if parameters.isc doesn't exist, don't try to switch to manager user

instance.go

@@ -260,6 +260,11 @@ func (i *Instance) managerSysProc() (*syscall.SysProcAttr, error) {
 
 	mgr, _, err := i.DetermineManager()
 	if err != nil {
+		var pIscErr *ParametersISCNotExistError
+		if errors.As(err, &pIscErr) {
+			log.WithError(pIscErr).Debug("cannot determine manager")
+			return nil, nil
+		}
 		return nil, err
 	}
 
@@ -644,11 +649,27 @@ func (i *Instance) ExecuteString(namespace string, code string) (string, error)
 	return i.Execute(namespace, b)
 }
 
+type ParametersISCNotExistError struct {
+	dir string
+	err error
+}
+
+func (e *ParametersISCNotExistError) Error() string {
+	return fmt.Sprintf("parameters.isc not found in %s. err: %s", e.dir, e.err.Error())
+}
+
+func (e *ParametersISCNotExistError) Unwrap() error {
+	return e.err
+}
+
 // ReadParametersISC will read the current instances parameters ISC file into a simple data structure.
 // It returns the ParametersISC data structure and any error encountered.
 func (i *Instance) ReadParametersISC() (ParametersISC, error) {
 	f, err := parameterReader(i.Directory, iscParametersFile)
 	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil, &ParametersISCNotExistError{dir: i.Directory, err: err}
+		}
 		return nil, err
 	}
 	defer f.Close()

instance_test.go

@@ -21,6 +21,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"os"
 	"os/user"
 	"syscall"
 	"time"
@@ -192,6 +193,7 @@ var _ = Describe("Instance", func() {
 			})
 		})
 	})
+
 	Describe("DetermineISCDatFileName", func() {
 		Context("The product is Cache", func() {
 			It("Returns the correct DAT filename", func() {
@@ -550,4 +552,23 @@ var _ = Describe("Instance", func() {
 			})
 		})
 	})
+
+	Describe("Update", func() {
+		// To test instance updates when running somewhere that doesn't actually have access to the
+		// parameters.isc file, such as `iscenv` wrapping `csession` or `iris`
+		Context("Valid qlist without parameters.isc", func() {
+			BeforeEach(func() {
+				parameterReader = func(directory string, file string) (io.ReadCloser, error) {
+					return nil, os.ErrNotExist
+				}
+				instance, err = InstanceFromQList(cacheqlist)
+				Expect(err).ToNot(HaveOccurred())
+			})
+
+			It("Does not return an error", func() {
+				err := instance.Update()
+				Expect(err).NotTo(HaveOccurred())
+			})
+		})
+	})
 })