9 files changed, 178 insertions, 22 deletions

diff --git a/docs/admin_api/rooms.md b/docs/admin_api/rooms.md
index 9aa489e4a3..7526956bec 100644
--- a/docs/admin_api/rooms.md
+++ b/docs/admin_api/rooms.md
@@ -302,6 +302,8 @@ The following fields are possible in the JSON response body:
 * `state_events` - Total number of state_events of a room. Complexity of the room.
 * `room_type` - The type of the room taken from the room's creation event; for example "m.space" if the room is a space.
   If the room does not define a type, the value will be `null`.
+* `forgotten` - Whether all local users have
+  [forgotten](https://spec.matrix.org/latest/client-server-api/#leaving-rooms) the room.
 
 The API is:
 
@@ -330,10 +332,13 @@ A response body like the following is returned:
   "guest_access": null,
   "history_visibility": "shared",
   "state_events": 93534,
-  "room_type": "m.space"
+  "room_type": "m.space",
+  "forgotten": false
 }
 ```
 
+_Changed in Synapse 1.66:_ Added the `forgotten` key to the response body.
+
 # Room Members API
 
 The Room Members admin API allows server admins to get a list of all members of a room.
diff --git a/docs/admin_api/user_admin_api.md b/docs/admin_api/user_admin_api.md
index 0871cfebf5..c1ca0c8a64 100644
--- a/docs/admin_api/user_admin_api.md
+++ b/docs/admin_api/user_admin_api.md
@@ -753,6 +753,7 @@ A response body like the following is returned:
       "device_id": "QBUAZIFURK",
       "display_name": "android",
       "last_seen_ip": "1.2.3.4",
+      "last_seen_user_agent": "Mozilla/5.0 (X11; Linux x86_64; rv:103.0) Gecko/20100101 Firefox/103.0",
       "last_seen_ts": 1474491775024,
       "user_id": "<user_id>"
     },
@@ -760,6 +761,7 @@ A response body like the following is returned:
       "device_id": "AUIECTSRND",
       "display_name": "ios",
       "last_seen_ip": "1.2.3.5",
+      "last_seen_user_agent": "Mozilla/5.0 (X11; Linux x86_64; rv:103.0) Gecko/20100101 Firefox/103.0",
       "last_seen_ts": 1474491775025,
       "user_id": "<user_id>"
     }
@@ -786,6 +788,8 @@ The following fields are returned in the JSON response body:
     Absent if no name has been set.
   - `last_seen_ip` - The IP address where this device was last seen.
     (May be a few minutes out of date, for efficiency reasons).
+  - `last_seen_user_agent` - The user agent of the device when it was last seen.
+    (May be a few minutes out of date, for efficiency reasons).
   - `last_seen_ts` - The timestamp (in milliseconds since the unix epoch) when this
     devices was last seen. (May be a few minutes out of date, for efficiency reasons).
   - `user_id` - Owner of  device.
@@ -837,6 +841,7 @@ A response body like the following is returned:
   "device_id": "<device_id>",
   "display_name": "android",
   "last_seen_ip": "1.2.3.4",
+  "last_seen_user_agent": "Mozilla/5.0 (X11; Linux x86_64; rv:103.0) Gecko/20100101 Firefox/103.0",
   "last_seen_ts": 1474491775024,
   "user_id": "<user_id>"
 }
@@ -858,6 +863,8 @@ The following fields are returned in the JSON response body:
   Absent if no name has been set.
 - `last_seen_ip` - The IP address where this device was last seen.
   (May be a few minutes out of date, for efficiency reasons).
+  - `last_seen_user_agent` - The user agent of the device when it was last seen.
+  (May be a few minutes out of date, for efficiency reasons).
 - `last_seen_ts` - The timestamp (in milliseconds since the unix epoch) when this
   devices was last seen. (May be a few minutes out of date, for efficiency reasons).
 - `user_id` - Owner of  device.
diff --git a/docs/auth_chain_difference_algorithm.md b/docs/auth_chain_difference_algorithm.md
index 30f72a70da..ebc9de25b8 100644
--- a/docs/auth_chain_difference_algorithm.md
+++ b/docs/auth_chain_difference_algorithm.md
@@ -34,13 +34,45 @@ the process of indexing it).
 ## Chain Cover Index
 
 Synapse computes auth chain differences by pre-computing a "chain cover" index
-for the auth chain in a room, allowing efficient reachability queries like "is
-event A in the auth chain of event B". This is done by assigning every event a
-*chain ID* and *sequence number* (e.g. `(5,3)`), and having a map of *links*
-between chains (e.g. `(5,3) -> (2,4)`) such that A is reachable by B (i.e. `A`
-is in the auth chain of `B`) if and only if either:
-
-1. A and B have the same chain ID and `A`'s sequence number is less than `B`'s
+for the auth chain in a room, allowing us to efficiently make reachability queries
+like "is event `A` in the auth chain of event `B`?". We could do this with an index
+that tracks all pairs `(A, B)` such that `A` is in the auth chain of `B`. However, this
+would be prohibitively large, scaling poorly as the room accumulates more state
+events.
+
+Instead, we break down the graph into *chains*. A chain is a subset of a DAG
+with the following property: for any pair of events `E` and `F` in the chain,
+the chain contains a path `E -> F` or a path `F -> E`. This forces a chain to be
+linear (without forks), e.g. `E -> F -> G -> ... -> H`. Each event in the chain
+is given a *sequence number* local to that chain. The oldest event `E` in the
+chain has sequence number 1. If `E` has a child `F` in the chain, then `F` has
+sequence number 2. If `E` has a grandchild `G` in the chain, then `G` has
+sequence number 3; and so on.
+
+Synapse ensures that each persisted event belongs to exactly one chain, and
+tracks how the chains are connected to one another. This allows us to
+efficiently answer reachability queries. Doing so uses less storage than
+tracking reachability on an event-by-event basis, particularly when we have
+fewer and longer chains. See
+
+> Jagadish, H. (1990). [A compression technique to materialize transitive closure](https://doi.org/10.1145/99935.99944).
+> *ACM Transactions on Database Systems (TODS)*, 15*(4)*, 558-598.
+
+for the original idea or
+
+> Y. Chen, Y. Chen, [An efficient algorithm for answering graph
+> reachability queries](https://doi.org/10.1109/ICDE.2008.4497498),
+> in: 2008 IEEE 24th International Conference on Data Engineering, April 2008,
+> pp. 893–902. (PDF available via [Google Scholar](https://scholar.google.com/scholar?q=Y.%20Chen,%20Y.%20Chen,%20An%20efficient%20algorithm%20for%20answering%20graph%20reachability%20queries,%20in:%202008%20IEEE%2024th%20International%20Conference%20on%20Data%20Engineering,%20April%202008,%20pp.%20893902.).)
+
+for a more modern take.
+
+In practical terms, the chain cover assigns every event a
+*chain ID* and *sequence number* (e.g. `(5,3)`), and maintains a map of *links*
+between events in chains (e.g. `(5,3) -> (2,4)`) such that `A` is reachable by `B`
+(i.e. `A` is in the auth chain of `B`) if and only if either:
+
+1. `A` and `B` have the same chain ID and `A`'s sequence number is less than `B`'s
    sequence number; or
 2. there is a link `L` between `B`'s chain ID and `A`'s chain ID such that
    `L.start_seq_no` <= `B.seq_no` and `A.seq_no` <= `L.end_seq_no`.
@@ -49,8 +81,9 @@ There are actually two potential implementations, one where we store links from
 each chain to every other reachable chain (the transitive closure of the links
 graph), and one where we remove redundant links (the transitive reduction of the
 links graph) e.g. if we have chains `C3 -> C2 -> C1` then the link `C3 -> C1`
-would not be stored. Synapse uses the former implementations so that it doesn't
-need to recurse to test reachability between chains.
+would not be stored. Synapse uses the former implementation so that it doesn't
+need to recurse to test reachability between chains. This trades-off extra storage
+in order to save CPU cycles and DB queries.
 
 ### Example
 
diff --git a/docs/message_retention_policies.md b/docs/message_retention_policies.md
index 8c88f93935..7f3e5359f1 100644
--- a/docs/message_retention_policies.md
+++ b/docs/message_retention_policies.md
@@ -8,7 +8,8 @@ and allow server and room admins to configure how long messages should
 be kept in a homeserver's database before being purged from it.
 **Please note that, as this feature isn't part of the Matrix
 specification yet, this implementation is to be considered as
-experimental.** 
+experimental. There are known bugs which may cause database corruption.
+Proceed with caution.** 
 
 A message retention policy is mainly defined by its `max_lifetime`
 parameter, which defines how long a message can be kept around after
diff --git a/docs/openid.md b/docs/openid.md
index d0ccf36f71..ce9b026228 100644
--- a/docs/openid.md
+++ b/docs/openid.md
@@ -174,7 +174,9 @@ oidc_providers:
 
 1. Create a regular web application for Synapse
 2. Set the Allowed Callback URLs to `[synapse public baseurl]/_synapse/client/oidc/callback`
-3. Add a rule to add the `preferred_username` claim.
+3. Add a rule with any name to add the `preferred_username` claim. 
+(See https://auth0.com/docs/customize/rules/create-rules for more information on how to create rules.)
+   
    <details>
     <summary>Code sample</summary>
 
diff --git a/docs/templates.md b/docs/templates.md
index f87692a453..453ac90dd8 100644
--- a/docs/templates.md
+++ b/docs/templates.md
@@ -9,7 +9,7 @@ in, allowing them to specify custom templates:
 
 ```yaml
 templates:
-  custom_templates_directory: /path/to/custom/templates/
+  custom_template_directory: /path/to/custom/templates/
 ```
 
 If this setting is not set, or the files named below are not found within the directory,
diff --git a/docs/upgrade.md b/docs/upgrade.md
index 47a74b67de..0ab5bfeaf0 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -89,6 +89,25 @@ process, for example:
     dpkg -i matrix-synapse-py3_1.3.0+stretch1_amd64.deb
     ```
 
+# Upgrading to v1.66.0
+
+## Delegation of email validation no longer supported
+
+As of this version, Synapse no longer allows the tasks of verifying email address
+ownership, and password reset confirmation, to be delegated to an identity server.
+This removal was previously planned for Synapse 1.64.0, but was
+[delayed](https://github.com/matrix-org/synapse/issues/13421) until now to give
+homeserver administrators more notice of the change.
+
+To continue to allow users to add email addresses to their homeserver accounts,
+and perform password resets, make sure that Synapse is configured with a working
+email server in the [`email` configuration
+section](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#email)
+(including, at a minimum, a `notif_from` setting.)
+
+Specifying an `email` setting under `account_threepid_delegates` will now cause
+an error at startup.
+
 # Upgrading to v1.64.0
 
 ## Deprecation of the ability to delegate e-mail verification to identity servers
diff --git a/docs/usage/administration/admin_faq.md b/docs/usage/administration/admin_faq.md
index 3dcad4bbef..7ba5a83f04 100644
--- a/docs/usage/administration/admin_faq.md
+++ b/docs/usage/administration/admin_faq.md
@@ -2,9 +2,9 @@
 
 How do I become a server admin?
 ---
-If your server already has an admin account you should use the user admin API to promote other accounts to become admins. See [User Admin API](../../admin_api/user_admin_api.md#Change-whether-a-user-is-a-server-administrator-or-not)
+If your server already has an admin account you should use the [User Admin API](../../admin_api/user_admin_api.md#Change-whether-a-user-is-a-server-administrator-or-not) to promote other accounts to become admins.
 
-If you don't have any admin accounts yet you won't be able to use the admin API so you'll have to edit the database manually. Manually editing the database is generally not recommended so once you have an admin account, use the admin APIs to make further changes.
+If you don't have any admin accounts yet you won't be able to use the admin API, so you'll have to edit the database manually. Manually editing the database is generally not recommended so once you have an admin account: use the admin APIs to make further changes.
 
 ```sql
 UPDATE users SET admin = 1 WHERE name = '@foo:bar.com';
@@ -32,9 +32,11 @@ What users are registered on my server?
 SELECT NAME from users;
 ```
 
-Manually resetting passwords:
+Manually resetting passwords
 ---
-See https://github.com/matrix-org/synapse/blob/master/README.rst#password-reset
+Users can reset their password through their client. Alternatively, a server admin
+can reset a user's password using the [admin API](../../admin_api/user_admin_api.md#reset-password).
+
 
 I have a problem with my server. Can I just delete my database and start again?
 ---
@@ -101,3 +103,83 @@ LIMIT 10;
 
 You can also use the [List Room API](../../admin_api/rooms.md#list-room-api)
 and `order_by` `state_events`.
+
+
+People can't accept room invitations from me
+---
+
+The typical failure mode here is that you send an invitation to someone
+to join a room or direct chat, but when they go to accept it, they get an
+error (typically along the lines of "Invalid signature"). They might see
+something like the following in their logs:
+
+    2019-09-11 19:32:04,271 - synapse.federation.transport.server - 288 - WARNING - GET-11752 - authenticate_request failed: 401: Invalid signature for server <server> with key ed25519:a_EqML: Unable to verify signature for <server>
+
+This is normally caused by a misconfiguration in your reverse-proxy. See [the reverse proxy docs](docs/reverse_proxy.md) and double-check that your settings are correct.
+
+
+Help!! Synapse is slow and eats all my RAM/CPU!
+-----------------------------------------------
+
+First, ensure you are running the latest version of Synapse, using Python 3
+with a [PostgreSQL database](../../postgres.md).
+
+Synapse's architecture is quite RAM hungry currently - we deliberately
+cache a lot of recent room data and metadata in RAM in order to speed up
+common requests. We'll improve this in the future, but for now the easiest
+way to either reduce the RAM usage (at the risk of slowing things down)
+is to set the almost-undocumented ``SYNAPSE_CACHE_FACTOR`` environment
+variable. The default is 0.5, which can be decreased to reduce RAM usage
+in memory constrained environments, or increased if performance starts to
+degrade.
+
+However, degraded performance due to a low cache factor, common on
+machines with slow disks, often leads to explosions in memory use due
+backlogged requests. In this case, reducing the cache factor will make
+things worse. Instead, try increasing it drastically. 2.0 is a good
+starting value.
+
+Using [libjemalloc](https://jemalloc.net) can also yield a significant
+improvement in overall memory use, and especially in terms of giving back
+RAM to the OS. To use it, the library must simply be put in the
+LD_PRELOAD environment variable when launching Synapse. On Debian, this
+can be done by installing the `libjemalloc1` package and adding this
+line to `/etc/default/matrix-synapse`:
+
+    LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libjemalloc.so.1
+
+This made a significant difference on Python 2.7 - it's unclear how
+much of an improvement it provides on Python 3.x.
+
+If you're encountering high CPU use by the Synapse process itself, you
+may be affected by a bug with presence tracking that leads to a
+massive excess of outgoing federation requests (see [discussion](https://github.com/matrix-org/synapse/issues/3971)). If metrics
+indicate that your server is also issuing far more outgoing federation
+requests than can be accounted for by your users' activity, this is a
+likely cause. The misbehavior can be worked around by disabling presence
+in the Synapse config file: [see here](../configuration/config_documentation.md#presence).
+
+
+Running out of File Handles
+---------------------------
+
+If Synapse runs out of file handles, it typically fails badly - live-locking
+at 100% CPU, and/or failing to accept new TCP connections (blocking the
+connecting client).  Matrix currently can legitimately use a lot of file handles,
+thanks to busy rooms like `#matrix:matrix.org` containing hundreds of participating
+servers.  The first time a server talks in a room it will try to connect
+simultaneously to all participating servers, which could exhaust the available
+file descriptors between DNS queries & HTTPS sockets, especially if DNS is slow
+to respond. (We need to improve the routing algorithm used to be better than
+full mesh, but as of March 2019 this hasn't happened yet).
+
+If you hit this failure mode, we recommend increasing the maximum number of
+open file handles to be at least 4096 (assuming a default of 1024 or 256).
+This is typically done by editing ``/etc/security/limits.conf``
+
+Separately, Synapse may leak file handles if inbound HTTP requests get stuck
+during processing - e.g. blocked behind a lock or talking to a remote server etc.
+This is best diagnosed by matching up the 'Received request' and 'Processed request'
+log lines and looking for any 'Processed request' lines which take more than
+a few seconds to execute. Please let us know at [`#synapse:matrix.org`](https://matrix.to/#/#synapse-dev:matrix.org) if
+you see this failure mode so we can help debug it, however.
diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md
index bc3d2bec6a..8ae018e628 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -444,7 +444,7 @@ Sub-options for each listener include:
    * `names`: a list of names of HTTP resources. See below for a list of valid resource names.
 
    * `compress`: set to true to enable gzip compression on HTTP bodies for this resource. This is currently only supported with the
-     `client`, `consent` and `metrics` resources.
+     `client`, `consent`, `metrics` and `federation` resources.
 
 * `additional_resources`: Only valid for an 'http' listener. A map of
    additional endpoints which should be loaded via dynamic modules.
@@ -849,7 +849,11 @@ which are older than the room's maximum retention period. Synapse will also
 filter events received over federation so that events that should have been
 purged are ignored and not stored again.
 
-The message retention policies feature is disabled by default.
+The message retention policies feature is disabled by default. Please be advised 
+that enabling this feature carries some risk. There are known bugs with the implementation
+which can cause database corruption. Setting retention to delete older history
+is less risky than deleting newer history but in general caution is advised when enabling this
+experimental feature. You can read more about this feature [here](../../message_retention_policies.md).
 
 This setting has the following sub-options:
 * `default_policy`: Default retention policy. If set, Synapse will apply it to rooms that lack the
@@ -2178,7 +2182,10 @@ their account.
 by the Matrix Identity Service API
 [specification](https://matrix.org/docs/spec/identity_service/latest).)
 
-*Updated in Synapse 1.64.0*: The `email` option is deprecated.
+*Deprecated in Synapse 1.64.0*: The `email` option is deprecated.
+
+*Removed in Synapse 1.66.0*: The `email` option has been removed.
+If present, Synapse will report a configuration error on startup.
 
 Example configuration:
 ```yaml
@@ -3348,7 +3355,7 @@ user_directory:
 For detailed instructions on user consent configuration, see [here](../../consent_tracking.md).
 
 Parts of this section are required if enabling the `consent` resource under
-`listeners`, in particular `template_dir` and `version`. # TODO: link `listeners`
+[`listeners`](#listeners), in particular `template_dir` and `version`.
 
 * `template_dir`: gives the location of the templates for the HTML forms.
   This directory should contain one subdirectory per language (eg, `en`, `fr`),
@@ -3360,7 +3367,7 @@ Parts of this section are required if enabling the `consent` resource under
    parameter.
 
 * `server_notice_content`: if enabled, will send a user a "Server Notice"
-   asking them to consent to the privacy policy. The `server_notices` section ##TODO: link
+   asking them to consent to the privacy policy. The [`server_notices` section](#server_notices)
    must also be configured for this to work. Notices will *not* be sent to
    guest users unless `send_server_notice_to_guests` is set to true.