docs: Add policy variables for resource and conditions (#10278)

Bonus fix adds LDAP policy variable and clarifies the
usage of policy variables for temporary credentials.

fixes #10197

pkg/bucket/policy/condition/key.go

@@ -137,6 +137,7 @@ var AllSupportedKeys = append([]Key{
 	AWSPrincipalType,
 	AWSUserID,
 	AWSUsername,
+	LDAPUser,
 	// Add new supported condition keys.
 }, JWTKeys...)
 
@@ -152,6 +153,7 @@ var CommonKeys = append([]Key{
 	AWSUserID,
 	AWSUsername,
 	S3XAmzContentSha256,
+	LDAPUser,
 }, JWTKeys...)
 
 func substFuncFromValues(values map[string][]string) func(string) string {
@@ -199,6 +201,8 @@ func (key Key) Name() string {
 		return strings.TrimPrefix(keyString, "aws:")
 	} else if strings.HasPrefix(keyString, "jwt:") {
 		return strings.TrimPrefix(keyString, "jwt:")
+	} else if strings.HasPrefix(keyString, "ldap:") {
+		return strings.TrimPrefix(keyString, "ldap:")
 	}
 	return strings.TrimPrefix(keyString, "s3:")
 }

pkg/bucket/policy/condition/ldap.go

@@ -0,0 +1,22 @@
+/*
+ * MinIO Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package condition
+
+const (
+	// LDAPUser - LDAP username, in MinIO this value is equal to your authenticating LDAP user.
+	LDAPUser Key = "ldap:user"
+)