seedwriter: check if the optionSnap channel is same as model assert

If we define `presence: optional` for an app snap in the model assertion, and the
default channel is `24/stable` for example, then when we pass `--snap=SNAP=24/stable`
to `snap prepare-image` with a model assertion that has model grade higher than
`dangerous`, I will get an error `error: cannot override channels, add devmode snaps,
local snaps, or extra snaps with a model of grade higher than dangerous`

However, I am expecting that snapd should check whether the channel for option snap
is same as the channel defined in model assertion, maybe snapd should only return
error if channel is not the same and the model grade is higher than dangerous

Signed-off-by: Aristo Chen <[email protected]>

seed/seedwriter/writer.go

@@ -427,9 +427,26 @@ func (w *Writer) SetOptionsSnaps(optSnaps []*OptionsSnap) error {
 			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 {
+
+			// Check whether the channel defined in the model assertion
+			// is same as the option snap(--snap=SNAP_NAME=CHANNEL)
+			modSnaps, err := w.modSnaps()
+			if err != nil {
 				return err
 			}
+			for _, modSnap := range modSnaps {
+				if sn.Name != modSnap.Name {
+					continue
+				}
+
+				if modSnap.Presence == "optional" && sn.Channel == modSnap.DefaultChannel {
+					continue
+				} else {
+					if err := w.policy.checkSnapChannel(ch, whichSnap); err != nil {
+						return err
+					}
+				}
+			}
 		}
 		if local {
 			if w.localSnaps == nil {

seed/seedwriter/writer_test.go

@@ -3316,6 +3316,102 @@ func (s *writerSuite) TestSnapsToDownloadCore20OptionalSnaps(c *C) {
 	c.Check(snaps[5].SnapName(), Equals, "optional20-b")
 }
 
+func (s *writerSuite) TestSnapsToDownloadCore20OptionalSnapsWithSameChannel(c *C) {
+	model := s.Brands.Model("my-brand", "my-model", map[string]interface{}{
+		"display-name": "my model",
+		"architecture": "amd64",
+		"base":         "core20",
+		"snaps": []interface{}{
+			map[string]interface{}{
+				"name":            "pc-kernel",
+				"id":              s.AssertedSnapID("pc-kernel"),
+				"type":            "kernel",
+				"default-channel": "20",
+			},
+			map[string]interface{}{
+				"name":            "pc",
+				"id":              s.AssertedSnapID("pc"),
+				"type":            "gadget",
+				"default-channel": "20",
+			},
+			map[string]interface{}{
+				"name":            "optional20-a",
+				"id":              s.AssertedSnapID("optional20-a"),
+				"presence":        "optional",
+				"default-channel": "20",
+			},
+		},
+	})
+
+	// validity
+	c.Assert(model.Grade(), Equals, asserts.ModelSigned)
+
+	s.makeSnap(c, "snapd", "")
+	s.makeSnap(c, "core20", "")
+	s.makeSnap(c, "pc-kernel=20", "")
+	s.makeSnap(c, "pc=20", "")
+	s.makeSnap(c, "optional20-a", "developerid")
+
+	s.opts.Label = "20191122"
+	w, err := seedwriter.New(model, s.opts)
+	c.Assert(err, IsNil)
+
+	err = w.SetOptionsSnaps([]*seedwriter.OptionsSnap{{Name: "optional20-a", Channel: "20"}})
+	c.Assert(err, IsNil)
+
+	err = w.Start(s.db, s.rf)
+	c.Assert(err, IsNil)
+
+	snaps, err := w.SnapsToDownload()
+	c.Assert(err, IsNil)
+	c.Check(snaps, HasLen, 5)
+	c.Check(snaps[4].SnapName(), Equals, "optional20-a")
+}
+
+func (s *writerSuite) TestSnapsToDownloadCore20OptionalSnapsWithDifferentChannel(c *C) {
+	model := s.Brands.Model("my-brand", "my-model", map[string]interface{}{
+		"display-name": "my model",
+		"architecture": "amd64",
+		"base":         "core20",
+		"snaps": []interface{}{
+			map[string]interface{}{
+				"name":            "pc-kernel",
+				"id":              s.AssertedSnapID("pc-kernel"),
+				"type":            "kernel",
+				"default-channel": "20",
+			},
+			map[string]interface{}{
+				"name":            "pc",
+				"id":              s.AssertedSnapID("pc"),
+				"type":            "gadget",
+				"default-channel": "20",
+			},
+			map[string]interface{}{
+				"name":            "optional20-a",
+				"id":              s.AssertedSnapID("optional20-a"),
+				"presence":        "optional",
+				"default-channel": "20",
+			},
+		},
+	})
+
+	// validity
+	c.Assert(model.Grade(), Equals, asserts.ModelSigned)
+
+	s.makeSnap(c, "snapd", "")
+	s.makeSnap(c, "core20", "")
+	s.makeSnap(c, "pc-kernel=20", "")
+	s.makeSnap(c, "pc=20", "")
+	s.makeSnap(c, "optional20-a", "developerid")
+
+	s.opts.Label = "20191122"
+	w, err := seedwriter.New(model, s.opts)
+	c.Assert(err, IsNil)
+
+	err = w.SetOptionsSnaps([]*seedwriter.OptionsSnap{{Name: "optional20-a", Channel: "22"}})
+	c.Check(err, ErrorMatches, `cannot override channels, add devmode snaps, local snaps, or extra snaps with a model of grade higher than dangerous`)
+}
+
 func (s *writerSuite) TestSeedSnapsWriteMetaCore20ExtraSnaps(c *C) {
 	model := s.Brands.Model("my-brand", "my-model", map[string]interface{}{
 		"display-name": "my model",