fetch-pack: wire up and enable auto filter logic

Previous commits have set up an infrastructure for `--filter=auto` to
automatically prepare a partial clone filter based on what the server
advertised and the client accepted.

Using that infrastructure, let's now enable the `--filter=auto` option
in `git clone` and `git fetch` by setting `allow_auto_filter` to 1.

Note that these small changes mean that when `git clone --filter=auto`
or `git fetch --filter=auto` are used, "auto" is automatically saved
as the partial clone filter for the server on the client. Therefore
subsequent calls to `git fetch` on the client will automatically use
this "auto" mode even without `--filter=auto`.

Let's also set `allow_auto_filter` to 1 in `transport.c`, as the
transport layer must be able to accept the "auto" filter spec even if
the invoking command hasn't fully parsed it yet.

When an "auto" filter is requested, let's have the "fetch-pack.c" code
in `do_fetch_pack_v2()` compute a filter and send it to the server.

In `do_fetch_pack_v2()` the logic also needs to check for the
"promisor-remote" capability and call `promisor_remote_reply()` to
parse advertised remotes and populate the list of those accepted (and
their filters).

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 45 insertions, 15 deletions

diff --git a/Documentation/fetch-options.adoc b/Documentation/fetch-options.adoc
index 1ef9807d00..a0cfb50d89 100644
--- a/Documentation/fetch-options.adoc
+++ b/Documentation/fetch-options.adoc
@@ -92,11 +92,20 @@ precedence over the `fetch.output` config option.
 	Use the partial clone feature and request that the server sends
 	a subset of reachable objects according to a given object filter.
 	When using `--filter`, the supplied _<filter-spec>_ is used for
-	the partial fetch. For example, `--filter=blob:none` will filter
-	out all blobs (file contents) until needed by Git. Also,
-	`--filter=blob:limit=<size>` will filter out all blobs of size
-	at least _<size>_. For more details on filter specifications, see
-	the `--filter` option in linkgit:git-rev-list[1].
+	the partial fetch.
++
+If `--filter=auto` is used, the filter specification is determined
+automatically by combining the filter specifications advertised by
+the server for the promisor remotes that the client accepts (see
+linkgit:gitprotocol-v2[5] and the `promisor.acceptFromServer`
+configuration option in linkgit:git-config[1]).
++
+For details on all other available filter specifications, see the
+`--filter=<filter-spec>` option in linkgit:git-rev-list[1].
++
+For example, `--filter=blob:none` will filter out all blobs (file
+contents) until needed by Git. Also, `--filter=blob:limit=<size>` will
+filter out all blobs of size at least _<size>_.
 
 ifndef::git-pull[]
 `--write-fetch-head`::
diff --git a/Documentation/git-clone.adoc b/Documentation/git-clone.adoc
index 57cdfb7620..0db2d1e5f0 100644
--- a/Documentation/git-clone.adoc
+++ b/Documentation/git-clone.adoc
@@ -187,11 +187,26 @@ objects from the source repository into a pack in the cloned repository.
 	Use the partial clone feature and request that the server sends
 	a subset of reachable objects according to a given object filter.
 	When using `--filter`, the supplied _<filter-spec>_ is used for
-	the partial clone filter. For example, `--filter=blob:none` will
-	filter out all blobs (file contents) until needed by Git. Also,
-	`--filter=blob:limit=<size>` will filter out all blobs of size
-	at least _<size>_. For more details on filter specifications, see
-	the `--filter` option in linkgit:git-rev-list[1].
+	the partial clone filter.
++
+If `--filter=auto` is used the filter specification is determined
+automatically through the 'promisor-remote' protocol (see
+linkgit:gitprotocol-v2[5]) by combining the filter specifications
+advertised by the server for the promisor remotes that the client
+accepts (see the `promisor.acceptFromServer` configuration option in
+linkgit:git-config[1]). This allows the server to suggest the optimal
+filter for the available promisor remotes.
++
+As with other filter specifications, the "auto" value is persisted in
+the configuration. This ensures that future fetches will continue to
+adapt to the server's current recommendation.
++
+For details on all other available filter specifications, see the
+`--filter=<filter-spec>` option in linkgit:git-rev-list[1].
++
+For example, `--filter=blob:none` will filter out all blobs (file
+contents) until needed by Git. Also, `--filter=blob:limit=<size>` will
+filter out all blobs of size at least _<size>_.
 
 `--also-filter-submodules`::
 	Also apply the partial clone filter to any submodules in the repository.
diff --git a/Documentation/gitprotocol-v2.adoc b/Documentation/gitprotocol-v2.adoc
index d93dd279ea..f985cb4c47 100644
--- a/Documentation/gitprotocol-v2.adoc
+++ b/Documentation/gitprotocol-v2.adoc
@@ -812,10 +812,15 @@ MUST appear first in each pr-fields, in that order.
 After these mandatory fields, the server MAY advertise the following
 optional fields in any order:
 
-`partialCloneFilter`:: The filter specification used by the remote.
+`partialCloneFilter`:: The filter specification for the remote. It
+corresponds to the "remote.<name>.partialCloneFilter" config setting.
 Clients can use this to determine if the remote's filtering strategy
-is compatible with their needs (e.g., checking if both use "blob:none").
-It corresponds to the "remote.<name>.partialCloneFilter" config setting.
+is compatible with their needs (e.g., checking if both use
+"blob:none"). Additionally they can use this through the
+`--filter=auto` option in linkgit:git-clone[1]. With that option, the
+filter specification of the clone will be automatically computed by
+combining the filter specifications of the promisor remotes the client
+accepts.
 
 `token`:: An authentication token that clients can use when
 connecting to the remote. It corresponds to the "remote.<name>.token"
@@ -828,8 +833,9 @@ future protocol extensions.
 
 The client can use information transmitted through these fields to
 decide if it accepts the advertised promisor remote. Also, the client
-can be configured to store the values of these fields (see
-"promisor.storeFields" in linkgit:git-config[1]).
+can be configured to store the values of these fields or use them
+to automatically configure the repository (see "promisor.storeFields"
+in linkgit:git-config[1] and `--filter=auto` in linkgit:git-clone[1]).
 
 Field values MUST be urlencoded.