commit d700dc7801a9e23ddda0d482eeb2b64ced3ca756 Author: Nick Mathewson nickm@torproject.org Date: Sat Nov 16 14:31:49 2019 -0500

Topic documentation on our publish-subscribe architecture. --- src/core/mainloop/mainloop_pubsub.h | 32 ++++++++ src/lib/pubsub/publish_subscribe.md | 141 ++++++++++++++++++++++++++++++++++++ src/mainpage.md | 3 +- 3 files changed, 175 insertions(+), 1 deletion(-)

diff --git a/src/core/mainloop/mainloop_pubsub.h b/src/core/mainloop/mainloop_pubsub.h index bd57c0c17..c02127401 100644 --- a/src/core/mainloop/mainloop_pubsub.h +++ b/src/core/mainloop/mainloop_pubsub.h @@ -14,9 +14,41 @@

struct pubsub_builder_t;

+/** + * Describe when and how messages are delivered on message channel. + * + * Every message channel must be associated with one of these strategies. + **/ typedef enum { + /** + * Never deliver messages automatically. + * + * If a message channel uses this strategy, then no matter now many + * messages are published on it, they are not delivered until something + * manually calls dispatch_flush() for that channel + **/ DELIV_NEVER=0, + /** + * Deliver messages promptly, via the event loop. + * + * If a message channel uses this strategy, then publishing a messages + * that channel activates an event that causes messages to be handled + * later in the mainloop. The messages will be processed at some point + * very soon, delaying only for pending IO events and the like. + * + * Generally this is the best choice for a delivery strategy, since + * it avoids stack explosion. + **/ DELIV_PROMPT, + /** + * Deliver messages immediately, skipping the event loop. + * + * Every event on this channel is flushed immediately after it is queued, + * using the stack. + * + * This delivery type should be used with caution, since it can cause + * unexpected call chains, resource starvation, and the like. + **/ DELIV_IMMEDIATE, } deliv_strategy_t;

diff --git a/src/lib/pubsub/publish_subscribe.md b/src/lib/pubsub/publish_subscribe.md new file mode 100644 index 000000000..019591bc1 --- /dev/null +++ b/src/lib/pubsub/publish_subscribe.md @@ -0,0 +1,141 @@ + +@page publish_subscribe Publish-subscribe message passing in Tor + +@tableofcontents + +## Introduction + +Tor has introduced a generic publish-subscribe mechanism for delivering +messages internally. It is meant to help us improve the modularity of +our code, by avoiding direct coupling between modules that don't +actually need to invoke one another. + +This publish-subscribe mechanism is *not* meant for handing +multithreading or multiprocess issues, thought we hope that eventually +it might be extended and adapted for that purpose. Instead, we use +publish-subscribe today to decouple modules that shouldn't be calling +each other directly. + +For example, there are numerous parts of our code that might need to +take action when a circuit is completed: a controller might need to be +informed, an onion service negotiation might need to be attached, a +guard might need to be marked as working, or a client connection might +need to be attached. But many of those actions occur at a higher layer +than circuit completion: calling them directly is a layering violation, +and makes our code harder to understand and analyze. + +But with message-passing, we can invert this layering violation: circuit +completion can become a "message" that the circuit code publishes, and +to which higher-level layers subscribe. This means that circuit +handling can be decoupled from higher-level modules, and stay nice and +simple. (@ref pubsub_notyet "1") + +> @anchor pubsub_notyet 1. Unfortunately, like most of our code, circuit +> handling is _not_ yet refactored to use publish-subscribe throughout. +> Instead, layer violations of the type described here are pretty common +> in Tor today. To see a small part of what happens when a circuit is +> completed today, have a look at circuit_build_no_more_hops() and its +> associated code. + +## Channels and delivery policies + +To work with messages, especially when refactoring existing code, you'll +need to understand "channels" and "delivery policies". + +Every message is delivered on a "message channel". Each channel +(conceptually) a queue-like structure that can support an arbitrarily +number of message types. Where channels vary is their delivery +mechanisms, and their guarantees about when messages are processed. + +Currently, three delivery policies are possible: + + - `DELIV_PROMPT` -- causes messages to be processed via a callback in + Tor's event loop. This is generally the best choice, since it + avoids unexpected growth of the stack. + + - `DELIV_IMMEDIATE` -- causes messages to be processed immediately + on the call stack when they are published. This choice grows the + stack, and can lead to unexpected complexity in the call graph. + We should only use it when necessary. + + - `DELIV_NEVER` -- causes messages not to be delivered by the message + dispatch system at all. Instead, some other part of the code must + call dispatch_flush() to get the messages delivered. + +## Layers: Dispatch vs publish-subsubscribe vs mainloop. + +At the lowest level, messages are sent via the "dispatcher" module in +@refdir{lib/dispatch}. For performance, this dispatcher works with a +untyped messages. Publishers, subscribers, channels, and messages are +distinguished by short integers. Associated data is handled as +dynamically-typed data pointers, and its types are also stored as short +integers. + +Naturally, this results in a type-unsafe C API, so most other modules +shouldn't invoke @refdir{lib/dispatch} directly. At a higher level, +@refdir{lib/pubsub} defines a set of functions and macros that make +messages named and type-safe. This is the one that other modules should +use when they want to send or receive a message. + +The two modules above do not handle message delivery. Instead, the +dispatch module takes a callback that it can invoke when a channel +becomes nonempty, and defines a dispatch_flush() function to deliver all +the messages queued in a channel. The work of actually making sure that +dispatch_flush() is called when appropriate falls to the main loop, +which needs to integrate the message dispatcher with the rest of our +events and callbacks. This work happens in mainloop_pubsub.c. + + +## How to publish and subscribe + +This section gives an overview of how to make new messages and how to +use them. For full details, see pubsub_macros.h. + +Before anybody can publish or subscribe to a message, the message must +be declared, typically in a header. This uses DECLARE_MESSAGE() or +DECLARE_MESSAGE_INT(). + +Only subsystems can publish or subscribe messages. For more information +about the subsystems architecture, see @ref initialization. + +To publish a message, you must: + - Include the header that declares the message. + - Declare a set of helper functions via DECLARE_PUBLISH(). These + must be visible wherever you call PUBLISH(). + - Call PUBLISH() to actually send a message. + - Connect your subsystem to the dispatcher by calling + DISPATCH_ADD_PUB() from your subsystem's subsys_fns_t.add_pubsub + callback. + +To subscribe to a message, you must: + - Include the header that declares the message. + - Declare a callback function to be invoked when the message is delivered. + - Use DISPATCH_SUBSCRIBE at file scope to define a set of wrapper + functions to call your callback function with the appropriate type. + - Connect your subsystem to the dispatcher by calling + DISPATCH_ADD_SUB() from your subsystem's subsys_fns_t.add_pubsub + callback. + +Again, the file-level documentation for pubsub_macros.h describes how to +declare a message, how to publish it, and how to subscribe to it. + +## Designing good messages + +**Frequency**: +The publish-subscribe system uses a few function calls +and allocations for each message sent. This makes it unsuitable for +very-high-bandwidth events, like "receiving a single data cell" or "a +socket has become writable." It's fine, however, for events that +ordinarily happen a bit less frequently than that, like a circuit +getting finished, a new connection getting opened, or so on. + +**Semantics**: +A message should declare that something has happened or is happening, +not that something in particular should be done. + +For example, suppose you want to set up a message so that onion services +clean up their replay caches whenever we're low on memory. The event +should be something like `memory_low`, not `clean_up_replay_caches`. +The latter name would imply that the publisher knew who was subscribing +to the message and what they intended to do about it, which would be a +layering violation. diff --git a/src/mainpage.md b/src/mainpage.md index a2c5ec630..63a5b0a3f 100644 --- a/src/mainpage.md +++ b/src/mainpage.md @@ -41,6 +41,8 @@ Tor repository.

@subpage time_periodic

+@subpage publish_subscribe +

@page intro A high-level overview

@@ -140,4 +142,3 @@ more connection types.

A 'Node' (node_t) is a view of a Tor instance's current knowledge and opinions about a Tor relay or bridge. -