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cmd/go/internal/modload: document mvsReqs.Max

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The version "" denotes the main module, which has no version. The
mvs.Reqs interface documentation hints this is allowed, but it's not
obvious from the implementation in modload.mvsReqs.Max.

Also, replace a related TODO with a comment in mvs.Downgrade.

Fixes #39042

Change-Id: I11e10908c9b3d8c2283eaa5c04bd8e1b936851fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/234003
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
This commit is contained in:
Jay Conrod 2020-05-14 15:03:26 -04:00
parent f8662a5a96
commit 8be0de1e7e
2 changed files with 24 additions and 5 deletions
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6
src/cmd/go/internal/modload/mvs.go
View File

@ -157,6 +157,12 @@ func (r *mvsReqs) required(mod module.Version) ([]module.Version, error) {
return r.modFileToList(f), nil
}
// Max returns the maximum of v1 and v2 according to semver.Compare.
//
// As a special case, the version "" is considered higher than all other
// versions. The main module (also known as the target) has no version and must
// be chosen over other versions of the same module in the module dependency
// graph.
func (*mvsReqs) Max(v1, v2 string) string {
if v1 != "" && semver.Compare(v1, v2) == -1 {
return v2

23
src/cmd/go/internal/mvs/mvs.go
View File

@ -115,7 +115,21 @@ func (e *BuildListError) Error() string {
}
// BuildList returns the build list for the target module.
// The first element is the target itself, with the remainder of the list sorted by path.
//
// target is the root vertex of a module requirement graph. For cmd/go, this is
// typically the main module, but note that this algorithm is not intended to
// be Go-specific: module paths and versions are treated as opaque values.
//
// reqs describes the module requirement graph and provides an opaque method
// for comparing versions.
//
// BuildList traverses the graph and returns a list containing the highest
// version for each visited module. The first element of the returned list is
// target itself; reqs.Max requires target.Version to compare higher than all
// other versions, so no other version can be selected. The remaining elements
// of the list are sorted by path.
//
// See https://research.swtch.com/vgo-mvs for details.
func BuildList(target module.Version, reqs Reqs) ([]module.Version, error) {
return buildList(target, reqs, nil)
}
@ -220,10 +234,9 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) (m
// The final list is the minimum version of each module found in the graph.
if v := min[target.Path]; v != target.Version {
// TODO(jayconrod): there is a special case in modload.mvsReqs.Max
// that prevents us from selecting a newer version of a module
// when the module has no version. This may only be the case for target.
// Should we always panic when target has a version?
// target.Version will be "" for modload, the main client of MVS.
// "" denotes the main module, which has no version. However, MVS treats
// version strings as opaque, so "" is not a special value here.
// See golang.org/issue/31491, golang.org/issue/29773.
panic(fmt.Sprintf("mistake: chose version %q instead of target %+v", v, target)) // TODO: Don't panic.
}