Merge branch 'saml-doc-update' into 'master'

Updated SAML and Omniauth docs

Updated SAML and Omniauth docs with information on the new behavior recently introduced in !2882

/cc @ernstvn @DouweM 

See merge request !2950

doc/integration/omniauth.md

 # OmniAuth
 
-GitLab leverages OmniAuth to allow users to sign in using Twitter, GitHub, and other popular services.
+GitLab leverages OmniAuth to allow users to sign in using Twitter, GitHub, and
+other popular services.
 
-Configuring OmniAuth does not prevent standard GitLab authentication or LDAP (if configured) from continuing to work. Users can choose to sign in using any of the configured mechanisms.
+Configuring OmniAuth does not prevent standard GitLab authentication or LDAP
+(if configured) from continuing to work. Users can choose to sign in using any
+of the configured mechanisms.
 
 - [Initial OmniAuth Configuration](#initial-omniauth-configuration)
 - [Supported Providers](#supported-providers)
@@ -28,17 +31,25 @@ contains some settings that are common for all providers.
 
 ## Initial OmniAuth Configuration
 
-Before configuring individual OmniAuth providers there are a few global settings that are in common for all providers that we need to consider.
+Before configuring individual OmniAuth providers there are a few global settings
+that are in common for all providers that we need to consider.
 
 - Omniauth needs to be enabled, see details below for example.
-- `allow_single_sign_on` defaults to `false`. If `false` users must be created manually or they will not be able to
-sign in via OmniAuth.
-- `block_auto_created_users` defaults to `true`. If `true` auto created users will be blocked by default and will
-have to be unblocked by an administrator before they are able to sign in.
-- **Note:** If you set `allow_single_sign_on` to `true` and `block_auto_created_users` to `false` please be aware
-that any user on the Internet will be able to successfully sign in to your GitLab without administrative approval.
-
-If you want to change these settings:
+- `allow_single_sign_on` allows you to specify the providers you want to allow to
+  automatically create an account. It defaults to `false`. If `false` users must
+  be created manually or they will not be able to sign in via OmniAuth.
+- `block_auto_created_users` defaults to `true`. If `true` auto created users will
+  be blocked by default and will have to be unblocked by an administrator before
+  they are able to sign in.
+
+>**Note:**
+If you set `block_auto_created_users` to `false`, make sure to only
+define providers under `allow_single_sign_on` that you are able to control, like
+SAML, Shibboleth, Crowd or Google, or set it to `false` otherwise any user on
+the Internet will be able to successfully sign in to your GitLab without
+administrative approval.
+
+To change these settings:
 
 * **For omnibus package**
 
@@ -48,11 +59,16 @@ If you want to change these settings:
     sudo editor /etc/gitlab/gitlab.rb
     ```
 
-    and change
+    and change:
 
-    ```
+    ```ruby
     gitlab_rails['omniauth_enabled'] = true
-    gitlab_rails['omniauth_allow_single_sign_on'] = false
+
+    # CAUTION!
+    # This allows users to login without having a user account first. Define the allowed providers
+    # using an array, e.g. ["saml", "twitter"], or as true/false to allow all providers or none.
+    # User accounts will be created automatically when authentication was successful.
+    gitlab_rails['omniauth_allow_single_sign_on'] = ['saml', 'twitter']
     gitlab_rails['omniauth_block_auto_created_users'] = true
     ```
 
@@ -66,43 +82,57 @@ If you want to change these settings:
     sudo -u git -H editor config/gitlab.yml
     ```
 
-    and change the following section
+    and change the following section:
 
-    ```
+    ```yaml
      ## OmniAuth settings
       omniauth:
         # Allow login via Twitter, Google, etc. using OmniAuth providers
         enabled: true
 
         # CAUTION!
-        # This allows users to login without having a user account first (default: false).
+        # This allows users to login without having a user account first. Define the allowed providers
+        # using an array, e.g. ["saml", "twitter"], or as true/false to allow all providers or none.
         # User accounts will be created automatically when authentication was successful.
-        allow_single_sign_on: false
+        allow_single_sign_on: ["saml", "twitter"]
+
         # Locks down those users until they have been cleared by the admin (default: true).
         block_auto_created_users: true
     ```
 
-Now we can choose one or more of the Supported Providers below to continue configuration.
+Now we can choose one or more of the Supported Providers listed above to continue
+the configuration process.
 
 ## Enable OmniAuth for an Existing User
 
-Existing users can enable OmniAuth for specific providers after the account is created. For example, if the user originally signed in with LDAP an OmniAuth provider such as Twitter can be enabled. Follow the steps below to enable an OmniAuth provider for an existing user.
+Existing users can enable OmniAuth for specific providers after the account is
+created. For example, if the user originally signed in with LDAP, an OmniAuth
+provider such as Twitter can be enabled. Follow the steps below to enable an
+OmniAuth provider for an existing user.
 
 1. Sign in normally - whether standard sign in, LDAP, or another OmniAuth provider.
 1. Go to profile settings (the silhouette icon in the top right corner).
 1. Select the "Account" tab.
 1. Under "Connected Accounts" select the desired OmniAuth provider, such as Twitter.
-1. The user will be redirected to the provider. Once the user authorized GitLab they will be redirected back to GitLab.
+1. The user will be redirected to the provider. Once the user authorized GitLab
+   they will be redirected back to GitLab.
 
 The chosen OmniAuth provider is now active and can be used to sign in to GitLab from then on.
 
 ## Using Custom Omniauth Providers
 
-GitLab uses [Omniauth](http://www.omniauth.org/) for authentication and already ships with a few providers preinstalled (e.g. LDAP, GitHub, Twitter). But sometimes that is not enough and you need to integrate with other authentication solutions. For these cases you can use the Omniauth provider.
+>**Note:**
+The following information only applies for installations from source.
+
+GitLab uses [Omniauth](http://www.omniauth.org/) for authentication and already ships
+with a few providers pre-installed (e.g. LDAP, GitHub, Twitter). But sometimes that
+is not enough and you need to integrate with other authentication solutions. For
+these cases you can use the Omniauth provider.
 
 ### Steps
 
-These steps are fairly general and you will need to figure out the exact details from the Omniauth provider's documentation.
+These steps are fairly general and you will need to figure out the exact details
+from the Omniauth provider's documentation.
 
 -   Stop GitLab:
 
@@ -128,8 +158,12 @@ These steps are fairly general and you will need to figure out the exact details
 
 ### Examples
 
-If you have successfully set up a provider that is not shipped with GitLab itself, please let us know.
+If you have successfully set up a provider that is not shipped with GitLab itself,
+please let us know.
 
-You can help others by reporting successful configurations and probably share a few insights or provide warnings for common errors or pitfalls by sharing your experience [in the public Wiki](https://github.com/gitlabhq/gitlab-public-wiki/wiki/Custom-omniauth-provider-configurations).
+You can help others by reporting successful configurations and probably share a
+few insights or provide warnings for common errors or pitfalls by sharing your
+experience [in the public Wiki](https://github.com/gitlabhq/gitlab-public-wiki/wiki/Custom-omniauth-provider-configurations).
 
-While we can't officially support every possible authentication mechanism out there, we'd like to at least help those with specific needs.
+While we can't officially support every possible authentication mechanism out there,
+we'd like to at least help those with specific needs.

doc/integration/saml.md

 # SAML OmniAuth Provider
 
-GitLab can be configured to act as a SAML 2.0 Service Provider (SP). This allows GitLab to consume assertions from a SAML 2.0 Identity Provider (IdP) such as Microsoft ADFS to authenticate users. 
+GitLab can be configured to act as a SAML 2.0 Service Provider (SP). This allows
+GitLab to consume assertions from a SAML 2.0 Identity Provider (IdP) such as
+Microsoft ADFS to authenticate users.
 
-First configure SAML 2.0 support in GitLab, then register the GitLab application in your SAML IdP:  
+First configure SAML 2.0 support in GitLab, then register the GitLab application
+in your SAML IdP:
 
-1.  Make sure GitLab is configured with HTTPS. See [Using HTTPS](../install/installation.md#using-https) for instructions.
+1.  Make sure GitLab is configured with HTTPS.
+    See [Using HTTPS](../install/installation.md#using-https) for instructions.
 
 1.  On your GitLab server, open the configuration file.
 
@@ -22,7 +26,40 @@ First configure SAML 2.0 support in GitLab, then register the GitLab application
       sudo -u git -H editor config/gitlab.yml
     ```
 
-1.  See [Initial OmniAuth Configuration](omniauth.md#initial-omniauth-configuration) for initial settings.
+1.  See [Initial OmniAuth Configuration](omniauth.md#initial-omniauth-configuration)
+    for initial settings.
+
+1.  To allow your users to use SAML to sign up without having to manually create
+    an account first, don't forget to add the following values to your configuration:
+
+    For omnibus package:
+
+    ```ruby
+      gitlab_rails['omniauth_allow_single_sign_on'] = ['saml']
+      gitlab_rails['omniauth_block_auto_created_users'] = false
+    ```
+
+    For installations from source:
+
+    ```yaml
+      allow_single_sign_on: ["saml"]
+      block_auto_created_users: false
+    ```
+
+1.  You can also automatically link SAML users with existing GitLab users if their
+    email addresses match by adding the following setting:
+
+    For omnibus package:
+
+    ```ruby
+      gitlab_rails['omniauth_auto_link_saml_user'] = true
+    ```
+
+    For installations from source:
+
+    ```yaml
+      auto_link_saml_user: true
+    ```
 
 1.  Add the provider configuration:
 
@@ -31,15 +68,15 @@ First configure SAML 2.0 support in GitLab, then register the GitLab application
     ```ruby
       gitlab_rails['omniauth_providers'] = [
         {
-          "name" => "saml",
-           args: {
+          name: 'saml',
+          args: {
                    assertion_consumer_service_url: 'https://gitlab.example.com/users/auth/saml/callback',
                    idp_cert_fingerprint: '43:51:43:a1:b5:fc:8b:b7:0a:3a:a9:b1:0f:66:73:a8',
                    idp_sso_target_url: 'https://login.example.com/idp',
                    issuer: 'https://gitlab.example.com',
                    name_identifier_format: 'urn:oasis:names:tc:SAML:2.0:nameid-format:transient'
                  },
-          "label" => "Company Login" # optional label for SAML login button, defaults to "Saml"
+          label: 'Company Login' # optional label for SAML login button, defaults to "Saml"
         }
       ]
     ```
@@ -47,34 +84,52 @@ First configure SAML 2.0 support in GitLab, then register the GitLab application
     For installations from source:
 
     ```yaml
-      - { name: 'saml',
-           args: {
-                   assertion_consumer_service_url: 'https://gitlab.example.com/users/auth/saml/callback',
-                   idp_cert_fingerprint: '43:51:43:a1:b5:fc:8b:b7:0a:3a:a9:b1:0f:66:73:a8',
-                   idp_sso_target_url: 'https://login.example.com/idp',
-                   issuer: 'https://gitlab.example.com',
-                   name_identifier_format: 'urn:oasis:names:tc:SAML:2.0:nameid-format:transient'
-                 } }
+      - {
+          name: 'saml',
+          args: {
+                 assertion_consumer_service_url: 'https://gitlab.example.com/users/auth/saml/callback',
+                 idp_cert_fingerprint: '43:51:43:a1:b5:fc:8b:b7:0a:3a:a9:b1:0f:66:73:a8',
+                 idp_sso_target_url: 'https://login.example.com/idp',
+                 issuer: 'https://gitlab.example.com',
+                 name_identifier_format: 'urn:oasis:names:tc:SAML:2.0:nameid-format:transient'
+               },
+          label: 'Company Login' # optional label for SAML login button, defaults to "Saml"
+        }
     ```
 
-1.  Change the value for 'assertion_consumer_service_url' to match the HTTPS endpoint of GitLab (append 'users/auth/saml/callback' to the HTTPS URL of your GitLab installation to generate the correct value). 
+1.  Change the value for `assertion_consumer_service_url` to match the HTTPS endpoint
+    of GitLab (append `users/auth/saml/callback` to the HTTPS URL of your GitLab
+    installation to generate the correct value).
 
-1.  Change the values of 'idp_cert_fingerprint', 'idp_sso_target_url', 'name_identifier_format' to match your IdP. Check [the omniauth-saml documentation](https://github.com/PracticallyGreen/omniauth-saml) for details on these options.
+1.  Change the values of `idp_cert_fingerprint`, `idp_sso_target_url`,
+    `name_identifier_format` to match your IdP. Check
+    [the omniauth-saml documentation](https://github.com/omniauth/omniauth-saml)
+    for details on these options.
 
-1.  Change the value of 'issuer' to a unique name, which will identify the application to the IdP.
+1.  Change the value of `issuer` to a unique name, which will identify the application
+    to the IdP.
 
 1.  Restart GitLab for the changes to take effect.
 
-1.  Register the GitLab SP in your SAML 2.0 IdP, using the application name specified in 'issuer'. 
+1.  Register the GitLab SP in your SAML 2.0 IdP, using the application name specified
+    in `issuer`.
 
-To ease configuration, most IdP accept a metadata URL for the application to provide configuration information to the IdP. To build the metadata URL for GitLab, append 'users/auth/saml/metadata' to the HTTPS URL of your GitLab installation, for instance:
+To ease configuration, most IdP accept a metadata URL for the application to provide
+configuration information to the IdP. To build the metadata URL for GitLab, append
+`users/auth/saml/metadata` to the HTTPS URL of your GitLab installation, for instance:
    ```
    https://gitlab.example.com/users/auth/saml/metadata
    ```
 
-At a minimum the IdP *must* provide a claim containing the user's email address, using claim name 'email' or 'mail'. The email will be used to automatically generate the GitLab username. GitLab will also use claims with name 'name', 'first_name', 'last_name' (see [the omniauth-saml gem](https://github.com/PracticallyGreen/omniauth-saml/blob/master/lib/omniauth/strategies/saml.rb) for supported claims).
+At a minimum the IdP *must* provide a claim containing the user's email address, using
+claim name `email` or `mail`. The email will be used to automatically generate the GitLab
+username. GitLab will also use claims with name `name`, `first_name`, `last_name`
+(see [the omniauth-saml gem](https://github.com/omniauth/omniauth-saml/blob/master/lib/omniauth/strategies/saml.rb)
+for supported claims).
 
-On the sign in page there should now be a SAML button below the regular sign in form. Click the icon to begin the authentication process. If everything goes well the user will be returned to GitLab and will be signed in.
+On the sign in page there should now be a SAML button below the regular sign in form.
+Click the icon to begin the authentication process. If everything goes well the user
+will be returned to GitLab and will be signed in.
 
 ## Troubleshooting
 
@@ -82,7 +137,7 @@ If you see a "500 error" in GitLab when you are redirected back from the SAML si
 this likely indicates that GitLab could not get the email address for the SAML user.
 
 Make sure the IdP provides a claim containing the user's email address, using claim name
-'email' or 'mail'. The email will be used to automatically generate the GitLab username.
+`email` or `mail`.
 
 If after signing in into your SAML server you are redirected back to the sign in page and
 no error is displayed, check your `production.log` file. It will most likely contain the