[PATCH] Config option to set the transport protocol version for network fetches

[Alex Riesen <alexander.riesen@cetitec.com>]

Affecting the transfers initiated by fetch and push,
the option allows to control network operations similar
to --ipv4 and --ipv6 options.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Alex Riesen <alexander.riesen@cetitec.com>
---

Jeff King, Wed, Sep 16, 2020 22:02:03 +0200:
> On Tue, Sep 15, 2020 at 03:54:28PM +0200, Alex Riesen wrote:
> 
> > Jeff King, Tue, Sep 15, 2020 15:05:06 +0200:
> > > On Tue, Sep 15, 2020 at 01:50:25PM +0200, Alex Riesen wrote:
> > > > Sure! Thinking about it, I actually would have preferred to have both: a
> > > > config option and a command-line option. So that I can set --ipv4 in, say,
> > > > ~/.config/git/config file, but still have the option to try --ipv6 from time
> > > > to time to check if the network setup magically fixed itself.
> > > > 
> > > > What would the preferred name for that config option be? fetch.ipv?
> > > 
> > > It looks like we've got similar options for clone/pull (which are really
> > > fetch under the hood of course) and push. We have the "transfer.*"
> > > namespace which applies to both already. So maybe "transfer.ipversion"
> > > or something?
> > 
> > Something like this?
> 
> That's the right direction, but I think we'd want to make sure it
> impacted all of the spots that allow switching. "clone" on the fetching
> side, but probably also "push".
> 
> Each of those commands could learn the same config, but it might be
> easier to just enforce it in the transport layer. Something like this
> maybe (compiled but not tested):

I have merged the patches.

> A few notes:
> 
>   - it cheats a little by noting that command-line options can't get it
>     to "ALL". It might be a bit cleaner if we have an explicit
>     TRANSPORT_FAMILY_UNSET, and then resolve the default at look-up
>     time.
> 
>   - it probably needs more "if (family)" spots in each command, which is
>     unfortunate (which again might go away if "UNSET" is 0).

As this is still being discussed, and because I still imagine the
transport protocol family as a list of allowed protocols, this part
is not in the patch below.

>   - I waffled on the name. transfer.* is where we put things that apply
>     to both push/fetch, but it's usually more related to the git layer.
>     This is more of a core networking decision, and if we added other
>     network programs, I'd expect them to respect it. So maybe "core." is
>     a better namespace.

I agree.

 Documentation/config/core.txt |  7 +++++++
 builtin/fetch.c               |  3 ++-
 builtin/push.c                |  3 ++-
 transport.c                   | 19 +++++++++++++++++++
 4 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
index 74619a9c03..dcb7db9799 100644
--- a/Documentation/config/core.txt
+++ b/Documentation/config/core.txt
@@ -626,3 +626,10 @@ core.abbrev::
 	in your repository, which hopefully is enough for
 	abbreviated object names to stay unique for some time.
 	The minimum length is 4.
+
+core.ipversion::
+	Limit the network operations to the specified version of the transport
+	protocol. Can be specified as `4` to allow IPv4 only, `6` for IPv6, or
+	`all` to allow all protocols.
+	See also linkgit:git-fetch[1] options `--ipv4` and `--ipv6`.
+	The default value is `all` to allow all protocols.
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 447d28ac29..41f82d61d7 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1248,7 +1248,8 @@ static struct transport *prepare_transport(struct remote *remote, int deepen)
 
 	transport = transport_get(remote, NULL);
 	transport_set_verbosity(transport, verbosity, progress);
-	transport->family = family;
+	if (family)
+		transport->family = family;
 	if (upload_pack)
 		set_option(transport, TRANS_OPT_UPLOADPACK, upload_pack);
 	if (keep)
diff --git a/builtin/push.c b/builtin/push.c
index bc94078e72..f7a40b65cd 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -343,7 +343,8 @@ static int push_with_options(struct transport *transport, struct refspec *rs,
 	char *anon_url = transport_anonymize_url(transport->url);
 
 	transport_set_verbosity(transport, verbosity, progress);
-	transport->family = family;
+	if (family)
+		transport->family = family;
 
 	if (receivepack)
 		transport_set_option(transport,
diff --git a/transport.c b/transport.c
index b41386eccb..e16c339f3e 100644
--- a/transport.c
+++ b/transport.c
@@ -922,6 +922,23 @@ static struct transport_vtable builtin_smart_vtable = {
 	disconnect_git
 };
 
+static enum transport_family default_transport_family(void)
+{
+	static const char key[] = "core.ipversion";
+	const char *v;
+
+	if (git_config_get_string_const(key, &v))
+		return TRANSPORT_FAMILY_ALL;
+	if (!strcmp(v, "all"))
+		return TRANSPORT_FAMILY_ALL;
+	if (!strcmp(v, "ipv4"))
+		return TRANSPORT_FAMILY_IPV4;
+	if (!strcmp(v, "ipv6"))
+		return TRANSPORT_FAMILY_IPV6;
+
+	die(_("%s: unknown value '%s'"), key, v);
+}
+
 struct transport *transport_get(struct remote *remote, const char *url)
 {
 	const char *helper;
@@ -951,6 +968,8 @@ struct transport *transport_get(struct remote *remote, const char *url)
 			helper = xstrndup(url, p - url);
 	}
 
+	ret->family = default_transport_family();
+
 	if (helper) {
 		transport_helper_init(ret, helper);
 	} else if (starts_with(url, "rsync:")) {
-- 
2.28.0.22.gfab0d4627e