[dev.typeparams] all: merge master (eb98272) into dev.typeparams

Merge List:

+ 2021-02-18 eb98272 cmd/go/internal/mvs: fix Downgrade to match Algorithm 4
+ 2021-02-18 3b7277d cmd/go: add a script test for artifacts resulting from 'go get -u'
+ 2021-02-18 f3c2208 cmd/go: add script tests for potential upgrades due to downgrades
+ 2021-02-18 a5c8a15 cmd/go/internal/mvs: clarify and annotate test cases
+ 2021-02-18 a76efea cmd/go/internal/mvs: don't emit duplicates from Req
+ 2021-02-18 609d82b cmd/dist: set GOARM=7 for windows/arm
+ 2021-02-18 f0be3cc runtime: unbreak linux/riscv64 following regabi merge
+ 2021-02-18 07ef313 runtime/cgo: add cast in C code to avoid C compiler warning

Change-Id: I8e58ad1e82a9ea313a99c1b11df5b341f80680d4

src/cmd/dist/util.go

@@ -389,6 +389,10 @@ func xgetgoarm() string {
 		// sense to auto-detect the setting.
 		return "7"
 	}
+	if goos == "windows" {
+		// windows/arm only works with ARMv7 executables.
+		return "7"
+	}
 	if gohostarch != "arm" || goos != gohostos {
 		// Conservative default for cross-compilation.
 		return "5"

src/cmd/go/internal/mvs/mvs.go

@@ -293,10 +293,15 @@ func Req(target module.Version, base []string, reqs Reqs) ([]module.Version, err
 	}
 	// First walk the base modules that must be listed.
 	var min []module.Version
+	haveBase := map[string]bool{}
 	for _, path := range base {
+		if haveBase[path] {
+			continue
+		}
 		m := module.Version{Path: path, Version: max[path]}
 		min = append(min, m)
 		walk(m)
+		haveBase[path] = true
 	}
 	// Now the reverse postorder to bring in anything else.
 	for i := len(postorder) - 1; i >= 0; i-- {
@@ -370,10 +375,19 @@ func Upgrade(target module.Version, reqs Reqs, upgrade ...module.Version) ([]mod
 // 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)
+	// Per https://research.swtch.com/vgo-mvs#algorithm_4:
+	// “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.”
+	//
+	// In order to generate those new requirements, we need to identify versions
+	// for every module in the build list — not just reqs.Required(target).
+	list, err := BuildList(target, reqs)
 	if err != nil {
 		return nil, err
 	}
+	list = list[1:] // remove target
+
 	max := make(map[string]string)
 	for _, r := range list {
 		max[r.Path] = r.Version
@@ -406,6 +420,9 @@ func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([
 		}
 		added[m] = true
 		if v, ok := max[m.Path]; ok && reqs.Max(m.Version, v) != v {
+			// m would upgrade an existing dependency — it is not a strict downgrade,
+			// and because it was already present as a dependency, it could affect the
+			// behavior of other relevant packages.
 			exclude(m)
 			return
 		}
@@ -422,6 +439,7 @@ func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([
 			// is transient (we couldn't download go.mod), return the error from
 			// Downgrade. Currently, we can't tell what kind of error it is.
 			exclude(m)
+			return
 		}
 		for _, r := range list {
 			add(r)
@@ -433,8 +451,8 @@ func Downgrade(target module.Version, reqs Reqs, downgrade ...module.Version) ([
 		}
 	}
 
-	var out []module.Version
-	out = append(out, target)
+	downgraded := make([]module.Version, 0, len(list)+1)
+	downgraded = append(downgraded, target)
 List:
 	for _, r := range list {
 		add(r)
@@ -461,10 +479,14 @@ List:
 			add(p)
 			r = p
 		}
-		out = append(out, r)
+		downgraded = append(downgraded, r)
 	}
 
-	return out, nil
+	return BuildList(target, &override{
+		target: target,
+		list:   downgraded,
+		Reqs:   reqs,
+	})
 }
 
 type override struct {

src/cmd/go/internal/mvs/mvs_test.go

@@ -28,10 +28,11 @@ D4: E2 F1
 D5: E2
 G1: C4
 A2: B1 C4 D4
-build A: A B1 C2 D4 E2 F1
-upgrade* A: A B1 C4 D5 E2 F1 G1
-upgrade A C4: A B1 C4 D4 E2 F1 G1
-downgrade A2 D2: A2 C4 D2
+build A:       A B1 C2 D4 E2 F1
+upgrade* A:    A B1 C4 D5 E2 F1 G1
+upgrade A C4:  A B1 C4 D4 E2 F1 G1
+build A2:     A2 B1 C4 D4 E2 F1 G1
+downgrade A2 D2: A2 C4 D2 E2 F1 G1
 
 name: trim
 A: B1 C2
@@ -68,7 +69,7 @@ B2: D1
 C: D2
 D1: E2
 D2: E1
-build A: A B1 C D2 E1
+build A:      A B1 C D2 E1
 upgrade A B2: A B2 C D2 E2
 
 name: cross1R
@@ -136,17 +137,17 @@ name: cross5
 A: D1
 D1: E2
 D2: E1
-build A: A D1 E2
-upgrade* A: A D2 E2
-upgrade A D2: A D2 E2
+build A:       A D1 E2
+upgrade* A:    A D2 E2
+upgrade A D2:  A D2 E2
 upgradereq A D2: D2 E2
 
 name: cross6
 A: D2
 D1: E2
 D2: E1
-build A: A D2 E1
-upgrade* A: A D2 E2
+build A:      A D2 E1
+upgrade* A:   A D2 E2
 upgrade A E2: A D2 E2
 
 name: cross7
@@ -175,7 +176,7 @@ B1: D1
 B2:
 C2:
 D2:
-build A: A B1 C1 D1
+build A:    A B1 C1 D1
 upgrade* A: A B2 C2 D2
 
 name: simplify
@@ -194,7 +195,7 @@ B4:
 B5.hidden:
 C2:
 C3:
-build A: A B1 C1
+build A:    A B1 C1
 upgrade* A: A B4 C3
 
 name: up2
@@ -206,15 +207,15 @@ B4:
 B5.hidden:
 C2:
 C3:
-build A: A B5.hidden C1
+build A:    A B5.hidden C1
 upgrade* A: A B5.hidden C3
 
 name: down1
 A: B2
 B1: C1
 B2: C2
-build A: A B2 C2
-downgrade A C1: A B1
+build A:        A B2 C2
+downgrade A C1: A B1 C1
 
 name: down2
 A: B2 E2
@@ -227,33 +228,75 @@ D2: B2
 E2: D2
 E1:
 F1:
-downgrade A F1: A B1 E1
+build A:        A B2 C2 D2 E2 F2
+downgrade A F1: A B1 C1 D1 E1 F1
+
+# https://research.swtch.com/vgo-mvs#algorithm_4:
+# “[D]owngrades are constrained to only downgrade packages, not also upgrade
+# them; if an upgrade before downgrade is needed, the user must ask for it
+# explicitly.”
+#
+# Here, downgrading B2 to B1 upgrades C1 to C2, and C2 does not depend on D2.
+# However, C2 would be an upgrade — not a downgrade — so B1 must also be
+# rejected.
+name: downcross1
+A: B2 C1
+B1: C2
+B2: C1
+C1: D2
+C2:
+D1:
+D2:
+build A:        A B2 C1 D2
+downgrade A D1: A       D1
+
+# https://research.swtch.com/vgo-mvs#algorithm_4:
+# “Unlike upgrades, downgrades must work by removing requirements, not adding
+# them.”
+#
+# However, downgrading a requirement may introduce a new requirement on a
+# previously-unrequired module. If each dependency's requirements are complete
+# (“tidy”), that can't change the behavior of any other package whose version is
+# not also being downgraded, so we should allow it.
+name: downcross2
+A: B2
+B1: C1
+B2: D2
+C1:
+D1:
+D2:
+build A:        A B2    D2
+downgrade A D1: A B1 C1 D1
 
 name: downcycle
 A: A B2
 B2: A
 B1:
+build A:        A B2
 downgrade A B1: A B1
 
 # golang.org/issue/25542.
 name: noprev1
 A: B4 C2
 B2.hidden:
 C2:
+build A:               A B4        C2
 downgrade A B2.hidden: A B2.hidden C2
 
 name: noprev2
 A: B4 C2
 B2.hidden:
 B1:
 C2:
+build A:               A B4        C2
 downgrade A B2.hidden: A B2.hidden C2
 
 name: noprev3
 A: B4 C2
 B3:
 B2.hidden:
 C2:
+build A:               A B4        C2
 downgrade A B2.hidden: A B2.hidden C2
 
 # Cycles involving the target.
@@ -264,9 +307,9 @@ A: B1
 B1: A1
 B2: A2
 B3: A3
-build A: A B1
+build A:      A B1
 upgrade A B2: A B2
-upgrade* A: A B3
+upgrade* A:   A B3
 
 # golang.org/issue/29773:
 # Requirements of older versions of the target
@@ -280,7 +323,7 @@ B2: A2
 C1: A2
 C2:
 D2:
-build A: A B1 C1 D1
+build A:    A B1 C1 D1
 upgrade* A: A B2 C2 D2
 
 # Cycles with multiple possible solutions.
@@ -293,23 +336,23 @@ B2: C2
 C1:
 C2: B2
 build M: M A1 B2 C2
-req M: A1 B2
-req M A: A1 B2
-req M C: A1 C2
+req M:     A1 B2
+req M A:   A1 B2
+req M C:   A1 C2
 
 # Requirement minimization.
 
 name: req1
 A: B1 C1 D1 E1 F1
 B1: C1 E1 F1
-req A: B1 D1
+req A:   B1    D1
 req A C: B1 C1 D1
 
 name: req2
 A: G1 H1
 G1: H1
 H1: G1
-req A: G1
+req A:   G1
 req A G: G1
 req A H: H1
 
@@ -326,7 +369,20 @@ M: Anone B1 D1 E1
 B1: Cnone D1
 E1: Fnone
 build M: M B1 D1 E1
-req M: B1 E1
+req M:     B1    E1
+
+name: reqdup
+M: A1 B1
+A1: B1
+B1:
+req M A A: A1
+
+name: reqcross
+M: A1 B1 C1
+A1: B1 C1
+B1: C1
+C1:
+req M A B: A1 B1
 `
 
 func Test(t *testing.T) {