cmd/go/internal/mvs: retain modules required by older versions

Fixes #29773
Updates #31248

Change-Id: Ic1923119c8cf3a60c586df1b270c3af0c9095f29
Reviewed-on: https://go-review.googlesource.com/c/go/+/186537
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -216,8 +216,8 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) (m
 		}
 	}
 
-	// Construct the list by traversing the graph again, replacing older
-	// modules with required minimum versions.
+	// 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
@@ -228,19 +228,18 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) (m
 	}
 
 	list := []module.Version{target}
-	listed := map[string]bool{target.Path: true}
-	for i := 0; i < len(list); i++ {
-		n := modGraph[list[i]]
+	for path, vers := range min {
+		if path != target.Path {
+			list = append(list, module.Version{Path: path, Version: vers})
+		}
+
+		n := modGraph[module.Version{Path: path, Version: vers}]
 		required := n.required
 		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
-			}
 		}
 	}
 

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

@@ -29,7 +29,7 @@ D5: E2
 G1: C4
 A2: B1 C4 D4
 build A: A B1 C2 D4 E2 F1
-upgrade* A: A B1 C4 D5 E2 G1
+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
 
@@ -38,7 +38,7 @@ A: B1 C2
 B1: D3
 C2: B2
 B2:
-build A: A B2 C2
+build A: A B2 C2 D3
 
 # Cross-dependency between D and E.
 # No matter how it arises, should get result of merging all build lists via max,
@@ -157,22 +157,33 @@ D1: E2
 E1: D2
 build A: A B C D2 E2
 
-# Upgrade from B1 to B2 should drop the transitive dep on D.
+# golang.org/issue/31248:
+# Even though we select X2, the requirement on I1
+# via X1 should be preserved.
+name: cross8
+M: A1 B1
+A1: X1
+B1: X2
+X1: I1
+X2: 
+build M: M A1 B1 I1 X2
+
+# Upgrade from B1 to B2 should not drop the transitive dep on D.
 name: drop
 A: B1 C1
 B1: D1
 B2:
 C2:
 D2:
 build A: A B1 C1 D1
-upgrade* A: A B2 C2
+upgrade* A: A B2 C2 D2
 
 name: simplify
 A: B1 C1
 B1: C2
 C1: D1
 C2:
-build A: A B1 C2
+build A: A B1 C2 D1
 
 name: up1
 A: B1 C1
@@ -254,8 +265,9 @@ build A: A B1
 upgrade A B2: A B2
 upgrade* A: A B3
 
+# golang.org/issue/29773:
 # Requirements of older versions of the target
-# must not be carried over.
+# must be carried over.
 name: cycle2
 A: B1
 A1: C1
@@ -265,8 +277,8 @@ B2: A2
 C1: A2
 C2:
 D2:
-build A: A B1
-upgrade* A: A B2
+build A: A B1 C1 D1
+upgrade* A: A B2 C2 D2
 
 # Requirement minimization.
 

src/cmd/go/testdata/script/mod_indirect.txt

@@ -0,0 +1,81 @@
+env GO111MODULE=on
+
+# golang.org/issue/31248: module requirements imposed by dependency versions
+# older than the selected version must still be taken into account.
+
+env GOFLAGS=-mod=readonly
+
+# Indirect dependencies required via older-than-selected versions must exist in
+# the module graph, but do not need to be listed explicitly in the go.mod file
+# (since they are implied).
+go mod graph
+stdout i@v0.1.0
+
+# The modules must also appear in the build list, not just the graph.
+go list -m all
+stdout '^i v0.1.0'
+
+# The packages provided by those dependencies must resolve.
+go list all
+stdout '^i$'
+
+-- go.mod --
+module main
+
+go 1.13
+
+require (
+	a v0.0.0
+	b v0.0.0
+	c v0.0.0
+)
+
+// Apply replacements so that the test can be self-contained.
+// (It's easier to see all of the modules here than to go
+// rooting around in testdata/mod.)
+replace (
+	a => ./a
+	b => ./b
+	c => ./c
+	x v0.1.0 => ./x1
+	x v0.2.0 => ./x2
+	i => ./i
+)
+-- main.go --
+package main
+
+import (
+	_ "a"
+	_ "b"
+	_ "c"
+)
+
+func main() {}
+-- a/go.mod --
+module a
+go 1.13
+require x v0.1.0
+-- a/a.go --
+package a
+-- b/go.mod --
+module b
+go 1.13
+require x v0.2.0
+-- b/b.go --
+package b
+-- c/go.mod --
+module c
+go 1.13
+-- c/c.go --
+package c
+import _ "i"
+-- x1/go.mod --
+module x
+go1.13
+require i v0.1.0
+-- x2/go.mod --
+module x
+go1.13
+-- i/go.mod --
+-- i/i.go --
+package i

src/cmd/go/testdata/script/mod_indirect_main.txt

@@ -0,0 +1,65 @@
+env GO111MODULE=on
+
+# Regression test for golang.org/issue/29773: 'go list -m' was not following
+# dependencies through older versions of the main module.
+
+go list -f '{{with .Module}}{{.Path}}{{with .Version}} {{.}}{{end}}{{end}}' all
+cmp stdout pkgmods.txt
+
+go list -m all
+cmp stdout mods.txt
+
+go mod graph
+cmp stdout graph.txt
+
+-- go.mod --
+module golang.org/issue/root
+
+go 1.12
+
+replace (
+	golang.org/issue/mirror v0.1.0 => ./mirror-v0.1.0
+	golang.org/issue/pkg v0.1.0 => ./pkg-v0.1.0
+	golang.org/issue/root v0.1.0 => ./root-v0.1.0
+)
+
+require golang.org/issue/mirror v0.1.0
+
+-- root.go --
+package root
+
+import _ "golang.org/issue/mirror"
+
+-- mirror-v0.1.0/go.mod --
+module golang.org/issue/mirror
+
+require golang.org/issue/root v0.1.0
+
+-- mirror-v0.1.0/mirror.go --
+package mirror
+
+import _ "golang.org/issue/pkg"
+
+-- pkg-v0.1.0/go.mod --
+module golang.org/issue/pkg
+
+-- pkg-v0.1.0/pkg.go --
+package pkg
+
+-- root-v0.1.0/go.mod --
+module golang.org/issue/root
+
+require golang.org/issue/pkg v0.1.0
+
+-- pkgmods.txt --
+golang.org/issue/mirror v0.1.0
+golang.org/issue/pkg v0.1.0
+golang.org/issue/root
+-- mods.txt --
+golang.org/issue/root
+golang.org/issue/mirror v0.1.0 => ./mirror-v0.1.0
+golang.org/issue/pkg v0.1.0 => ./pkg-v0.1.0
+-- graph.txt --
+golang.org/issue/root golang.org/issue/mirror@v0.1.0
+golang.org/issue/mirror@v0.1.0 golang.org/issue/root@v0.1.0
+golang.org/issue/root@v0.1.0 golang.org/issue/pkg@v0.1.0