fix(apiserver): propagate submit user as job execuser (bug-1780)

modelJobToSubmit() never set job_startinfo_t.execuser when converting
a submitted job to the C struct sent to the controller/worker, so
every job ran as the worker's 'nobody' default and failed to exec
(exit 126). Default execuser to the submitting user via a new
resolveExecUser() pure helper, and free the newly allocated C string.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Hb6Edtj4KbS8CrgEWYdA2E

src/frontend/apiserver/backend/convert.go

@@ -273,6 +273,9 @@ func cClusterConfToClusterInfo(conf *C.cluster_config_t) *models.ClusterInfo {
 func modelJobToSubmit(job *models.JobInfo) (*C.xce_job_submit_t, *C.job_submit_data_t, *C.job_startinfo_t) {
 	si := (*C.job_startinfo_t)(C.calloc(1, C.sizeof_job_startinfo_t))
 	si.cmdline = C.CString(job.Command)
+	if execUser := resolveExecUser(job.ExecUser, job.SubmitUser); execUser != "" {
+		si.execuser = C.CString(execUser)
+	}
 	if job.LaunchType != "" {
 		si.launch_type = C.CString(job.LaunchType)
 	}
@@ -324,6 +327,7 @@ func modelJobToSubmit(job *models.JobInfo) (*C.xce_job_submit_t, *C.job_submit_d
 func freeJobSubmit(js *C.xce_job_submit_t, sd *C.job_submit_data_t, si *C.job_startinfo_t) {
 	if si != nil {
 		C.free(unsafe.Pointer(si.cmdline))
+		C.free(unsafe.Pointer(si.execuser))
 		C.free(unsafe.Pointer(si.launch_type))
 		C.free(unsafe.Pointer(si.job_pre_exec))
 		C.free(unsafe.Pointer(si.job_post_exec))

src/frontend/apiserver/backend/convert_pure.go

@@ -104,6 +104,18 @@ func WorkerDefaultOutputPath(uniqueJobID int64, taskIdx int) string {
 	return fmt.Sprintf("/tmp/job_%s_task%d.out", FormatJobID(uniqueJobID), taskIdx)
 }
 
+// resolveExecUser returns the effective exec user for a job submission: the
+// caller-specified ExecUser if present, otherwise the submitting user
+// (bug-1780: an unset execuser in the startjob request reaches xcecd as
+// empty, which controller_connector.c defaults to the literal Linux user
+// 'nobody' -- a user with no home/shell that then fails to exec the job).
+func resolveExecUser(execUser, submitUser string) string {
+	if execUser != "" {
+		return execUser
+	}
+	return submitUser
+}
+
 // rawStartInfo mirrors the C job_startinfo_t fields used by the Go model.
 type rawStartInfo struct {
 	cmdline    string

src/frontend/apiserver/backend/convert_test.go

@@ -6,6 +6,28 @@ import (
 	"testing"
 )
 
+// bug-1780: modelJobToSubmit must default a job's exec user to its
+// submitting user when the caller didn't specify one explicitly, so
+// xcecd never falls back to running the job as the literal Linux user
+// 'nobody' (which has no home/shell and fails to exec the command).
+func TestResolveExecUser_DefaultsToSubmitUserWhenEmpty(t *testing.T) {
+	if got, want := resolveExecUser("", "alice"), "alice"; got != want {
+		t.Errorf("resolveExecUser(\"\", %q): got %q, want %q", "alice", got, want)
+	}
+}
+
+func TestResolveExecUser_PrefersExplicitExecUser(t *testing.T) {
+	if got, want := resolveExecUser("bob", "alice"), "bob"; got != want {
+		t.Errorf("resolveExecUser(%q, %q): got %q, want %q", "bob", "alice", got, want)
+	}
+}
+
+func TestResolveExecUser_EmptyWhenBothEmpty(t *testing.T) {
+	if got, want := resolveExecUser("", ""), ""; got != want {
+		t.Errorf("resolveExecUser(\"\", \"\"): got %q, want %q", got, want)
+	}
+}
+
 func TestRawJobToModel_BasicFields(t *testing.T) {
 	r := rawJobData{
 		jobID:   123,