working of the system offline and which is not
// included in snap.yaml or for which the store is the canonical
// source overriding snap.yaml content.
//
// It can be marshalled and will be stored in the system state for
// each currently installed snap revision so it needs to be evolved
// carefully.
//
// Information that can be taken directly from snap.yaml or that comes
// from the store but is not required for working offline should not
// end up in SideInfo.
type SideInfo struct {
	RealName          string   `yaml:"name,omitempty" json:"name,omitempty"`
	SnapID            string   `yaml:"snap-id" json:"snap-id"`
	Revision          Revision `yaml:"revision" json:"revision"`
	Channel           string   `yaml:"channel,omitempty" json:"channel,omitempty"`
	Contact           string   `yaml:"contact,omitempty" json:"contact,omitempty"`
	EditedTitle       string   `yaml:"title,omitempty" json:"title,omitempty"`
	EditedSummary     string   `yaml:"summary,omitempty" json:"summary,omitempty"`
	EditedDescription string   `yaml:"description,omitempty" json:"description,omitempty"`
	Private           bool     `yaml:"private,omitempty" json:"private,omitempty"`
	Paid              bool     `yaml:"paid,omitempty" json:"paid,omitempty"`
}

// Info provides information about snaps.
type Info struct {
	SuggestedName string
	InstanceKey   string
	Version       string
	SnapType      Type
	Architectures []string
	Assumes       []string

	OriginalTitle       string
	OriginalSummary     string
	OriginalDescription string

	Environment strutil.OrderedMap

	LicenseAgreement string
	LicenseVersion   string
	License          string
	Epoch            Epoch
	Base             string
	Confinement      ConfinementType
	Apps             map[string]*AppInfo
	LegacyAliases    map[string]*AppInfo // FIXME: eventually drop this
	Hooks            map[string]*HookInfo
	Plugs            map[string]*PlugInfo
	Slots            map[string]*SlotInfo

	// Plugs or slots with issues (they are not included in Plugs or Slots)
	BadInterfaces map[string]string // slot or plug => message

	// The information in all the remaining fields is not sourced from the snap
	// blob itself.
	SideInfo

	// Broken marks whether the snap is broken and the reason.
	Broken string

	// The information in these fields is ephemeral, available only from the
	// store.
	DownloadInfo

	Prices  map[string]float64
	MustBuy bool

	Publisher StoreAccount

	Media   MediaInfos
	Website string

	StoreURL string

	// The flattended channel map with $track/$risk
	Channels map[string]*ChannelSnapInfo

	// The ordered list of tracks that contain channels
	Tracks []string

	Layout map[string]*Layout

	// The list of common-ids from all apps of the snap
	CommonIDs []string

	// List of system users (usernames) this snap may use. The group of the same
	// name must also exist.
	SystemUsernames map[string]*SystemUsernameInfo
}

// StoreAccount holds information about a store account, for example of snap
// publisher.
type StoreAccount struct {
	ID          string `json:"id"`
	Username    string `json:"username"`
	DisplayName string `json:"display-name"`
	Validation  string `json:"validation,omitempty"`
}

// Layout describes a single element of the layout section.
type Layout struct {
	Snap *Info

	Path     string      `json:"path"`
	Bind     string      `json:"bind,omitempty"`
	BindFile string      `json:"bind-file,omitempty"`
	Type     string      `json:"type,omitempty"`
	User     string      `json:"user,omitempty"`
	Group    string      `json:"group,omitempty"`
	Mode     os.FileMode `json:"mode,omitempty"`
	Symlink  string      `json:"symlink,omitempty"`
}

// String returns a simple textual representation of a layout.
func (l *Layout) String() string {
	var buf bytes.Buffer
	fmt.Fprintf(&buf, "%s: ", l.Path)
	switch {
	case l.Bind != "":
		fmt.Fprintf(&buf, "bind %s", l.Bind)
	case l.BindFile != "":
		fmt.Fprintf(&buf, "bind-file %s", l.BindFile)
	case l.Symlink != "":
		fmt.Fprintf(&buf, "symlink %s", l.Symlink)
	case l.Type != "":
		fmt.Fprintf(&buf, "type %s", l.Type)
	default:
		fmt.Fprintf(&buf, "???")
	}
	if l.User != "root" && l.User != "" {
		fmt.Fprintf(&buf, ", user: %s", l.User)
	}
	if l.Group != "root" && l.Group != "" {
		fmt.Fprintf(&buf, ", group: %s", l.Group)
	}
	if l.Mode != 0755 {
		fmt.Fprintf(&buf, ", mode: %#o", l.Mode)
	}
	return buf.String()
}

// ChannelSnapInfo is the minimum information that can be used to clearly
// distinguish different revisions of the same snap.
type ChannelSnapInfo struct {
	Revision    Revision        `json:"revision"`
	Confinement ConfinementType `json:"confinement"`
	Version     string          `json:"version"`
	Channel     string          `json:"channel"`
	Epoch       Epoch           `json:"epoch"`
	Size        int64           `json:"size"`
	ReleasedAt  time.Time       `json:"released-at"`
}

// InstanceName returns the blessed name of the snap decorated with instance
// key, if any.
func (s *Info) InstanceName() string {
	return InstanceName(s.SnapName(), s.InstanceKey)
}

// SnapName returns the global blessed name of the snap.
func (s *Info) SnapName() string {
	if s.RealName != "" {
		return s.RealName
	}
	return s.SuggestedName
}

// Filename returns the name of the snap with the revision number,
// as used on the filesystem. This is the equivalent of
// filepath.Base(s.MountFile()).
func (s *Info) Filename() string {
	return filepath.Base(s.MountFile())
}

// SnapRevision returns the revision of the snap.
func (s *Info) SnapRevision() Revision {
	return s.Revision
}

// ID implements naming.SnapRef.
func (s *Info) ID() string {
	return s.SnapID
}

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

// Title returns the blessed title for the snap.
func (s *Info) Title() string {
	if s.EditedTitle != "" {
		return s.EditedTitle
	}
	return s.OriginalTitle
}

// Summary returns the blessed summary for the snap.
func (s *Info) Summary() string {
	if s.EditedSummary != "" {
		return s.EditedSummary
	}
	return s.OriginalSummary
}

// Description returns the blessed description for the snap.
func (s *Info) Description() string {
	if s.EditedDescription != "" {
		return s.EditedDescription
	}
	return s.OriginalDescription
}

// Type returns the type of the snap, including additional snap ID check
// for the legacy snapd snap definitions.
func (s *Info) Type() Type {
	if s.SnapType == TypeApp && IsSnapd(s.SnapID) {
		return TypeSnapd
	}
	return s.SnapType
}

// MountDir returns the base directory of the snap where it gets mounted.
func (s *Info) MountDir() string {
	return MountDir(s.InstanceName(), s.Revision)
}

// MountFile returns the path where the snap file that is mounted is installed.
func (s *Info) MountFile() string {
	return MountFile(s.InstanceName(), s.Revision)
}

// HooksDir returns the directory containing the snap's hooks.
func (s *Info) HooksDir() string {
	return HooksDir(s.InstanceName(), s.Revision)
}

// DataDir returns the data directory of the snap.
func (s *Info) DataDir() string {
	return DataDir(s.InstanceName(), s.Revision)
}

// UserDataDir returns the user-specific data directory of the snap.
func (s *Info) UserDataDir(home string) string {
	return UserDataDir(home, s.InstanceName(), s.Revision)
}

// UserCommonDataDir returns the user-specific data directory common across
// revision of the snap.
func (s *Info) UserCommonDataDir(home string) string {
	return UserCommonDataDir(home, s.InstanceName())
}

// CommonDataDir returns the data directory common across revisions of the snap.
func (s *Info) CommonDataDir() string {
	return CommonDataDir(s.InstanceName())
}

// DataHomeDir returns the per user data directory of the snap.
func (s *Info) DataHomeDir() string {
	return filepath.Join(dirs.SnapDataHomeGlob, s.InstanceName(), s.Revision.String())
}

// CommonDataHomeDir returns the per user data directory common across revisions
// of the snap.
func (s *Info) CommonDataHomeDir() string {
	return filepath.Join(dirs.SnapDataHomeGlob, s.InstanceName(), "common")
}

// UserXdgRuntimeDir returns the XDG_RUNTIME_DIR directory of the snap for a
// particular user.
func (s *Info) UserXdgRuntimeDir(euid sys.UserID) string {
	return UserXdgRuntimeDir(euid, s.InstanceName())
}

// XdgRuntimeDirs returns the XDG_RUNTIME_DIR directories for all users of the
// snap.
func (s *Info) XdgRuntimeDirs() string {
	return filepath.Join(dirs.XdgRuntimeDirGlob, fmt.Sprintf("snap.%s", s.InstanceName()))
}

// NeedsDevMode returns whether the snap needs devmode.
func (s *Info) NeedsDevMode() bool {
	return s.Confinement == DevModeConfinement
}

// NeedsClassic  returns whether the snap needs classic confinement consent.
func (s *Info) NeedsClassic() bool {
	return s.Confinement == ClassicConfinement
}

// Services returns a list of the apps that have "daemon" set.
func (s *Info) Services() []*AppInfo {
	svcs := make([]*AppInfo, 0, len(s.Apps))
	for _, app := range s.Apps {
		if !app.IsService() {
			continue
		}
		svcs = append(svcs, app)
	}

	return svcs
}

// ExpandSnapVariables resolves $SNAP, $SNAP_DATA and $SNAP_COMMON inside the
// snap's mount namespace.
func (s *Info) ExpandSnapVariables(path string) string {
	return os.Expand(path, func(v string) string {
		switch v {
		case "SNAP":
			// NOTE: We use dirs.CoreSnapMountDir here as the path used will be
			// always inside the mount namespace snap-confine creates and there
			// we will always have a /snap directory available regardless if the
			// system we're running on supports this or not.
			return filepath.Join(dirs.CoreSnapMountDir, s.SnapName(), s.Revision.String())
		case "SNAP_DATA":
			return DataDir(s.SnapName(), s.Revision)
		case "SNAP_COMMON":
			return CommonDataDir(s.SnapName())
		}
		return ""
	})
}

// InstallDate returns the "install date" of the snap.
//
// If the snap is not active, it'll return a zero time; otherwise
// it'll return the modtime of the "current" symlink. Sneaky.
func (s *Info) InstallDate() time.Time {
	dir, rev := filepath.Split(s.MountDir())
	cur := filepath.Join(dir, "current")
	tag, err := os.Readlink(cur)
	if err == nil && tag == rev {
		if st, err := os.Lstat(cur); err == nil {
			return st.ModTime()
		}
	}
	return time.Time{}
}

// IsActive returns whether this snap revision is active.
func (s *Info) IsActive() bool {
	dir, rev := filepath.Split(s.MountDir())
	cur := filepath.Join(dir, "current")
	tag, err := os.Readlink(cur)
	return err == nil && tag == rev
}

// BadInterfacesSummary returns a summary of the problems of bad plugs
// and slots in the snap.
func BadInterfacesSummary(snapInfo *Info) string {
	inverted := make(map[string][]string)
	for name, reason := range snapInfo.BadInterfaces {
		inverted[reason] = append(inverted[reason], name)
	}
	var buf bytes.Buffer
	fmt.Fprintf(&buf, "snap %q has bad plugs or slots: ", snapInfo.InstanceName())
	reasons := make([]string, 0, len(inverted))
	for reason := range inverted {
		reasons = append(reasons, reason)
	}
	sort.Strings(reasons)
	for _, reason := range reasons {
		names := inverted[reason]
		sort.Strings(names)
		for i, name := range names {
			if i > 0 {
				buf.WriteString(", ")
			}
			buf.WriteString(name)
		}
		fmt.Fprintf(&buf, " (%s); ", reason)
	}
	return strings.TrimSuffix(buf.String(), "; ")
}

// DesktopPrefix returns the prefix string for the desktop files that
// belongs to the given snapInstance. We need to do something custom
// here because a) we need to be compatible with the world before we had
// parallel installs b) we can't just use the usual "_" parallel installs
// separator because that is already used as the separator between snap
// and desktop filename.
func (s *Info) DesktopPrefix() string {
	if s.InstanceKey == "" {
		return s.SnapName()
	}
	// we cannot use the usual "_" separator because that is also used
	// to separate "$snap_$desktopfile"
	return fmt.Sprintf("%s+%s", s.SnapName(), s.InstanceKey)
}

// DownloadInfo contains the information to download a snap.
// It can be marshalled.
type DownloadInfo struct {
	AnonDownloadURL string `json:"anon-download-url,omitempty"`
	DownloadURL     string `json:"download-url,omitempty"`

	Size     int64  `json:"size,omitempty"`
	Sha3_384 string `json:"sha3-384,omitempty"`

	// The server can include information about available deltas for a given
	// snap at a specific revision during refresh. Currently during refresh the
	// server will provide single matching deltas only, from the clients
	// revision to the target revision when available, per requested format.
	Deltas []DeltaInfo `json:"deltas,omitempty"`
}

// DeltaInfo contains the information to download a delta
// from one revision to another.
type DeltaInfo struct {
	FromRevision    int    `json:"from-revision,omitempty"`
	ToRevision      int    `json:"to-revision,omitempty"`
	Format          string `json:"format,omitempty"`
	AnonDownloadURL string `json:"anon-download-url,omitempty"`
	DownloadURL     string `json:"download-url,omitempty"`
	Size            int64  `json:"size,omitempty"`
	Sha3_384        string `json:"sha3-384,omitempty"`
}

// sanity check that Info is a PlaceInfo
var _ PlaceInfo = (*Info)(nil)

// PlugInfo provides information about a plug.
type PlugInfo struct {
	Snap *Info

	Name      string
	Interface string
	Attrs     map[string]interface{}
	Label     string
	Apps      map[string]*AppInfo
	Hooks     map[string]*HookInfo
}

func lookupAttr(attrs map[string]interface{}, path string) (interface{}, bool) {
	var v interface{}
	comps := strings.FieldsFunc(path, func(r rune) bool { return r == '.' })
	if len(comps) == 0 {
		return nil, false
	}
	v = attrs
	for _, comp := range comps {
		m, ok := v.(map[string]interface{})
		if !ok {
			return nil, false
		}
		v, ok = m[comp]
		if !ok {
			return nil, false
		}
	}

	return v, true
}

func getAttribute(snapName string, ifaceName string, attrs map[string]interface{}, key string, val interface{}) error {
	v, ok := lookupAttr(attrs, key)
	if !ok {
		return fmt.Errorf("snap %q does not have attribute %q for interface %q", snapName, key, ifaceName)
	}

	rt := reflect.TypeOf(val)
	if rt.Kind() != reflect.Ptr || val == nil {
		return fmt.Errorf("internal error: cannot get %q attribute of interface %q with non-pointer value", key, ifaceName)
	}

	if reflect.TypeOf(v) != rt.Elem() {
		return fmt.Errorf("snap %q has interface %q with invalid value type for %q attribute", snapName, ifaceName, key)
	}
	rv := reflect.ValueOf(val)
	rv.Elem().Set(reflect.ValueOf(v))

	return nil
}

func (plug *PlugInfo) Attr(key string, val interface{}) error {
	return getAttribute(plug.Snap.InstanceName(), plug.Interface, plug.Attrs, key, val)
}

func (plug *PlugInfo) Lookup(key string) (interface{}, bool) {
	return lookupAttr(plug.Attrs, key)
}

// SecurityTags returns security tags associated with a given plug.
func (plug *PlugInfo) SecurityTags() []string {
	tags := make([]string, 0, len(plug.Apps)+len(plug.Hooks))
	for _, app := range plug.Apps {
		tags = append(tags, app.SecurityTag())
	}
	for _, hook := range plug.Hooks {
		tags = append(tags, hook.SecurityTag())
	}
	sort.Strings(tags)
	return tags
}

// String returns the representation of the plug as snap:plug string.
func (plug *PlugInfo) String() string {
	return fmt.Sprintf("%s:%s", plug.Snap.InstanceName(), plug.Name)
}

func (slot *SlotInfo) Attr(key string, val interface{}) error {
	return getAttribute(slot.Snap.InstanceName(), slot.Interface, slot.Attrs, key, val)
}

func (slot *SlotInfo) Lookup(key string) (interface{}, bool) {
	return lookupAttr(slot.Attrs, key)
}

// SecurityTags returns security tags associated with a given slot.
func (slot *SlotInfo) SecurityTags() []string {
	tags := make([]string, 0, len(slot.Apps))
	for _, app := range slot.Apps {
		tags = append(tags, app.SecurityTag())
	}
	for _, hook := range slot.Hooks {
		tags = append(tags, hook.SecurityTag())
	}
	sort.Strings(tags)
	return tags
}

// String returns the representation of the slot as snap:slot string.
func (slot *SlotInfo) String() string {
	return fmt.Sprintf("%s:%s", slot.Snap.InstanceName(), slot.Name)
}

func gatherDefaultContentProvider(providerSnapsToContentTag map[string][]string, plug *PlugInfo) {
	if plug.Interface == "content" {
		var dprovider string
		if err := plug.Attr("default-provider", &dprovider); err == nil && dprovider != "" {
			// usage can be "snap:slot" but slot
			// is ignored/unused
			name := strings.Split(dprovider, ":")[0]
			var contentTag string
			plug.Attr("content", &contentTag)
			tags := providerSnapsToContentTag[name]
			if tags == nil {
				tags = []string{contentTag}
			} else {
				if !strutil.SortedListContains(tags, contentTag) {
					tags = append(tags, contentTag)
					sort.Strings(tags)
				}
			}
			providerSnapsToContentTag[name] = tags
		}
	}
}

// DefaultContentProviders returns the set of default provider snaps
// requested by the given plugs, mapped to their content tags.
func DefaultContentProviders(plugs []*PlugInfo) (providerSnapsToContentTag map[string][]string) {
	providerSnapsToContentTag = make(map[string][]string)
	for _, plug := range plugs {
		gatherDefaultContentProvider(providerSnapsToContentTag, plug)
	}
	return providerSnapsToContentTag
}

// SlotInfo provides information about a slot.
type SlotInfo struct {
	Snap *Info

	Name      string
	Interface string
	Attrs     map[string]interface{}
	Label     string
	Apps      map[string]*AppInfo
	Hooks     map[string]*HookInfo

	// HotplugKey is a unique key built by the slot's interface
	// using properties of a hotplugged device so that the same
	// slot may be made available if the device is reinserted.
	// It's empty for regular slots.
	HotplugKey HotplugKey
}

// SocketInfo provides information on application sockets.
type SocketInfo struct {
	App *AppInfo

	Name         string
	ListenStream string
	SocketMode   os.FileMode
}

// TimerInfo provides information on application timer.
type TimerInfo struct {
	App *AppInfo

	Timer string
}

// StopModeType is the type for the "stop-mode:" of a snap app
type StopModeType string

// KillAll returns if the stop-mode means all processes should be killed
// when the service is stopped or just the main process.
func (st StopModeType) KillAll() bool {
	return string(st) == "" || strings.HasSuffix(string(st), "-all")
}

// KillSignal returns the signal that should be used to kill the process
// (or an empty string if no signal is needed).
func (st StopModeType) KillSignal() string {
	if st.Validate() != nil || st == "" {
		return ""
	}
	return strings.ToUpper(strings.TrimSuffix(string(st), "-all"))
}

// Validate ensures that the StopModeType has an valid value.
func (st StopModeType) Validate() error {
	switch st {
	case "", "sigterm", "sigterm-all", "sighup", "sighup-all", "sigusr1", "sigusr1-all", "sigusr2", "sigusr2-all":
		// valid
		return nil
	}
	return fmt.Errorf(`"stop-mode" field contains invalid value %q`, st)
}

// AppInfo provides information about an app.
type AppInfo struct {
	Snap *Info

	Name          string
	LegacyAliases []string // FIXME: eventually drop this
	Command       string
	CommandChain  []string
	CommonID      string

	Daemon          string
	DaemonScope     DaemonScope
	StopTimeout     timeout.Timeout
	StartTimeout    timeout.Timeout
	WatchdogTimeout timeout.Timeout
	StopCommand     string
	ReloadCommand   string
	PostStopCommand string
	RestartCond     RestartCondition
	RestartDelay    timeout.Timeout
	Completer       string
	RefreshMode     string
	StopMode        StopModeType

	// TODO: this should go away once we have more plumbing and can change
	// things vs refactor
	// https://github.com/snapcore/snapd/pull/794#discussion_r58688496
	BusName     string
	ActivatesOn []*SlotInfo

	Plugs   map[string]*PlugInfo
	Slots   map[string]*SlotInfo
	Sockets map[string]*SocketInfo

	Environment strutil.OrderedMap

	// list of other service names that this service will start after or
	// before
	After  []string
	Before []string

	Timer *TimerInfo

	Autostart string
}

// ScreenshotInfo provides information about a screenshot.
type ScreenshotInfo struct {
	URL    string `json:"url,omitempty"`
	Width  int64  `json:"width,omitempty"`
	Height int64  `json:"height,omitempty"`
	Note   string `json:"note,omitempty"`
}

type MediaInfo struct {
	Type   string `json:"type"`
	URL    string `json:"url"`
	Width  int64  `json:"width,omitempty"`
	Height int64  `json:"height,omitempty"`
}

type MediaInfos []MediaInfo

func (mis MediaInfos) IconURL() string {
	for _, mi := range mis {
		if mi.Type == "icon" {
			return mi.URL
		}
	}
	return ""
}

// HookInfo provides information about a hook.
type HookInfo struct {
	Snap *Info

	Name  string
	Plugs map[string]*PlugInfo
	Slots map[string]*SlotInfo

	Environment  strutil.OrderedMap
	CommandChain []string

	Explicit bool
}

// SystemUsernameInfo provides information about a system username (ie, a
// UNIX user and group with the same name). The scope defines visibility of the
// username wrt the snap and the system. Defined scopes:
// - shared    static, snapd-managed user/group shared between host and all
//             snaps
// - private   static, snapd-managed user/group private to a particular snap
//             (currently not implemented)
// - external  dynamic user/group shared between host and all snaps (currently
//             not implented)
type SystemUsernameInfo struct {
	Name  string
	Scope string
	Attrs map[string]interface{}
}

// File returns the path to the *.socket file
func (socket *SocketInfo) File() string {
	return filepath.Join(socket.App.serviceDir(), socket.App.SecurityTag()+"."+socket.Name+".socket")
}

// File returns the path to the *.timer file
func (timer *TimerInfo) File() string {
	return filepath.Join(timer.App.serviceDir(), timer.App.SecurityTag()+".timer")
}

func (app *AppInfo) String() string {
	return JoinSnapApp(app.Snap.InstanceName(), app.Name)
}

// SecurityTag returns application-specific security tag.
//
// Security tags are used by various security subsystems as "profile names" and
// sometimes also as a part of the file name.
func (app *AppInfo) SecurityTag() string {
	return AppSecurityTag(app.Snap.InstanceName(), app.Name)
}

// DesktopFile returns the path to the installed optional desktop file for the
// application.
func (app *AppInfo) DesktopFile() string {
	return filepath.Join(dirs.SnapDesktopFilesDir, fmt.Sprintf("%s_%s.desktop", app.Snap.DesktopPrefix(), app.Name))
}

// WrapperPath returns the path to wrapper invoking the app binary.
func (app *AppInfo) WrapperPath() string {
	return filepath.Join(dirs.SnapBinariesDir, JoinSnapApp(app.Snap.InstanceName(), app.Name))
}

// CompleterPath returns the path to the completer snippet for the app binary.
func (app *AppInfo) CompleterPath() string {
	return filepath.Join(dirs.CompletersDir, JoinSnapApp(app.Snap.InstanceName(), app.Name))
}

func (app *AppInfo) launcherCommand(command string) string {
	if command != "" {
		command = " " + command
	}
	if app.Name == app.Snap.SnapName() {
		return fmt.Sprintf("/usr/bin/snap run%s %s", command, app.Snap.InstanceName())
	}
	return fmt.Sprintf("/usr/bin/snap run%s %s.%s", command, app.Snap.InstanceName(), app.Name)
}

// LauncherCommand returns the launcher command line to use when invoking the
// app binary.
func (app *AppInfo) LauncherCommand() string {
	if app.Timer != nil {
		return app.launcherCommand(fmt.Sprintf("--timer=%q", app.Timer.Timer))
	}
	return app.launcherCommand("")
}

// LauncherStopCommand returns the launcher command line to use when invoking
// the app stop command binary.
func (app *AppInfo) LauncherStopCommand() string {
	return app.launcherCommand("--command=stop")
}

// LauncherReloadCommand returns the launcher command line to use when invoking
// the app stop command binary.
func (app *AppInfo) LauncherReloadCommand() string {
	return app.launcherCommand("--command=reload")
}

// LauncherPostStopCommand returns the launcher command line to use when
// invoking the app post-stop command binary.
func (app *AppInfo) LauncherPostStopCommand() string {
	return app.launcherCommand("--command=post-stop")
}

// ServiceName returns the systemd service name for the daemon app.
func (app *AppInfo) ServiceName() string {
	return app.SecurityTag() + ".service"
}

func (app *AppInfo) serviceDir() string {
	switch app.DaemonScope {
	case SystemDaemon:
		return dirs.SnapServicesDir
	case UserDaemon:
		return dirs.SnapUserServicesDir
	default:
		panic("unknown daemon scope")
	}
}

// ServiceFile returns the systemd service file path for the daemon app.
func (app *AppInfo) ServiceFile() string {
	return filepath.Join(app.serviceDir(), app.ServiceName())
}

// IsService returns whether app represents a daemon/service.
func (app *AppInfo) IsService() bool {
	return app.Daemon != ""
}

// EnvChain returns the chain of environment overrides, possibly with
// expandable $ vars, specific for the app.
func (app *AppInfo) EnvChain() []osutil.ExpandableEnv {
	return []osutil.ExpandableEnv{
		{OrderedMap: &app.Snap.Environment},
		{OrderedMap: &app.Environment},
	}
}

// SecurityTag returns the hook-specific security tag.
//
// Security tags are used by various security subsystems as "profile names" and
// sometimes also as a part of the file name.
func (hook *HookInfo) SecurityTag() string {
	return HookSecurityTag(hook.Snap.InstanceName(), hook.Name)
}

// EnvChain returns the chain of environment overrides, possibly with
// expandable $ vars, specific for the hook.
func (hook *HookInfo) EnvChain() []osutil.ExpandableEnv {
	return []osutil.ExpandableEnv{
		{OrderedMap: &hook.Snap.Environment},
		{OrderedMap: &hook.Environment},
	}
}

func infoFromSnapYamlWithSideInfo(meta []byte, si *SideInfo, strk *scopedTracker) (*Info, error) {
	info, err := infoFromSnapYaml(meta, strk)
	if err != nil {
		return nil, err
	}

	if si != nil {
		info.SideInfo = *si
	}

	return info, nil
}

// BrokenSnapError describes an error that refers to a snap that warrants the
// "broken" note.
type BrokenSnapError interface {
	error
	Broken() string
}

type NotFoundError struct {
	Snap     string
	Revision Revision
	// Path encodes the path that triggered the not-found error. It may refer to
	// a file inside the snap or to the snap file itself.
	Path string
}

func (e NotFoundError) Error() string {
	if e.Path != "" {
		return fmt.Sprintf("cannot find installed snap %q at revision %s: missing file %s", e.Snap, e.Revision, e.Path)
	}
	return fmt.Sprintf("cannot find installed snap %q at revision %s", e.Snap, e.Revision)
}

func (e NotFoundError) Broken() string {
	return e.Error()
}

type invalidMetaError struct {
	Snap     string
	Revision Revision
	Msg      string
}

func (e invalidMetaError) Error() string {
	return fmt.Sprintf("cannot use installed snap %q at revision %s: %s", e.Snap, e.Revision, e.Msg)
}

func (e invalidMetaError) Broken() string {
	return e.Error()
}

func MockSanitizePlugsSlots(f func(snapInfo *Info)) (restore func()) {
	old := SanitizePlugsSlots
	SanitizePlugsSlots = f
	return func() { SanitizePlugsSlots = old }
}

var SanitizePlugsSlots = func(snapInfo *Info) {
	panic("SanitizePlugsSlots function not set")
}

// ReadInfo reads the snap information for the installed snap with the given
// name and given side-info.
func ReadInfo(name string, si *SideInfo) (*Info, error) {
	snapYamlFn := filepath.Join(MountDir(name, si.Revision), "meta", "snap.yaml")
	meta, err := ioutil.ReadFile(snapYamlFn)
	if os.IsNotExist(err) {
		return nil, &NotFoundError{Snap: name, Revision: si.Revision, Path: snapYamlFn}
	}
	if err != nil {
		return nil, err
	}

	strk := new(scopedTracker)
	info, err := infoFromSnapYamlWithSideInfo(meta, si, strk)
	if err != nil {
		return nil, &invalidMetaError{Snap: name, Revision: si.Revision, Msg: err.Error()}
	}

	_, instanceKey := SplitInstanceName(name)
	info.InstanceKey = instanceKey

	err = addImplicitHooks(info)
	if err != nil {
		return nil, &invalidMetaError{Snap: name, Revision: si.Revision, Msg: err.Error()}
	}

	bindImplicitHooks(info, strk)

	mountFile := MountFile(name, si.Revision)
	st, err := os.Lstat(mountFile)
	if os.IsNotExist(err) {
		// This can happen when "snap try" mode snap is moved around. The mount
		// is still in place (it's a bind mount, it doesn't care about the
		// source moving) but the symlink in /var/lib/snapd/snaps is now
		// dangling.
		return nil, &NotFoundError{Snap: name, Revision: si.Revision, Path: mountFile}
	}
	if err != nil {
		return nil, err
	}
	// If the file is a regular file than it must be a squashfs file that is
	// used as the backing store for the snap. The size of that file is the
	// size of the snap.
	if st.Mode().IsRegular() {
		info.Size = st.Size()
	}

	return info, nil
}

// ReadCurrentInfo reads the snap information from the installed snap in
// 'current' revision
func ReadCurrentInfo(snapName string) (*Info, error) {
	curFn := filepath.Join(dirs.SnapMountDir, snapName, "current")
	realFn, err := os.Readlink(curFn)
	if err != nil {
		return nil, fmt.Errorf("cannot find current revision for snap %s: %s", snapName, err)
	}
	rev := filepath.Base(realFn)
	revision, err := ParseRevision(rev)
	if err != nil {
		return nil, fmt.Errorf("cannot read revision %s: %s", rev, err)
	}

	return ReadInfo(snapName, &SideInfo{Revision: revision})
}

// ReadInfoFromSnapFile reads the snap information from the given Container and
// completes it with the given side-info if this is not nil.
func ReadInfoFromSnapFile(snapf Container, si *SideInfo) (*Info, error) {
	meta, err := snapf.ReadFile("meta/snap.yaml")
	if err != nil {
		return nil, err
	}

	strk := new(scopedTracker)
	info, err := infoFromSnapYamlWithSideInfo(meta, si, strk)
	if err != nil {
		return nil, err
	}

	info.Size, err = snapf.Size()
	if err != nil {
		return nil, err
	}

	err = addImplicitHooksFromContainer(info, snapf)
	if err != nil {
		return nil, err
	}

	bindImplicitHooks(info, strk)

	err = Validate(info)
	if err != nil {
		return nil, err
	}

	return info, nil
}

// InstallDate returns the "install date" of the snap.
//
// If the snap is not active, it'll return a zero time; otherwise it'll return
// the modtime of the "current" symlink.
func InstallDate(name string) time.Time {
	cur := filepath.Join(dirs.SnapMountDir, name, "current")
	if st, err := os.Lstat(cur); err == nil {
		return st.ModTime()
	}
	return time.Time{}
}

// SplitSnapApp will split a string of the form `snap.app` into the `snap` and
// the `app` part. It also deals with the special case of snapName == appName.
func SplitSnapApp(snapApp string) (snap, app string) {
	l := strings.SplitN(snapApp, ".", 2)
	if len(l) < 2 {
		return l[0], InstanceSnap(l[0])
	}
	return l[0], l[1]
}

// JoinSnapApp produces a full application wrapper name from the `snap` and the
// `app` part. It also deals with the special case of snapName == appName.
func JoinSnapApp(snap, app string) string {
	storeName, instanceKey := SplitInstanceName(snap)
	if storeName == app {
		return InstanceName(app, instanceKey)
	}
	return fmt.Sprintf("%s.%s", snap, app)
}

// InstanceSnap splits the instance name and returns the name of the snap.
func InstanceSnap(instanceName string) string {
	snapName, _ := SplitInstanceName(instanceName)
	return snapName
}

// SplitInstanceName splits the instance name and returns the snap name and the
// instance key.
func SplitInstanceName(instanceName string) (snapName, instanceKey string) {
	split := strings.SplitN(instanceName, "_", 2)
	snapName = split[0]
	if len(split) > 1 {
		instanceKey = split[1]
	}
	return snapName, instanceKey
}

// InstanceName takes the snap name and the instance key and returns an instance
// name of the snap.
func InstanceName(snapName, instanceKey string) string {
	if instanceKey != "" {
		return fmt.Sprintf("%s_%s", snapName, instanceKey)
	}
	return snapName
}

// ByType supports sorting the given slice of snap info by types. The most
// important types will come first.
type ByType []*Info

func (r ByType) Len() int      { return len(r) }
func (r ByType) Swap(i, j int) { r[i], r[j] = r[j], r[i] }
func (r ByType) Less(i, j int) bool {
	return r[i].Type().SortsBefore(r[j].Type())
}

// SortServices sorts the apps based on their Before and After specs, such that
// starting the services in the returned ordering will satisfy all specs.
func SortServices(apps []*AppInfo) (sorted []*AppInfo, err error) {
	nameToApp := make(map[string]*AppInfo, len(apps))
	for _, app := range apps {
		nameToApp[app.Name] = app
	}

	// list of successors of given app
	successors := make(map[string][]*AppInfo, len(apps))
	// count of predecessors (i.e. incoming edges) of given app
	predecessors := make(map[string]int, len(apps))

	for _, app := range apps {
		for _, other := range app.After {
			predecessors[app.Name]++
			successors[other] = append(successors[other], app)
		}
		for _, other := range app.Before {
			predecessors[other]++
			successors[app.Name] = append(successors[app.Name], nameToApp[other])
		}
	}

	// list of apps without predecessors (no incoming edges)
	queue := make([]*AppInfo, 0, len(apps))
	for _, app := range apps {
		if predecessors[app.Name] == 0 {
			queue = append(queue, app)
		}
	}

	// Kahn:
	// see https://dl.acm.org/citation.cfm?doid=368996.369025
	//     https://en.wikipedia.org/wiki/Topological_sorting%23Kahn%27s_algorithm
	//
	// Apps without predecessors are 'top' nodes. On each iteration, take
	// the next 'top' node, and decrease the predecessor count of each
	// successor app. Once that successor app has no more predecessors, take
	// it out of the predecessors set and add it to the queue of 'top'
	// nodes.
	for len(queue) > 0 {
		app := queue[0]
		queue = queue[1:]
		for _, successor := range successors[app.Name] {
			predecessors[successor.Name]--
			if predecessors[successor.Name] == 0 {
				delete(predecessors, successor.Name)
				queue = append(queue, successor)
			}
		}
		sorted = append(sorted, app)
	}

	if len(predecessors) != 0 {
		// apps with predecessors unaccounted for are a part of
		// dependency cycle
		unsatisifed := bytes.Buffer{}
		for name := range predecessors {
			if unsatisifed.Len() > 0 {
				unsatisifed.WriteString(", ")
			}
			unsatisifed.WriteString(name)
		}
		return nil, fmt.Errorf("applications are part of a before/after cycle: %s", unsatisifed.String())
	}
	return sorted, nil
}

// AppInfoBySnapApp supports sorting the given slice of app infos by
// (instance name, app name).
type AppInfoBySnapApp []*AppInfo

func (a AppInfoBySnapApp) Len() int      { return len(a) }
func (a AppInfoBySnapApp) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
func (a AppInfoBySnapApp) Less(i, j int) bool {
	iName := a[i].Snap.InstanceName()
	jName := a[j].Snap.InstanceName()
	if iName == jName {
		return a[i].Name < a[j].Name
	}
	return iName < jName
}