7 files changed, 153 insertions, 8 deletions
diff --git a/docs/modules/account_validity_callbacks.md b/docs/modules/account_validity_callbacks.md
index 80684b7828..836bda70bf 100644
--- a/docs/modules/account_validity_callbacks.md
+++ b/docs/modules/account_validity_callbacks.md
@@ -22,6 +22,11 @@ If the module returns `True`, the current request will be denied with the error
 `ORG_MATRIX_EXPIRED_ACCOUNT` and the HTTP status code 403. Note that this doesn't
 invalidate the user's access token.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `None`, Synapse falls through to the next one. The value of the first
+callback that does not return `None` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `on_user_registration`
 
 ```python
@@ -31,3 +36,5 @@ async def on_user_registration(user: str) -> None
 Called after successfully registering a user, in case the module needs to perform extra
 operations to keep track of them. (e.g. add them to a database table). The user is
 represented by their Matrix user ID.
+
+If multiple modules implement this callback, Synapse runs them all in order.
diff --git a/docs/modules/index.md b/docs/modules/index.md
index 3fda8cb7f0..0a868b309f 100644
--- a/docs/modules/index.md
+++ b/docs/modules/index.md
@@ -2,6 +2,11 @@
 
 Synapse supports extending its functionality by configuring external modules.
 
+**Note**: When using third-party modules, you effectively allow someone else to run
+custom code on your Synapse homeserver. Server admins are encouraged to verify the
+provenance of the modules they use on their homeserver and make sure the modules aren't
+running malicious code on their instance.
+
 ## Using modules
 
 To use a module on Synapse, add it to the `modules` section of the configuration file:
@@ -18,17 +23,31 @@ modules:
 Each module is defined by a path to a Python class as well as a configuration. This
 information for a given module should be available in the module's own documentation.
 
-**Note**: When using third-party modules, you effectively allow someone else to run
-custom code on your Synapse homeserver. Server admins are encouraged to verify the
-provenance of the modules they use on their homeserver and make sure the modules aren't
-running malicious code on their instance.
+## Using multiple modules
+
+The order in which modules are listed in this section is important. When processing an
+action that can be handled by several modules, Synapse will always prioritise the module
+that appears first (i.e. is the highest in the list). This means:
+
+* If several modules register the same callback, the callback registered by the module
+  that appears first is used.
+* If several modules try to register a handler for the same HTTP path, only the handler
+  registered by the module that appears first is used. Handlers registered by the other
+  module(s) are ignored and Synapse will log a warning message about them.
+
+Note that Synapse doesn't allow multiple modules implementing authentication checkers via
+the password auth provider feature for the same login type with different fields. If this
+happens, Synapse will refuse to start.
+
+## Current status
 
-Also note that we are currently in the process of migrating module interfaces to this
-system. While some interfaces might be compatible with it, others still require
-configuring modules in another part of Synapse's configuration file.
+We are currently in the process of migrating module interfaces to this system. While some
+interfaces might be compatible with it, others still require configuring modules in
+another part of Synapse's configuration file.
 
 Currently, only the following pre-existing interfaces are compatible with this new system:
 
 * spam checker
 * third-party rules
 * presence router
+* password auth providers
diff --git a/docs/modules/password_auth_provider_callbacks.md b/docs/modules/password_auth_provider_callbacks.md
index 36417dd39e..bb921def88 100644
--- a/docs/modules/password_auth_provider_callbacks.md
+++ b/docs/modules/password_auth_provider_callbacks.md
@@ -44,6 +44,15 @@ instead.
 
 If the authentication is unsuccessful, the module must return `None`.
 
+If multiple modules register an auth checker for the same login type but with different
+fields, Synapse will refuse to start.
+
+If multiple modules register an auth checker for the same login type with the same fields,
+then the callbacks will be executed in order, until one returns a Matrix User ID (and
+optionally a callback). In that case, the return value of that callback will be accepted
+and subsequent callbacks will not be fired. If every callback returns `None`, then the
+authentication fails.
+
 ### `check_3pid_auth`
 
 ```python
@@ -67,7 +76,13 @@ If the authentication is successful, the module must return the user's Matrix ID
 `@alice:example.com`) and optionally a callback to be called with the response to the `/login` request.
 If the module doesn't wish to return a callback, it must return None instead.
 
-If the authentication is unsuccessful, the module must return None.
+If the authentication is unsuccessful, the module must return `None`.
+
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `None`, Synapse falls through to the next one. The value of the first
+callback that does not return `None` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback. If every callback return `None`,
+the authentication is denied.
 
 ### `on_logged_out`
 
@@ -82,6 +97,8 @@ Called during a logout request for a user. It is passed the qualified user ID, t
 deactivated device (if any: access tokens are occasionally created without an associated
 device ID), and the (now deactivated) access token.
 
+If multiple modules implement this callback, Synapse runs them all in order.
+
 ## Example
 
 The example module below implements authentication checkers for two different login types: 
diff --git a/docs/modules/presence_router_callbacks.md b/docs/modules/presence_router_callbacks.md
index 4abcc9af47..349e185bd6 100644
--- a/docs/modules/presence_router_callbacks.md
+++ b/docs/modules/presence_router_callbacks.md
@@ -24,6 +24,10 @@ must return a dictionary that maps from Matrix user IDs (which can be local or r
 
 Synapse will then attempt to send the specified presence updates to each user when possible.
 
+If multiple modules implement this callback, Synapse merges all the dictionaries returned
+by the callbacks. If multiple callbacks return a dictionary containing the same key,
+Synapse concatenates the sets associated with this key from each dictionary. 
+
 ### `get_interested_users`
 
 ```python
@@ -44,6 +48,12 @@ query. The returned users can be local or remote.
 Alternatively the callback can return `synapse.module_api.PRESENCE_ALL_USERS`
 to indicate that the user should receive updates from all known users.
 
+If multiple modules implement this callback, they will be considered in order. Synapse
+calls each callback one by one, and use a concatenation of all the `set`s returned by the
+callbacks. If one callback returns `synapse.module_api.PRESENCE_ALL_USERS`, Synapse uses
+this value instead. If this happens, Synapse does not call any of the subsequent
+implementations of this callback.
+
 ## Example
 
 The example below is a module that implements both presence router callbacks, and ensures
diff --git a/docs/modules/spam_checker_callbacks.md b/docs/modules/spam_checker_callbacks.md
index 787e99074a..7d954cbe94 100644
--- a/docs/modules/spam_checker_callbacks.md
+++ b/docs/modules/spam_checker_callbacks.md
@@ -19,6 +19,11 @@ either a `bool` to indicate whether the event must be rejected because of spam,
 to indicate the event must be rejected because of spam and to give a rejection reason to
 forward to clients.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `False`, Synapse falls through to the next one. The value of the first
+callback that does not return `False` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `user_may_join_room`
 
 ```python
@@ -34,6 +39,11 @@ currently has a pending invite in the room.
 This callback isn't called if the join is performed by a server administrator, or in the
 context of a room creation.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `user_may_invite`
 
 ```python
@@ -44,6 +54,11 @@ Called when processing an invitation. The module must return a `bool` indicating
 the inviter can invite the invitee to the given room. Both inviter and invitee are
 represented by their Matrix user ID (e.g. `@alice:example.com`).
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `user_may_send_3pid_invite`
 
 ```python
@@ -79,6 +94,11 @@ await user_may_send_3pid_invite(
 **Note**: If the third-party identifier is already associated with a matrix user ID,
 [`user_may_invite`](#user_may_invite) will be used instead.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `user_may_create_room`
 
 ```python
@@ -88,6 +108,11 @@ async def user_may_create_room(user: str) -> bool
 Called when processing a room creation request. The module must return a `bool` indicating
 whether the given user (represented by their Matrix user ID) is allowed to create a room.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `user_may_create_room_with_invites`
 
 ```python
@@ -117,6 +142,11 @@ corresponding list(s) will be empty.
 since no invites are sent when cloning a room. To cover this case, modules also need to
 implement `user_may_create_room`.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `user_may_create_room_alias`
 
 ```python
@@ -127,6 +157,11 @@ Called when trying to associate an alias with an existing room. The module must
 `bool` indicating whether the given user (represented by their Matrix user ID) is allowed
 to set the given alias.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `user_may_publish_room`
 
 ```python
@@ -137,6 +172,11 @@ Called when trying to publish a room to the homeserver's public rooms directory.
 module must return a `bool` indicating whether the given user (represented by their
 Matrix user ID) is allowed to publish the given room.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `check_username_for_spam`
 
 ```python
@@ -154,6 +194,11 @@ is represented as a dictionary with the following keys:
 The module is given a copy of the original dictionary, so modifying it from within the
 module cannot modify a user's profile when included in user directory search results.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `False`, Synapse falls through to the next one. The value of the first
+callback that does not return `False` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `check_registration_for_spam`
 
 ```python
@@ -179,6 +224,12 @@ The arguments passed to this callback are:
   used during the registration process.
 * `auth_provider_id`: The identifier of the SSO authentication provider, if any.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `RegistrationBehaviour.ALLOW`, Synapse falls through to the next one.
+The value of the first callback that does not return `RegistrationBehaviour.ALLOW` will
+be used. If this happens, Synapse will not call any of the subsequent implementations of
+this callback.
+
 ### `check_media_file_for_spam`
 
 ```python
@@ -191,6 +242,11 @@ async def check_media_file_for_spam(
 Called when storing a local or remote file. The module must return a boolean indicating
 whether the given file can be stored in the homeserver's media store.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `False`, Synapse falls through to the next one. The value of the first
+callback that does not return `False` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ## Example
 
 The example below is a module that implements the spam checker callback
diff --git a/docs/modules/third_party_rules_callbacks.md b/docs/modules/third_party_rules_callbacks.md
index 2ba6f39453..5371e7f807 100644
--- a/docs/modules/third_party_rules_callbacks.md
+++ b/docs/modules/third_party_rules_callbacks.md
@@ -44,6 +44,11 @@ dictionary, and modify the returned dictionary accordingly.
 Note that replacing the event only works for events sent by local users, not for events
 received over federation.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `on_create_room`
 
 ```python
@@ -63,6 +68,12 @@ the request is a server admin.
 Modules can modify the `request_content` (by e.g. adding events to its `initial_state`),
 or deny the room's creation by raising a `module_api.errors.SynapseError`.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns without raising an exception, Synapse falls through to the next one. The
+room creation will be forbidden as soon as one of the callbacks raises an exception. If
+this happens, Synapse will not call any of the subsequent implementations of this
+callback.
+
 ### `check_threepid_can_be_invited`
 
 ```python
@@ -76,6 +87,11 @@ async def check_threepid_can_be_invited(
 Called when processing an invite via a third-party identifier (i.e. email or phone number).
 The module must return a boolean indicating whether the invite can go through.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ### `check_visibility_can_be_modified`
 
 ```python
@@ -90,6 +106,11 @@ Called when changing the visibility of a room in the local public room directory
 visibility is a string that's either "public" or "private". The module must return a
 boolean indicating whether the change can go through.
 
+If multiple modules implement this callback, they will be considered in order. If a
+callback returns `True`, Synapse falls through to the next one. The value of the first
+callback that does not return `True` will be used. If this happens, Synapse will not call
+any of the subsequent implementations of this callback.
+
 ## Example
 
 The example below is a module that implements the third-party rules callback
diff --git a/docs/modules/writing_a_module.md b/docs/modules/writing_a_module.md
index 4f2fec8dc9..7764e06692 100644
--- a/docs/modules/writing_a_module.md
+++ b/docs/modules/writing_a_module.md
@@ -12,6 +12,21 @@ configuration associated with the module in Synapse's configuration file.
 See the documentation for the `ModuleApi` class
 [here](https://github.com/matrix-org/synapse/blob/master/synapse/module_api/__init__.py).
 
+## When Synapse runs with several modules configured
+
+If Synapse is running with other modules configured, the order each module appears in
+within the `modules` section of the Synapse configuration file might restrict what it can
+or cannot register. See [this section](index.html#using-multiple-modules) for more
+information.
+
+On top of the rules listed in the link above, if a callback returns a value that should
+cause the current operation to fail (e.g. if a callback checking an event returns with a
+value that should cause the event to be denied), Synapse will fail the operation and
+ignore any subsequent callbacks that should have been run after this one.
+
+The documentation for each callback mentions how Synapse behaves when
+multiple modules implement it.
+
 ## Handling the module's configuration
 
 A module can implement the following static method: