1 files changed, 368 insertions, 0 deletions
diff --git a/libgo/go/cmd/go/internal/mvs/mvs.go b/libgo/go/cmd/go/internal/mvs/mvs.go
new file mode 100644
index 00000000000..8ec9162dabc
--- /dev/null
+++ b/libgo/go/cmd/go/internal/mvs/mvs.go
@@ -0,0 +1,368 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package mvs implements Minimal Version Selection.
+// See https://research.swtch.com/vgo-mvs.
+package mvs
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/module"
+	"cmd/go/internal/par"
+)
+
+// A Reqs is the requirement graph on which Minimal Version Selection (MVS) operates.
+//
+// The version strings are opaque except for the special version "none"
+// (see the documentation for module.Version). In particular, MVS does not
+// assume that the version strings are semantic versions; instead, the Max method
+// gives access to the comparison operation.
+//
+// It must be safe to call methods on a Reqs from multiple goroutines simultaneously.
+// Because a Reqs may read the underlying graph from the network on demand,
+// the MVS algorithms parallelize the traversal to overlap network delays.
+type Reqs interface {
+	// Required returns the module versions explicitly required by m itself.
+	// The caller must not modify the returned list.
+	Required(m module.Version) ([]module.Version, error)
+
+	// Max returns the maximum of v1 and v2 (it returns either v1 or v2).
+	//
+	// For all versions v, Max(v, "none") must be v,
+	// and for the tanget passed as the first argument to MVS functions,
+	// Max(target, v) must be target.
+	//
+	// Note that v1 < v2 can be written Max(v1, v2) != v1
+	// and similarly v1 <= v2 can be written Max(v1, v2) == v2.
+	Max(v1, v2 string) string
+
+	// Upgrade returns the upgraded version of m,
+	// for use during an UpgradeAll operation.
+	// If m should be kept as is, Upgrade returns m.
+	// If m is not yet used in the build, then m.Version will be "none".
+	// More typically, m.Version will be the version required
+	// by some other module in the build.
+	//
+	// If no module version is available for the given path,
+	// Upgrade returns a non-nil error.
+	// TODO(rsc): Upgrade must be able to return errors,
+	// but should "no latest version" just return m instead?
+	Upgrade(m module.Version) (module.Version, error)
+
+	// Previous returns the version of m.Path immediately prior to m.Version,
+	// or "none" if no such version is known.
+	Previous(m module.Version) (module.Version, error)
+}
+
+type MissingModuleError struct {
+	Module module.Version
+}
+
+func (e *MissingModuleError) Error() string {
+	return fmt.Sprintf("missing module: %v", e.Module)
+}
+
+// BuildList returns the build list for the target module.
+func BuildList(target module.Version, reqs Reqs) ([]module.Version, error) {
+	return buildList(target, reqs, nil)
+}
+
+func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) module.Version) ([]module.Version, error) {
+	// Explore work graph in parallel in case reqs.Required
+	// does high-latency network operations.
+	var work par.Work
+	work.Add(target)
+	var (
+		mu       sync.Mutex
+		min      = map[string]string{target.Path: target.Version}
+		firstErr error
+	)
+	work.Do(10, func(item interface{}) {
+		m := item.(module.Version)
+		required, err := reqs.Required(m)
+
+		mu.Lock()
+		if err != nil && firstErr == nil {
+			firstErr = err
+		}
+		if firstErr != nil {
+			mu.Unlock()
+			return
+		}
+		if v, ok := min[m.Path]; !ok || reqs.Max(v, m.Version) != v {
+			min[m.Path] = m.Version
+		}
+		mu.Unlock()
+
+		for _, r := range required {
+			if r.Path == "" {
+				base.Errorf("Required(%v) returned zero module in list", m)
+				continue
+			}
+			work.Add(r)
+		}
+
+		if upgrade != nil {
+			u := upgrade(m)
+			if u.Path == "" {
+				base.Errorf("Upgrade(%v) returned zero module", m)
+				return
+			}
+			work.Add(u)
+		}
+	})
+
+	if firstErr != nil {
+		return nil, firstErr
+	}
+	if v := min[target.Path]; v != target.Version {
+		panic(fmt.Sprintf("mistake: chose version %q instead of target %+v", v, target)) // TODO: Don't panic.
+	}
+
+	list := []module.Version{target}
+	listed := map[string]bool{target.Path: true}
+	for i := 0; i < len(list); i++ {
+		m := list[i]
+		required, err := reqs.Required(m)
+		if err != nil {
+			return nil, err
+		}
+		for _, r := range required {
+			v := min[r.Path]
+			if r.Path != target.Path && reqs.Max(v, r.Version) != v {
+				panic(fmt.Sprintf("mistake: version %q does not satisfy requirement %+v", v, r)) // TODO: Don't panic.
+			}
+			if !listed[r.Path] {
+				list = append(list, module.Version{Path: r.Path, Version: v})
+				listed[r.Path] = true
+			}
+		}
+	}
+
+	tail := list[1:]
+	sort.Slice(tail, func(i, j int) bool {
+		return tail[i].Path < tail[j].Path
+	})
+	return list, nil
+}
+
+// Req returns the minimal requirement list for the target module
+// that results in the given build list, with the constraint that all
+// module paths listed in base must appear in the returned list.
+func Req(target module.Version, list []module.Version, base []string, reqs Reqs) ([]module.Version, error) {
+	// Note: Not running in parallel because we assume
+	// that list came from a previous operation that paged
+	// in all the requirements, so there's no I/O to overlap now.
+
+	// Compute postorder, cache requirements.
+	var postorder []module.Version
+	reqCache := map[module.Version][]module.Version{}
+	reqCache[target] = nil
+	var walk func(module.Version) error
+	walk = func(m module.Version) error {
+		_, ok := reqCache[m]
+		if ok {
+			return nil
+		}
+		required, err := reqs.Required(m)
+		if err != nil {
+			return err
+		}
+		reqCache[m] = required
+		for _, m1 := range required {
+			if err := walk(m1); err != nil {
+				return err
+			}
+		}
+		postorder = append(postorder, m)
+		return nil
+	}
+	for _, m := range list {
+		if err := walk(m); err != nil {
+			return nil, err
+		}
+	}
+
+	// Walk modules in reverse post-order, only adding those not implied already.
+	have := map[string]string{}
+	walk = func(m module.Version) error {
+		if v, ok := have[m.Path]; ok && reqs.Max(m.Version, v) == v {
+			return nil
+		}
+		have[m.Path] = m.Version
+		for _, m1 := range reqCache[m] {
+			walk(m1)
+		}
+		return nil
+	}
+	max := map[string]string{}
+	for _, m := range list {
+		if v, ok := max[m.Path]; ok {
+			max[m.Path] = reqs.Max(m.Version, v)
+		} else {
+			max[m.Path] = m.Version
+		}
+	}
+	// First walk the base modules that must be listed.
+	var min []module.Version
+	for _, path := range base {
+		m := module.Version{Path: path, Version: max[path]}
+		min = append(min, m)
+		walk(m)
+	}
+	// Now the reverse postorder to bring in anything else.
+	for i := len(postorder) - 1; i >= 0; i-- {
+		m := postorder[i]
+		if max[m.Path] != m.Version {
+			// Older version.
+			continue
+		}
+		if have[m.Path] != m.Version {
+			min = append(min, m)
+			walk(m)
+		}
+	}
+	sort.Slice(min, func(i, j int) bool {
+		return min[i].Path < min[j].Path
+	})
+	return min, nil
+}
+
+// UpgradeAll returns a build list for the target module
+// in which every module is upgraded to its latest version.
+func UpgradeAll(target module.Version, reqs Reqs) ([]module.Version, error) {
+	return buildList(target, reqs, func(m module.Version) module.Version {
+		if m.Path == target.Path {
+			return target
+		}
+
+		latest, err := reqs.Upgrade(m)
+		if err != nil {
+			panic(err) // TODO
+		}
+		m.Version = latest.Version
+		return m
+	})
+}
+
+// Upgrade returns a build list for the target module
+// in which the given additional modules are upgraded.
+func Upgrade(target module.Version, reqs Reqs, upgrade ...module.Version) ([]module.Version, error) {
+	list, err := reqs.Required(target)
+	if err != nil {
+		panic(err) // TODO
+	}
+	// TODO: Maybe if an error is given,
+	// rerun with BuildList(upgrade[0], reqs) etc
+	// to find which ones are the buggy ones.
+	list = append([]module.Version(nil), list...)
+	list = append(list, upgrade...)
+	return BuildList(target, &override{target, list, reqs})
+}
+
+// Downgrade returns a build list for the target module
+// in which the given additional modules are downgraded.
+//
+// The versions to be downgraded may be unreachable from reqs.Latest and
+// reqs.Previous, but the methods of reqs must otherwise handle such versions
+// correctly.
+func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([]module.Version, error) {
+	list, err := reqs.Required(target)
+	if err != nil {
+		panic(err) // TODO
+	}
+	max := make(map[string]string)
+	for _, r := range list {
+		max[r.Path] = r.Version
+	}
+	for _, d := range downgrade {
+		if v, ok := max[d.Path]; !ok || reqs.Max(v, d.Version) != d.Version {
+			max[d.Path] = d.Version
+		}
+	}
+
+	var (
+		added    = make(map[module.Version]bool)
+		rdeps    = make(map[module.Version][]module.Version)
+		excluded = make(map[module.Version]bool)
+	)
+	var exclude func(module.Version)
+	exclude = func(m module.Version) {
+		if excluded[m] {
+			return
+		}
+		excluded[m] = true
+		for _, p := range rdeps[m] {
+			exclude(p)
+		}
+	}
+	var add func(module.Version)
+	add = func(m module.Version) {
+		if added[m] {
+			return
+		}
+		added[m] = true
+		if v, ok := max[m.Path]; ok && reqs.Max(m.Version, v) != v {
+			exclude(m)
+			return
+		}
+		list, err := reqs.Required(m)
+		if err != nil {
+			panic(err) // TODO
+		}
+		for _, r := range list {
+			add(r)
+			if excluded[r] {
+				exclude(m)
+				return
+			}
+			rdeps[r] = append(rdeps[r], m)
+		}
+	}
+
+	var out []module.Version
+	out = append(out, target)
+List:
+	for _, r := range list {
+		add(r)
+		for excluded[r] {
+			p, err := reqs.Previous(r)
+			if err != nil {
+				return nil, err // TODO
+			}
+			// If the target version is a pseudo-version, it may not be
+			// included when iterating over prior versions using reqs.Previous.
+			// Insert it into the right place in the iteration.
+			// If v is excluded, p should be returned again by reqs.Previous on the next iteration.
+			if v := max[r.Path]; reqs.Max(v, r.Version) != v && reqs.Max(p.Version, v) != p.Version {
+				p.Version = v
+			}
+			if p.Version == "none" {
+				continue List
+			}
+			add(p)
+			r = p
+		}
+		out = append(out, r)
+	}
+
+	return out, nil
+}
+
+type override struct {
+	target module.Version
+	list   []module.Version
+	Reqs
+}
+
+func (r *override) Required(m module.Version) ([]module.Version, error) {
+	if m == r.target {
+		return r.list, nil
+	}
+	return r.Reqs.Required(m)
+}