cmd/go/internal/mvs: fix Downgrade to match Algorithm 4
mvs.Downgrade is pretty clearly intended to match Algorithm 4 from the
MVS blog post (https://research.swtch.com/vgo-mvs#algorithm_4).
Per the blog post:
“Downgrading one module may require downgrading other modules, but we
want to downgrade as few other modules as possible. … To avoid an
unnecessary downgrade to E 1.1, we must also add a new requirement on
E 1.2. We can apply Algorithm R to find the minimal set of new
requirements to write to go.mod.”
mvs.Downgrade does not match that behavior today: it fails to retain
the selected versions of transitive dependencies that are not implied
by downgraded direct dependencies of the target (module E in the
post). This bug is currently masked by the fact that we only call
Downgrade today with a *modload.mvsReqs, for which the Required method
happens to return the complete build list — rather than only the
direct dependencies as documented for the mvs.Reqs interface.
For #36460
Change-Id: If9c8f413b156b5f67c02787d9359394e169951b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/287633
Trust: Bryan C. Mills <bcmills@google.com> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>