diff mercurial/help/internals/wireprotocolv2.txt @ 39637:c7a7c7e844e5

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

--- a/mercurial/help/internals/wireprotocolv2.txt	Wed Aug 22 14:51:11 2018 -0700
+++ b/mercurial/help/internals/wireprotocolv2.txt	Wed Sep 05 09:09:52 2018 -0700
@@ -258,6 +258,80 @@
 
 On success, returns a bytestring containing the resolved node.
 
+manifestdata
+------------
+
+Obtain various data related to manifests (which are lists of files in
+a revision).
+
+The command accepts the following arguments:
+
+fields
+   (set of bytestring) Which data associated with manifests to fetch.
+   The following values are recognized:
+
+   parents
+      Parent nodes for the manifest.
+
+   revision
+      The raw revision data for the manifest.
+
+nodes
+   (array of bytestring) Manifest nodes whose data to retrieve.
+
+tree
+   (bytestring) Path to manifest to retrieve. The empty bytestring represents
+   the root manifest. All other values represent directories/trees within
+   the repository.
+
+TODO allow specifying revisions via alternate means (such as from changeset
+revisions or ranges)
+TODO consider recursive expansion of manifests (with path filtering for
+narrow use cases)
+TODO more control over whether to emit fulltexts or deltas
+
+The response bytestream starts with a CBOR map describing the data that
+follows. It has the following bytestring keys:
+
+totalitems
+   (unsigned integer) Total number of manifest revisions whose data is
+   being returned.
+
+Following the header map is a series of 0 or more CBOR values. The first
+value is always a map describing a manifest revision. If this map has the
+``deltasize`` or ``revisionsize`` keys, a bytestring containing the delta
+or revision, respectively, will immediately follow the map. Otherwise
+the next value will be a map describing the next manifest revision.
+
+Each map has the following bytestring keys:
+
+node
+   (bytestring) The node of the manifest revision whose data is represented.
+
+deltabasenode
+   (bytestring) The node that the delta representation of this revision is
+   computed against. Only present if the ``revision`` field is requested and
+   a delta is being emitted.
+
+deltasize
+   (unsigned integer) The size of the delta data that follows this map.
+   Only present if the ``revision`` field is requested and a delta is
+   being emitted.
+
+parents
+   (array of bytestring) The nodes of the parents of this manifest revision.
+   Only present if the ``parents`` field is requested.
+
+revisionsize
+   (unsigned integer) The size of the fulltext revision data that follows
+   this map. Only present if the ``revision`` field is requested and a fulltext
+   revision is being emitted.
+
+When ``revision`` data is requested, the server chooses to emit either fulltext
+revision data or a delta. What the server decides can be inferred by looking
+for the presence of the ``deltasize`` or ``revisionsize`` keys in the map.
+Servers MUST NOT define both keys.
+
 pushkey
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