feat(harness): add subject entrypoint overrides

cmd/harness/main.go

@@ -744,18 +744,25 @@ func versionCmd() *cobra.Command {
 	}
 }
 
-// applyCasePin overlays a case's subject_image/subject_version pin onto a
-// registry-resolved Subject. The global --image/--version CLI flags are applied
-// later in runner.applySubjectOverrides, so an explicit CLI flag still wins:
+// applyCasePin overlays a case's subject_image/subject_version/subject_entrypoint
+// pin onto a registry-resolved Subject. The global --image/--version CLI flags are
+// applied later in runner.applySubjectOverrides, so an explicit CLI flag still
+// wins:
 //
 //	CLI --image/--version  >  case subject_image/subject_version  >  registry default
+//
+// subject_entrypoint has no CLI counterpart: it is a property of what the case
+// tests (see TestCase.SubjectEntrypoint), not of which build is under test.
 func applyCasePin(s config.Subject, tc *config.TestCase) config.Subject {
 	if tc.SubjectImage != "" {
 		s = s.WithImage(tc.SubjectImage)
 	}
 	if tc.SubjectVersion != "" {
 		s = s.WithVersion(tc.SubjectVersion)
 	}
+	if len(tc.SubjectEntrypoint) > 0 {
+		s = s.WithEntrypoint(tc.SubjectEntrypoint)
+	}
 	return s
 }
 

internal/config/case.go

@@ -135,6 +135,17 @@ type TestCase struct {
 	SubjectImage   string `yaml:"subject_image"`
 	SubjectVersion string `yaml:"subject_version"`
 
+	// SubjectEntrypoint replaces the registry's entrypoint for this case, with
+	// the registry Command still appended as arguments. Same precedence and the
+	// same non-strict-decode tolerance as SubjectImage.
+	//
+	// Needed where the case is about the subject's own process lifecycle: the
+	// director self-update cases stop and restart the service they are updating,
+	// which ends the container when the service is PID 1. Pointing the entrypoint
+	// at a wrapper that runs the service as a CHILD keeps the container alive
+	// across that restart, so "the service came back" is observable at all.
+	SubjectEntrypoint []string `yaml:"subject_entrypoint"`
+
 	Subjects       []string                 `yaml:"subjects"`
 	Configurations map[string]Configuration `yaml:"configurations"`
 	Correctness    CorrectnessConfig        `yaml:"correctness"`

internal/config/subject.go

@@ -336,3 +336,11 @@ func (s Subject) WithImage(img string) Subject {
 	s.Image = img
 	return s
 }
+
+// WithEntrypoint returns a copy of the Subject with the entrypoint overridden.
+// Command is left alone, so the registry's arguments still reach the new
+// entrypoint — a wrapper script gets them as "$@".
+func (s Subject) WithEntrypoint(entrypoint []string) Subject {
+	s.Entrypoint = entrypoint
+	return s
+}