If a user passes --advertise-tags=foo,bar (with no colons in any
segment), automatically prepend "tag:" client-side so it goes on the
wire as "tag:foo,tag:bar". Segments that already contain a colon are
left untouched and must be fully-qualified ("tag:foo"), which keeps
the door open for future colon-bearing syntax.
This was originally added in cd07437ad (2020-10-28) and then reverted
in 1be01ddc6 (2020-11-10) over forward-compatibility concerns. But
then it was realized in 2026-04-29 that this was always safe for
future extensiblity anyway (tags can't contain colons-- tag:foo:bar is
invalid anyway, per the 2020 CheckTag restrictions). So if we wanted
to perhaps some hypothetical --advertise-tags=tagset:setfoo or "group:foo",
we'd still have syntax to do, as it can't conflict with tag:group:foo.
Avery signed off on this on Slack: "Ok, I withdraw my objection to
auto-qualifying tag names in advertise-tags and I hope I won't regret
it :)"
Updates #861
Change-Id: I06935b0d3ae909894c95c9c2e185b7d6a219ff32
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
815bb291c9