diff hgext/clonebundles.py @ 26644:74de1c59f71c

clonebundles: filter on bundle specification

Not all clients are capable of reading every bundle. Currently, content
negotiation to ensure a server sends a client a compatible bundle
format is performed at request time. The response bundle is dynamically
generated at request time, so this works fine.

Clone bundles are statically generated *before* the request. This means
that a modern server could produce bundles that a legacy client isn't
capable of reading. Without some kind of "type hint" in the clone
bundles manifest, a client may attempt to download an incompatible
bundle. Furthermore, a client may not realize a bundle is incompatible
until it has processed part of the bundle (imagine consuming a 1 GB
changegroup bundle2 part only to discover the bundle2 part afterwards is
incompatibl). This would waste time and resources. And it isn't very
user friendly.

Clone bundle manifests thus need to advertise the *exact* format of the
hosted bundles so clients may filter out entries that they don't know
how to read. This patch introduces that mechanism.

We introduce the BUNDLESPEC attribute to declare the "bundle
specification" of the entry. Bundle specifications are parsed using
exchange.parsebundlespecification, which uses the same strings as the
"--type" argument to `hg bundle`. The supported bundle specifications
are well defined and backwards compatible.

When a client encounters a BUNDLESPEC that is invalid or unsupported, it
silently ignores the entry.

--- a/hgext/clonebundles.py	Tue Oct 13 10:41:54 2015 -0700
+++ b/hgext/clonebundles.py	Tue Oct 13 11:45:30 2015 -0700
@@ -32,7 +32,23 @@
 
 Metadata Attributes:
 
-TBD
+BUNDLESPEC
+   A "bundle specification" string that describes the type of the bundle.
+
+   These are string values that are accepted by the "--type" argument of
+   `hg bundle`.
+
+   The values are parsed in strict mode, which means they must be of the
+   "<compression>-<type>" form. See
+   mercurial.exchange.parsebundlespec() for more details.
+
+   Clients will automatically filter out specifications that are unknown or
+   unsupported so they won't attempt to download something that likely won't
+   apply.
+
+   The actual value doesn't impact client behavior beyond filtering:
+   clients will still sniff the bundle type from the header of downloaded
+   files.
 """
 
 from mercurial import (