List of fixed items:
```
integration/helpers.go:1279:2 gosimple S1000: should use for range instead of for { select {} }
integration/integration_test.go:144:5 gosimple S1009: should omit nil check; len() for nil slices is defined as zero
integration/integration_test.go:173:5 gosimple S1009: should omit nil check; len() for nil slices is defined as zero
integration/integration_test.go:296:28 gosimple S1019: should use make(chan error) instead
integration/integration_test.go:570:41 gosimple S1019: should use make(chan interface{}) instead
integration/integration_test.go:685:40 gosimple S1019: should use make(chan interface{}) instead
integration/integration_test.go:759:33 gosimple S1019: should use make(chan string) instead
lib/auth/init_test.go:62:2 gosimple S1021: should merge variable declaration with assignment on next line
lib/auth/tls_test.go:1658:22 gosimple S1024: should use time.Until instead of t.Sub(time.Now())
lib/backend/dynamo/dynamodbbk.go:420:5 gosimple S1004: should use !bytes.Equal(expected.Key, replaceWith.Key) instead
lib/backend/dynamo/dynamodbbk.go:656:12 gosimple S1039: unnecessary use of fmt.Sprintf
lib/backend/etcdbk/etcd.go:458:5 gosimple S1004: should use !bytes.Equal(expected.Key, replaceWith.Key) instead
lib/backend/firestore/firestorebk.go:407:5 gosimple S1004: should use !bytes.Equal(expected.Key, replaceWith.Key) instead
lib/backend/lite/lite.go:317:5 gosimple S1004: should use !bytes.Equal(expected.Key, replaceWith.Key) instead
lib/backend/lite/lite.go:336:6 gosimple S1004: should use !bytes.Equal(value, expected.Value) instead
lib/backend/memory/memory.go:365:5 gosimple S1004: should use !bytes.Equal(expected.Key, replaceWith.Key) instead
lib/backend/memory/memory.go:376:5 gosimple S1004: should use !bytes.Equal(existingItem.Value, expected.Value) instead
lib/backend/test/suite.go:327:10 gosimple S1024: should use time.Until instead of t.Sub(time.Now())
lib/client/api.go:1410:9 gosimple S1003: should use strings.ContainsRune(name, ':') instead
lib/client/api.go:2355:32 gosimple S1019: should use make([]ForwardedPort, len(spec)) instead
lib/client/keyagent_test.go:85:2 gosimple S1021: should merge variable declaration with assignment on next line
lib/client/player.go:54:33 gosimple S1019: should use make(chan int) instead
lib/config/configuration.go:1024:52 gosimple S1019: should use make(services.CommandLabels) instead
lib/config/configuration.go:1025:44 gosimple S1019: should use make(map[string]string) instead
lib/config/configuration.go:930:21 gosimple S1003: should use strings.Contains(clf.Roles, defaults.RoleNode) instead
lib/config/configuration.go:931:22 gosimple S1003: should use strings.Contains(clf.Roles, defaults.RoleAuthService) instead
lib/config/configuration.go:932:23 gosimple S1003: should use strings.Contains(clf.Roles, defaults.RoleProxy) instead
lib/service/supervisor.go:387:2 gosimple S1001: should use copy() instead of a loop
lib/tlsca/parsegen.go:140:9 gosimple S1034: assigning the result of this type assertion to a variable (switch generalKey := generalKey.(type)) could eliminate type assertions in switch cases
lib/utils/certs.go:140:9 gosimple S1034: assigning the result of this type assertion to a variable (switch generalKey := generalKey.(type)) could eliminate type assertions in switch cases
lib/utils/certs.go:167:40 gosimple S1010: should omit second index in slice, s[a:len(s)] is identical to s[a:]
lib/utils/certs.go:204:5 gosimple S1004: should use !bytes.Equal(certificateChain[0].SubjectKeyId, certificateChain[0].AuthorityKeyId) instead
lib/utils/parse/parse.go:116:45 gosimple S1003: should use strings.Contains(variable, "}}") instead
lib/utils/parse/parse.go:116:6 gosimple S1003: should use strings.Contains(variable, "{{") instead
lib/utils/socks/socks.go:192:10 gosimple S1025: should use String() instead of fmt.Sprintf
lib/utils/socks/socks.go:199:10 gosimple S1025: should use String() instead of fmt.Sprintf
lib/web/apiserver.go:1054:18 gosimple S1024: should use time.Until instead of t.Sub(time.Now())
lib/web/apiserver.go:1954:9 gosimple S1039: unnecessary use of fmt.Sprintf
tool/tsh/tsh.go:1193:14 gosimple S1024: should use time.Until instead of t.Sub(time.Now())
```
This commit fixes#3369, refs #3374
It adds support for kuberenetes_users section in roles,
allowing Teleport proxy to impersonate user identities.
It also extends variable interpolation syntax by adding
suffix and prefix to variables and function `email.local`:
Example:
```yaml
kind: role
version: v3
metadata:
name: admin
spec:
allow:
# extract email local part from the email claim
logins: ['{{email.local(external.email)}}']
# impersonate a kubernetes user with IAM prefix
kubernetes_users: ['IAM#{{external.email}}']
# the deny section uses the identical format as the 'allow' section.
# the deny rules always override allow rules.
deny: {}
```
Some notes on email.local behavior:
* This is the only function supported in the template variables for now
* In case if the email.local will encounter invalid email address,
it will interpolate to empty value, will be removed from resulting
output.
Changes in impersonation behavior:
* By default, if no kubernetes_users is set, which is a majority of cases,
user will impersonate themselves, which is the backwards-compatible behavior.
* As long as at least one `kubernetes_users` is set, the forwarder will start
limiting the list of users allowed by the client to impersonate.
* If the users' role set does not include actual user name, it will be rejected,
otherwise there will be no way to exclude the user from the list).
* If the `kuberentes_users` role set includes only one user
(quite frequently that's the real intent), teleport will default to it,
otherwise it will refuse to select.
This will enable the use case when `kubernetes_users` has just one field to
link the user identity with the IAM role, for example `IAM#{{external.email}}`
* Previous versions of the forwarding proxy were denying all external
impersonation headers, this commit allows 'Impesrsonate-User' and
'Impersonate-Group' header values that are allowed by role set.
* Previous versions of the forwarding proxy ignored 'Deny' section of the roles
when applied to impersonation, this commit fixes that - roles with deny
kubernetes_users and kubernetes_groups section will not allow
impersonation of those users and groups.
Andrew Lytvynov