Expand $SDK variable in mount plug targets

docs/reference/definition-files/workshop-definition.rst

@@ -267,6 +267,7 @@ Mount interface plugs can't belong to the :ref:`system SDK <ref_system_sdk>`.
 They are described by the following attributes:
 
 .. @artefact mount interface attributes
+.. @artefact $SDK
 
 .. list-table::
    :header-rows: 1
@@ -280,7 +281,9 @@ They are described by the following attributes:
    * - :samp:`workshop-target` (required)
      - string
      - A path inside the workshop
-       to be used as the plug's target directory.
+       to be used as the plug's target directory;
+       :file:`/project/` or :envvar:`$SDK`-based paths can be used;
+       :envvar:`$SDK` expands into the SDK's installation path in the workshop.
 
    * - :samp:`mode`
      - integer
@@ -332,7 +335,7 @@ They are described by the following attributes:
      - string
      - A path inside the workshop
        to be used as the slot's source directory;
-       :file:`/project` or :envvar:`$SDK`-based paths can be used;
+       :file:`/project/` or :envvar:`$SDK`-based paths can be used;
        :envvar:`$SDK` expands into the SDK's installation path in the workshop.
 
 

docs/reference/sdks.rst

@@ -239,7 +239,8 @@ Mount interface
 
 .. @artefact mount interface
 
-A mount plug in the definition must specify the plug name, the interface, the target directory, its permissions and ownership, and optionally whether to be read-only:
+A mount plug in the definition must specify the plug name, the interface, and the target directory.
+The plug can specify permissions and ownership for the target, and whether it is read-only:
 
 .. code-block:: yaml
    :caption: sdk.yaml
@@ -254,9 +255,12 @@ A mount plug in the definition must specify the plug name, the interface, the ta
         gid: <GROUP ID> # optional
         read-only: <true | false> # optional
 
+.. @artefact $SDK
 
 This mounts a directory automatically created by |ws_markup| on the host
 to the :samp:`workshop-target` directory.
+The :envvar:`$SDK` variable can be used to refer to the SDK installation path
+inside the workshop.
 The host directory will be created under the path
 designated by the :envvar:`$XDG_DATA_HOME` variable.
 The workshop directory will be created using the given :samp:`mode`, :samp:`uid`, and :samp:`gid`.

internal/interfaces/builtin/mount.go

@@ -93,16 +93,9 @@ func (iface *mountInterface) BeforePreparePlug(plug *sdk.PlugInfo) error {
 		}
 	}
 
-	target, ok := plug.Attrs["workshop-target"].(string)
-	if !ok || len(target) == 0 {
-		return fmt.Errorf("mount plug must contain target path")
-	}
-
-	if !filepath.IsAbs(target) {
-		return fmt.Errorf(`mount plug "workshop-target" must be absolute: %q`, target)
-	}
-	if filepath.Clean(target) != target {
-		return fmt.Errorf(`mount plug "workshop-target" is not clean: %q`, target)
+	path, err := parseMountPath(plug.Attrs, "plug", "workshop-target", plug.Sdk.Name)
+	if err != nil {
+		return err
 	}
 
 	if _, err := parseBool(plug.Attrs, "read-only", false); err != nil {
@@ -111,7 +104,7 @@ func (iface *mountInterface) BeforePreparePlug(plug *sdk.PlugInfo) error {
 
 	var fallbackUid, fallbackGid int64
 	for _, prefix := range []string{"/home/workshop", "/project", "/run/user/1000"} {
-		if target == prefix || strings.HasPrefix(target, prefix+string(filepath.Separator)) {
+		if path == prefix || strings.HasPrefix(path, prefix+string(filepath.Separator)) {
 			fallbackUid = workshop.Uid
 			fallbackGid = workshop.Gid
 			break
@@ -206,20 +199,43 @@ func (iface *mountInterface) BeforePrepareSlot(slot *sdk.SlotInfo) error {
 			return fmt.Errorf(`unknown attribute for mount interface slot: %q`, name)
 		}
 	}
-	source, exist := slot.Attrs["workshop-source"]
+
+	_, err := parseMountPath(slot.Attrs, "slot", "workshop-source", slot.Sdk.Name)
+	return err
+}
+
+func parseMountPath(attrs map[string]interface{}, kind, key string, sk string) (string, error) {
+	attr, exist := attrs[key]
 	if !exist {
-		return fmt.Errorf("mount slot must contain source path")
+		return "", fmt.Errorf("mount %s must contain %q", kind, key)
 	}
-	path, ok := source.(string)
+	template, ok := attr.(string)
 	if !ok {
-		return fmt.Errorf(`mount slot "workshop-source" is not a string (found %T)`, source)
+		return "", fmt.Errorf(`mount %s %q is not a string (found %T)`, kind, key, attr)
+	}
+
+	path, err := expandMountPath(template, sk)
+	if err != nil {
+		return "", err
 	}
 
+	if !filepath.IsAbs(path) {
+		return "", fmt.Errorf(`mount %s %q must be absolute: %q`, kind, key, path)
+	}
+	if filepath.Clean(path) != path {
+		return "", fmt.Errorf(`mount %s %q is not clean: %q`, kind, key, path)
+	}
+
+	attrs[key] = path
+	return path, nil
+}
+
+func expandMountPath(template string, sk string) (string, error) {
 	var err error
-	path = os.Expand(path, func(s string) string {
+	path := os.Expand(template, func(s string) string {
 		switch s {
 		case "SDK":
-			return sdk.SdkDir(slot.Sdk.Name)
+			return sdk.SdkDir(sk)
 		case "$":
 			// Unescape $$ -> $.
 			return "$"
@@ -228,19 +244,11 @@ func (iface *mountInterface) BeforePrepareSlot(slot *sdk.SlotInfo) error {
 			return ""
 		}
 	})
-	if err != nil {
-		return err
-	}
 
-	if !filepath.IsAbs(path) {
-		return fmt.Errorf(`mount slot "workshop-source" must be absolute: %q`, path)
-	}
-	if filepath.Clean(path) != path {
-		return fmt.Errorf(`mount slot "workshop-source" is not clean: %q`, path)
+	if err != nil {
+		return "", err
 	}
-
-	slot.Attrs["workshop-source"] = path
-	return nil
+	return path, nil
 }
 
 func (iface *mountInterface) setPlugAttrs(mount *workshop.Mount, plug *interfaces.ConnectedPlug) error {

internal/interfaces/builtin/mount_test.go

@@ -224,6 +224,47 @@ plugs:
 	c.Check(plug.Attrs["gid"], check.Equals, int64(0))
 }
 
+func (s *mountSuite) TestSanitizePlugSDK(c *check.C) {
+	const mockSdkYaml = `name: mount-slot-sdk
+base: ubuntu@22.04
+plugs:
+ mount-plug:
+  interface: mount
+  workshop-target: $SDK
+`
+	info := sdk.MockInfo(c, mockSdkYaml, s.projectId, "ws")
+	plug := info.Plugs["mount-plug"]
+	c.Assert(interfaces.BeforePreparePlug(s.iface, plug), check.IsNil)
+	c.Check(plug.Attrs["workshop-target"], check.Equals, "/var/lib/workshop/sdk/mount-slot-sdk")
+}
+
+func (s *mountSuite) TestSanitizePlugSDKSubdir(c *check.C) {
+	const mockSdkYaml = `name: mount-slot-sdk
+base: ubuntu@22.04
+plugs:
+ mount-plug:
+  interface: mount
+  workshop-target: ${SDK}/lib/x86_64-linux-gnu
+`
+	info := sdk.MockInfo(c, mockSdkYaml, s.projectId, "ws")
+	plug := info.Plugs["mount-plug"]
+	c.Assert(interfaces.BeforePreparePlug(s.iface, plug), check.IsNil)
+	c.Check(plug.Attrs["workshop-target"], check.Equals, "/var/lib/workshop/sdk/mount-slot-sdk/lib/x86_64-linux-gnu")
+}
+
+func (s *mountSuite) TestSanitizePlugSDKUnclean(c *check.C) {
+	const mockSdkYaml = `name: mount-slot-sdk
+base: ubuntu@22.04
+plugs:
+ mount-plug:
+  interface: mount
+  workshop-target: $SDK/
+`
+	info := sdk.MockInfo(c, mockSdkYaml, s.projectId, "ws")
+	plug := info.Plugs["mount-plug"]
+	c.Assert(interfaces.BeforePreparePlug(s.iface, plug), check.ErrorMatches, `mount plug "workshop-target" is not clean: "/var/lib/workshop/sdk/mount-slot-sdk/"`)
+}
+
 func (s *mountSuite) TestSanitizePlugSimpleNoTarget(c *check.C) {
 	const mockSdkYaml = `name: mount-slot-sdk
 base: ubuntu@22.04
@@ -233,7 +274,7 @@ plugs:
 `
 	info := sdk.MockInfo(c, mockSdkYaml, s.projectId, "ws")
 	plug := info.Plugs["mount-plug"]
-	c.Assert(interfaces.BeforePreparePlug(s.iface, plug), check.ErrorMatches, "mount plug must contain target path")
+	c.Assert(interfaces.BeforePreparePlug(s.iface, plug), check.ErrorMatches, `mount plug must contain "workshop-target"`)
 }
 
 func (s *mountSuite) TestSanitizePlugSimpleTargetRelative(c *check.C) {
@@ -452,7 +493,7 @@ slots:
 `
 	info := sdk.MockInfo(c, mockSdkYaml, s.projectId, "ws")
 	slot := info.Slots["mount-slot"]
-	c.Assert(interfaces.BeforePrepareSlot(s.iface, slot), check.ErrorMatches, "mount slot must contain source path")
+	c.Assert(interfaces.BeforePrepareSlot(s.iface, slot), check.ErrorMatches, `mount slot must contain "workshop-source"`)
 }
 
 func (s *mountSuite) TestSanitizeSlotAbsSourceFails(c *check.C) {

internal/interfaces/builtin/tunnel.go

@@ -278,20 +278,20 @@ func (iface *tunnelInterface) MountConnectedPlug(spec *lxd_device.Specification,
 	}
 	switch entry.Direction {
 	case workshop.HostToWorkshop:
-		if err := expandPath(&entry.Listen, spec.User); err != nil {
+		if err := expandProxyPath(&entry.Listen, spec.User); err != nil {
 			return err
 		}
 		if err := authorizePath(entry.Listen, spec.User); err != nil {
 			return err
 		}
-		if err := expandPath(&entry.Connect, &workshop.User); err != nil {
+		if err := expandProxyPath(&entry.Connect, &workshop.User); err != nil {
 			return err
 		}
 	case workshop.WorkshopToHost:
-		if err := expandPath(&entry.Listen, &workshop.User); err != nil {
+		if err := expandProxyPath(&entry.Listen, &workshop.User); err != nil {
 			return err
 		}
-		if err := expandPath(&entry.Connect, spec.User); err != nil {
+		if err := expandProxyPath(&entry.Connect, spec.User); err != nil {
 			return err
 		}
 	}
@@ -366,7 +366,7 @@ func checkListenPort(listen workshop.ProxyTarget, direction workshop.ProxyDirect
 	return nil
 }
 
-func expandPath(target *workshop.ProxyTarget, user *user.User) error {
+func expandProxyPath(target *workshop.ProxyTarget, user *user.User) error {
 	if target.Protocol != "unix" {
 		return nil
 	}