Ignore ErrNotExists when fixing permissions (#27846) (#27877)

* Ignore ErrNotExists when fixing permissions on Windows

* Ignore ErrNotExist for unix instances

(cherry picked from commit 13e3466)

Co-authored-by: Michel Laterman <82832767+michel-laterman@users.noreply.github.com>

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -84,6 +84,8 @@
 - Change output.elasticsearch.proxy_disabled flag to output.elasticsearch.proxy_disable so fleet uses it. {issue}27670[27670] {pull}27671[27671]
 - Add validation for certificate flags to ensure they are absolute paths. {pull}27779[27779]
 - Migrate state on upgrade {pull}27825[27825]
+- Add "_monitoring" suffix to monitoring instance names to remove ambiguity with the status command. {issue}25449[25449]
+- Ignore ErrNotExists when fixing permissions. {issue}27836[27836] {pull}27846[27846]
 
 ==== New features
 

x-pack/elastic-agent/pkg/agent/install/perms_unix.go

@@ -7,6 +7,7 @@
 package install
 
 import (
+	"errors"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -29,6 +30,8 @@ func recursiveRootPermissions(path string) error {
 			}
 			// remove any world permissions from the file
 			err = os.Chmod(name, info.Mode().Perm()&0770)
+		} else if errors.Is(err, fs.ErrNotExist) {
+			return nil
 		}
 		return err
 	})

x-pack/elastic-agent/pkg/agent/install/perms_windows.go

@@ -7,6 +7,7 @@
 package install
 
 import (
+	"errors"
 	"io/fs"
 	"path/filepath"
 
@@ -30,6 +31,8 @@ func recursiveSystemAdminPermissions(path string) error {
 				inherit = false
 			}
 			err = systemAdministratorsOnly(name, inherit)
+		} else if errors.Is(err, fs.ErrNotExist) {
+			return nil
 		}
 		return err
 	})