cmd/go: fix spurious edges in mod -graph output

The mod -graph output was showing every dependency
as an edge from the main module, instead of showing only
the things that are listed in go.mod.

Fixes #26489.

Change-Id: I248fedb1fc9225e2a7a9ddc2f4a84520b3a96138
Reviewed-on: https://go-review.googlesource.com/125657Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/mod.go

@@ -496,7 +496,7 @@ func modPrintJSON() {
 
 // modPrintGraph prints the -graph output.
 func modPrintGraph() {
-	reqs := modload.Reqs()
+	reqs := modload.MinReqs()
 
 	format := func(m module.Version) string {
 		if m.Version == "" {

src/cmd/go/internal/modload/build.go

@@ -91,6 +91,7 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 			Version: m.Version,
 			Main:    true,
 			Dir:     ModRoot,
+			GoMod:   filepath.Join(ModRoot, "go.mod"),
 		}
 	}
 
@@ -114,7 +115,15 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 				m.Version = q.Version
 				m.Time = &q.Time
 			}
-			dir, err := modfetch.DownloadDir(module.Version{Path: m.Path, Version: m.Version})
+
+			mod := module.Version{Path: m.Path, Version: m.Version}
+			gomod, err := modfetch.CachePath(mod, "mod")
+			if err == nil {
+				if info, err := os.Stat(gomod); err == nil && info.Mode().IsRegular() {
+					m.GoMod = gomod
+				}
+			}
+			dir, err := modfetch.DownloadDir(mod)
 			if err == nil {
 				if info, err := os.Stat(dir); err == nil && info.IsDir() {
 					m.Dir = dir
@@ -142,6 +151,7 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 		}
 		complete(info.Replace)
 		info.Dir = info.Replace.Dir
+		info.GoMod = filepath.Join(info.Dir, "go.mod")
 		info.Error = nil // ignore error loading original module version (it has been replaced)
 	}
 

src/cmd/go/internal/modload/init.go

@@ -474,6 +474,22 @@ func AllowWriteGoMod() {
 	allowWriteGoMod = true
 }
 
+// MinReqs returns a Reqs with minimal dependencies of Target,
+// as will be written to go.mod.
+func MinReqs() mvs.Reqs {
+	var direct []string
+	for _, m := range buildList[1:] {
+		if loaded.direct[m.Path] {
+			direct = append(direct, m.Path)
+		}
+	}
+	min, err := mvs.Req(Target, buildList, direct, Reqs())
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+	return &mvsReqs{buildList: append([]module.Version{Target}, min...)}
+}
+
 // WriteGoMod writes the current build list back to go.mod.
 func WriteGoMod() {
 	if !allowWriteGoMod {
@@ -483,13 +499,8 @@ func WriteGoMod() {
 	modfetch.WriteGoSum()
 
 	if loaded != nil {
-		var direct []string
-		for _, m := range buildList[1:] {
-			if loaded.direct[m.Path] {
-				direct = append(direct, m.Path)
-			}
-		}
-		min, err := mvs.Req(Target, buildList, direct, Reqs())
+		reqs := MinReqs()
+		min, err := reqs.Required(Target)
 		if err != nil {
 			base.Fatalf("go: %v", err)
 		}

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

+env GO111MODULE=on
+
+go mod -graph
+stdout '^m rsc.io/quote@v1.5.2$'
+stdout '^rsc.io/quote@v1.5.2 rsc.io/sampler@v1.3.0$'
+! stdout '^m rsc.io/sampler@v1.3.0$'
+
+-- go.mod --
+module m
+require rsc.io/quote v1.5.2