fix: work around issue with user.Lookup not returning UnknownUserError (

#427)

Per golang/go#67912, under some circumstances
(on my machine) user.Lookup isn't returning UnknownUserError when it
should. So before this fix I get the following test failures:

```
$ go test ./internals/osutil
...
FAIL: user_test.go:44: userSuite.TestRealUser

user_test.go:71:
    c.Assert(err, check.IsNil)
... value *errors.errorString = &errors.errorString{s:"user: lookup username guy: no such file or directory"} ("user: lookup username guy: no such file or directory")
...
```

And:

```
$ go test ./internals/daemon
...
----------------------------------------------------------------------
FAIL: api_files_test.go:1128: filesSuite.TestWriteErrors

api_files_test.go:1195:
    checkFileResult(c, r.Result[4], pathUserNotFound, "generic-file-error", ".*unknown user.*")
api_files_test.go:273:
    c.Check(r.Error.Message, Matches, errorMsg)
... value string = "cannot look up user and group: user: lookup username user-not-found: no such file or directory"
... regex string = ".*unknown user.*"

api_files_test.go:1196:
    checkFileResult(c, r.Result[5], pathGroupNotFound, "generic-file-error", ".*unknown group.*")
api_files_test.go:273:
    c.Check(r.Error.Message, Matches, errorMsg)
... value string = "cannot look up user and group: user: lookup groupname group-not-found: no such file or directory"
... regex string = ".*unknown group.*"
...
```

internals/osutil/user.go

@@ -19,6 +19,8 @@ import (
 	"os"
 	"os/user"
 	"strconv"
+	"strings"
+	"syscall"
 
 	"github.com/canonical/pebble/internals/osutil/sys"
 )
@@ -27,6 +29,8 @@ var (
 	userCurrent     = user.Current
 	userLookup      = user.Lookup
 	userLookupGroup = user.LookupGroup
+
+	enoentMessage = syscall.ENOENT.Error()
 )
 
 // RealUser finds the user behind a sudo invocation when root, if applicable
@@ -56,6 +60,12 @@ func RealUser() (*user.User, error) {
 	if _, ok := err.(user.UnknownUserError); ok {
 		return cur, nil
 	}
+	// Workaround for https://github.com/golang/go/issues/67912, until our
+	// minimum Go version has a fix for that. In short, user.Lookup sometimes
+	// doesn't return UnknownUserError when it should.
+	if err != nil && strings.Contains(err.Error(), enoentMessage) {
+		return cur, nil
+	}
 	if err != nil {
 		return nil, err
 	}
@@ -90,6 +100,10 @@ func NormalizeUidGid(uid, gid *int, username, group string) (*int, *int, error)
 	if username != "" {
 		u, err := userLookup(username)
 		if err != nil {
+			if strings.Contains(err.Error(), enoentMessage) {
+				// Better error message to work around https://github.com/golang/go/issues/67912
+				return nil, nil, user.UnknownUserError(username)
+			}
 			return nil, nil, err
 		}
 		n, _ := strconv.Atoi(u.Uid)
@@ -107,6 +121,10 @@ func NormalizeUidGid(uid, gid *int, username, group string) (*int, *int, error)
 	if group != "" {
 		g, err := userLookupGroup(group)
 		if err != nil {
+			if strings.Contains(err.Error(), enoentMessage) {
+				// Better error message to work around https://github.com/golang/go/issues/67912
+				return nil, nil, user.UnknownGroupError(group)
+			}
 			return nil, nil, err
 		}
 		n, _ := strconv.Atoi(g.Gid)