bundle-uri: store fetch.bundleCreationToken

When a bundle list specifies the "creationToken" heuristic, the Git
client downloads the list and then starts downloading bundles in
descending creationToken order. This process stops as soon as all
downloaded bundles can be applied to the repository (because all
required commits are present in the repository or in the downloaded
bundles).

When checking the same bundle list twice, this strategy requires
downloading the bundle with the maximum creationToken again, which is
wasteful. The creationToken heuristic promises that the client will not
have a use for that bundle if its creationToken value is at most the
previous creationToken value.

To prevent these wasteful downloads, create a fetch.bundleCreationToken
config setting that the Git client sets after downloading bundles. This
value allows skipping that maximum bundle download when this config
value is the same value (or larger).

To test that this works correctly, we can insert some "duplicate"
fetches into existing tests and demonstrate that only the bundle list is
downloaded.

The previous logic for downloading bundles by creationToken worked even
if the bundle list was empty, but now we have logic that depends on the
first entry of the list. Terminate early in the (non-sensical) case of
an empty bundle list.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/Documentation/config/fetch.txt b/Documentation/config/fetch.txt
index 244f44d460..568f0f75b3 100644
--- a/Documentation/config/fetch.txt
+++ b/Documentation/config/fetch.txt
@@ -104,3 +104,19 @@ fetch.bundleURI::
 	linkgit:git-clone[1]. `git clone --bundle-uri` will set the
 	`fetch.bundleURI` value if the supplied bundle URI contains a bundle
 	list that is organized for incremental fetches.
++
+If you modify this value and your repository has a `fetch.bundleCreationToken`
+value, then remove that `fetch.bundleCreationToken` value before fetching from
+the new bundle URI.
+
+fetch.bundleCreationToken::
+	When using `fetch.bundleURI` to fetch incrementally from a bundle
+	list that uses the "creationToken" heuristic, this config value
+	stores the maximum `creationToken` value of the downloaded bundles.
+	This value is used to prevent downloading bundles in the future
+	if the advertised `creationToken` is not strictly larger than this
+	value.
++
+The creation token values are chosen by the provider serving the specific
+bundle URI. If you modify the URI at `fetch.bundleURI`, then be sure to
+remove the value for the `fetch.bundleCreationToken` value before fetching.