feat(config): add assignment inheritance via 'extends' with abstract bases

config/assignment.go

@@ -27,6 +27,16 @@ func GetAssignmentConfig(course, assignment string, onlyForStudentsOrGroups ...s
 			Msg("configuration for assignment not found")
 	}
 
+	// Abstract assignments are bases for `extends` only and must not be used
+	// directly. Checked before resolving inheritance so the (own) flag is read,
+	// not an inherited one.
+	if assignmentIsAbstract(course, assignment) {
+		log.Fatal().
+			Str("course", course).
+			Str("assignment", assignment).
+			Msg("assignment is abstract (a base for 'extends') and cannot be used directly")
+	}
+
 	// Resolve `extends` inheritance before reading any fields so the rest of
 	// the config loading sees the merged, effective configuration.
 	resolveAssignmentInheritance(course, assignment)

config/inheritance.go

@@ -16,6 +16,19 @@ import (
 //	  assignmentpath: blatt-10   # ... and override only what differs
 const inheritKey = "extends"
 
+// abstractKey marks an assignment as an abstract base that exists only to be
+// inherited from via `extends`. Abstract assignments cannot be operated on
+// directly (generate, protect, clone, …). The flag itself is never inherited.
+const abstractKey = "abstract"
+
+// assignmentIsAbstract reports whether the assignment declares `abstract: true`
+// on itself. It reads the assignment's own value and must be called before
+// resolveAssignmentInheritance writes the merged config back into viper, so an
+// inherited abstract flag never makes a concrete child abstract.
+func assignmentIsAbstract(course, assignment string) bool {
+	return viper.GetBool(course + "." + assignment + "." + abstractKey)
+}
+
 // resolveAssignmentInheritance resolves the `extends` chain for the given
 // assignment and writes the merged, effective configuration back into viper at
 // the assignment key. After this call the rest of the config loading reads the
@@ -35,7 +48,9 @@ func resolveAssignmentInheritance(course, assignment string) {
 	}
 
 	merged := mergedAssignmentMap(course, assignment, map[string]bool{})
+	// Meta keys must never leak into the effective config or be inherited.
 	delete(merged, inheritKey)
+	delete(merged, abstractKey)
 	viper.Set(assignmentKey, merged)
 }
 

config/inheritance_test.go

@@ -180,6 +180,42 @@ func TestInheritance_NoExtendsIsUnaffected(t *testing.T) {
 	}
 }
 
+func TestAssignmentIsAbstract(t *testing.T) {
+	baseAssignment(t)
+
+	// blatt09 (from baseAssignment) is concrete.
+	if assignmentIsAbstract("mpd", "blatt09") {
+		t.Fatal("blatt09 should not be abstract")
+	}
+
+	// A declared abstract base.
+	viper.Set("mpd.defaults", true)
+	viper.Set("mpd.defaults.abstract", true)
+	viper.Set("mpd.defaults.per", "student")
+	if !assignmentIsAbstract("mpd", "defaults") {
+		t.Fatal("defaults should be abstract")
+	}
+
+	// A child extending an abstract base must NOT itself become abstract:
+	// the flag is read from the own value before inheritance is resolved.
+	viper.Set("mpd.blatt10", true)
+	viper.Set("mpd.blatt10.extends", "defaults")
+	viper.Set("mpd.blatt10.assignmentpath", "blatt-10")
+	if assignmentIsAbstract("mpd", "blatt10") {
+		t.Fatal("blatt10 extends an abstract base but must not be abstract itself")
+	}
+
+	// And after full resolution the abstract flag must not leak into the
+	// effective config.
+	cfg := GetAssignmentConfig("mpd", "blatt10")
+	if cfg.Per != PerStudent {
+		t.Fatalf("Per = %q, want inherited student", cfg.Per)
+	}
+	if viper.GetBool("mpd.blatt10.abstract") {
+		t.Fatal("abstract flag leaked into resolved blatt10 config")
+	}
+}
+
 func TestDeepMerge(t *testing.T) {
 	parent := map[string]any{
 		"a": 1,

docs/configuration.md

@@ -211,6 +211,37 @@ Notes:
 - Inheritance chains are allowed (`blatt11` → `blatt10` → `blatt09`); values are
   resolved transitively. Cycles and missing parents are reported as errors.
 
+#### Abstract base assignments
+
+A common pattern is a base entry that holds shared configuration and is only ever
+inherited from, never generated. Mark it with `abstract: true` — much like an
+abstract class. Any command targeting it directly (`generate`, `protect`,
+`clone`, …) is rejected with an error.
+
+```yaml
+defaults:
+  abstract: true          # base only — `glabs generate mpd defaults` is refused
+  per: student
+  mergeRequest:
+    mergeMethod: semi_linear
+    squashOption: never
+    pipeline: true
+  branches:
+    - name: main
+      mergeOnly: true
+
+blatt01:
+  extends: defaults       # inherits everything above
+  assignmentpath: blatt-01
+  description: Blatt 1
+  startercode:
+    url: git@gitlab.lrz.de:mpd/startercode/blatt-01.git
+    fromBranch: template
+```
+
+The `abstract` flag is **not inherited**: `blatt01` above stays a normal,
+generatable assignment.
+
 ### Access level values
 
 | Level | Value | Permissions |