Gregory Szorc <gregory.szorc@gmail.com> [Wed, 05 Sep 2018 09:10:17 -0700] rev 39655
wireprotov2: define and implement "filedata" command
Continuing our trend of implementing *data commands for retrieving
information about specific repository data primitives, this commit
implements a command for retrieving data about an individual tracked
file.
The command is very similar to "manifestdata." The only significant
difference is that we have a standalone function for obtaining
storage for a tracked file. This is to provide a monkeypatch point
for extensions to implement path-based access control.
With this API available, wire protocol version 2 now exposes all
data primitives necessary to implement a full clone. Of course,
since "filedata" can only resolve data for a single path at a time,
clients would need to issue N commands to perform a full clone. On
the Firefox repository, this would be ~461k commands. We'll likely
need to implement a file data retrieval command that supports
multiple paths. But that can be implemented later.
Differential Revision: https://phab.mercurial-scm.org/D4490
Gregory Szorc <gregory.szorc@gmail.com> [Wed, 05 Sep 2018 09:09:57 -0700] rev 39654
exchangev2: fetch manifest revisions
Now that the server has support for retrieving manifest data, we can
implement the client bits to call it.
We teach the changeset fetching code to capture the manifest revisions
that are encountered on incoming changesets. We then feed this into a
new function which filters out known manifests and then batches up
manifest data requests to the server.
This is different from the previous wire protocol in a few notable
ways.
First, the client fetches manifest data separately and explicitly.
Before, we'd ask the server for data pertaining to some changesets
(via a "getbundle" command) and manifests (and files) would be sent
automatically. Providing an API for looking up just manifest data
separately gives clients much more flexibility for manifest management.
For example, a client may choose to only fetch manifest data on demand
instead of prefetching it (i.e. partial clone).
Second, we send N commands to the server for manifest retrieval instead
of 1. This property has a few nice side-effects. One is that the
deterministic nature of the requests lends itself to server-side
caching. For example, say the remote has 50,000 manifests. If the
server is configured to cache responses, each time a new commit
arrives, you will have a cache miss and need to regenerate all outgoing
data. But if you makes N requests requesting 10,000 manifests each,
a new commit will still yield cache hits on the initial, unchanged
manifest batches/requests.
A derived benefit from these properties is that resumable clone is
conceptually simpler to implement. When making a monolithic request
for all of the repository data, recovering from an interrupted clone
is hard because the server was in the driver's seat and was maintaining
state about all the data that needed transferred. With the client
driving fetching, the client can persist the set of unfetched entities
and retry/resume a fetch if something goes wrong. Or we can fetch all
data N changesets at a time and slowly build up a repository. This
approach is drastically easier to implement when we have server APIs
exposing low-level repository primitives (such as manifests and files).
We don't yet support tree manifests. But it should be possible to
implement that with the existing wire protocol command.
Differential Revision: https://phab.mercurial-scm.org/D4489
Gregory Szorc <gregory.szorc@gmail.com> [Wed, 05 Sep 2018 09:09:52 -0700] rev 39653
wireprotov2: define and implement "manifestdata" command
The added command can be used for obtaining manifest data.
Given a manifest path and set of manifest nodes, data about
manifests can be retrieved.
Unlike changeset data, we wish to emit deltas to describe
manifest revisions. So the command uses the relatively new
API for building delta requests and emitting them.
The code calls into deltaparent(), which I'm not very keen of.
There's still work to be done in delta generation land so
implementation details of storage (e.g. exactly one delta
is stored/available) don't creep into higher levels. But we
can worry about this later (there is already a TODO on
imanifestorage tracking this).
On the subject of parent deltas, the server assumes parent revisions
exist on the receiving end. This is obviously wrong for shallow
clone. I've added TODOs to add a mechanism to the command to
allow clients to specify desired behavior. This shouldn't be
too difficult to implement.
Another big change is that the client must explicitly request
manifest nodes to retrieve. This is a major departure from
"getbundle," where the server derives relevant manifests as it
iterates changesets and sends them automatically. As implemented,
the client must transmit each requested node to the server. At
20 bytes per node, we're looking at 2 MB per 100,000 nodes. Plus
wire encoding overhead. This isn't ideal for clients with limited
upload bandwidth. I plan to address this in the future by allowing
alternate mechanisms for defining the revisions to retrieve. One
idea is to define a range of changeset revisions whose manifest
revisions to retrieve (similar to how "changesetdata" works).
We almost certainly want an API to look up an individual manifest
by node. And that's where I've chosen to start with the implementation.
Again, a theme of this early exchangev2 work is I want to start by
building primitives for accessing raw repository data first and see
how far we can get with those before we need more complexity.
Differential Revision: https://phab.mercurial-scm.org/D4488
Gregory Szorc <gregory.szorc@gmail.com> [Wed, 22 Aug 2018 14:51:11 -0700] rev 39652
wireprotov2: add TODOs around extending changesetdata fields
Extensions will inevitably want to extend the set of changeset
data/fields that can be requested. We'll need to implement support
for extending this in the future. Add some TODOs to track that.
Differential Revision: https://phab.mercurial-scm.org/D4487
Gregory Szorc <gregory.szorc@gmail.com> [Wed, 29 Aug 2018 17:03:19 -0700] rev 39651
exchangev2: fetch and apply bookmarks
This is pretty similar to phases data. We collect bookmarks data
as we process records. Then at the end we make a call to the
bookmarks subsystem to reflect the remote's bookmarks.
Like phases, the code for handling bookmarks is vastly simpler
than the previous wire protocol code because the server always
transfers the full set of bookmarks when bookmarks are requested.
We don't have to keep track of whether we requested bookmarks or
not.
Differential Revision: https://phab.mercurial-scm.org/D4486
Gregory Szorc <gregory.szorc@gmail.com> [Thu, 23 Aug 2018 18:14:19 -0700] rev 39650
wireprotov2: add bookmarks to "changesetdata" command
Like we did for phases, we want to emit bookmarks data attached
to each changeset.
The approach here is very similar to phases: we emit bookmarks
data inline with requested revision data. But we emit
records for nodes that weren't requested as well so consumers have
access to the full set of defined bookmarks.
Differential Revision: https://phab.mercurial-scm.org/D4485
Gregory Szorc <gregory.szorc@gmail.com> [Wed, 12 Sep 2018 10:01:58 -0700] rev 39649
exchangev2: fetch and apply phases data
Now that the server supports emitting phases data, we can request it
and apply it on the client.
Because we may receive phases-only updates from the server, we no
longer conditionally perform the "changesetdata" command depending
on whether there are revisions to fetch. In the previous wire
protocol, this case would result in us falling back to performing
"listkeys" commands to look up phases, bookmarks, etc data. But
since "changesetdata" is smart enough to handle metadata only
fetches, we can keep things consistent.
It's worth noting that because of the unified approach to changeset
data retrieval, phase handling code in wire proto v2 exchange is
drastically simpler. Contrast with all the code in exchange.py
dealing with all the variations for obtaining phases data.
Differential Revision: https://phab.mercurial-scm.org/D4484
Gregory Szorc <gregory.szorc@gmail.com> [Tue, 28 Aug 2018 18:19:23 -0700] rev 39648
wireprotov2: add phases to "changesetdata" command
This commit teaches the "changesetdata" wire protocol command
to emit the phase state for each changeset.
This is a different approach from existing phase transfer in a
few ways. Previously, if there are no new revisions (or we're
not using bundle2), we perform a "listkeys" request to retrieve
phase heads. And when revision data is being transferred
with bundle2, phases data is encoded in a standalone bundle2 part.
In both cases, phases data is logically decoupled from the changeset
data and is encountered/applied after changeset revision data
is received.
The new wire protocol purposefully tries to more tightly associate
changeset metadata (phases, bookmarks, obsolescence markers, etc)
with the changeset revision and index data itself, rather than
have it live as a separate entity that must be fetched and
processed separately. I reckon that one reason we didn't do this
before was it was difficult to add new data types/fields without
breaking existing consumers. By using CBOR maps to transfer
changeset data and putting clients in control of what fields are
requested / present in those maps, we can easily add additional
changeset data while maintaining backwards compatibility. I believe
this to be a superior approach to the problem.
That being said, for performance reasons, we may need to resort
to alternative mechanisms for transferring data like phases. But
for now, I think giving the wire protocol the ability to transfer
changeset metadata next to the changeset itself is a powerful feature
because it is a raw, changeset-centric data API. And if you build
simple APIs for accessing the fundamental units of repository data,
you enable client-side experimentation (partial clone, etc). If it
turns out that we need specialized APIs or mechanisms for transferring
data like phases, we can build in those APIs later. For now, I'd
like to see how far we can get on simple APIs.
It's worth noting that when phase data is being requested, the
server will also emit changeset records for nodes in the bases
specified by the "noderange" argument. This is to ensure that
phase-only updates for nodes the client has are available to the
client, even if no new changesets will be transferred.
Differential Revision: https://phab.mercurial-scm.org/D4483