cmd/go: add a -go flag to 'go mod graph'

For #46366

Change-Id: I8417e6e4dbb8cb56ff7afc16893a01b7bb938217
Reviewed-on: https://go-review.googlesource.com/c/go/+/329529
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

doc/go1.17.html

@@ -187,6 +187,13 @@ <h4 id="lazy-loading">Lazy module loading</h4>
   features.
 </p>
 
+<p><!-- golang.org/issue/46366 -->
+  The <code>go</code> <code>mod</code> <code>graph</code> subcommand also
+  supports the <code>-go</code> flag, which causes it to report the graph as
+  seen by the indicated Go version, showing dependencies that may otherwise be
+  pruned out by lazy loading.
+</p>
+
 <h4 id="module-deprecation-comments">Module deprecation comments</h4>
 
 <p><!-- golang.org/issue/40357 -->

src/cmd/go/internal/modcmd/graph.go

@@ -18,20 +18,29 @@ import (
 )
 
 var cmdGraph = &base.Command{
-	UsageLine: "go mod graph",
+	UsageLine: "go mod graph [-go=version]",
 	Short:     "print module requirement graph",
 	Long: `
 Graph prints the module requirement graph (with replacements applied)
 in text form. Each line in the output has two space-separated fields: a module
 and one of its requirements. Each module is identified as a string of the form
 path@version, except for the main module, which has no @version suffix.
 
+The -go flag causes graph to report the module graph as loaded by by the
+given Go version, instead of the version indicated by the 'go' directive
+in the go.mod file.
+
 See https://golang.org/ref/mod#go-mod-graph for more about 'go mod graph'.
 	`,
 	Run: runGraph,
 }
 
+var (
+	graphGo goVersionFlag
+)
+
 func init() {
+	cmdGraph.Flag.Var(&graphGo, "go", "")
 	base.AddModCommonFlags(&cmdGraph.Flag)
 }
 
@@ -41,7 +50,7 @@ func runGraph(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	modload.ForceUseModules = true
 	modload.RootMode = modload.NeedRoot
-	mg := modload.LoadModGraph(ctx)
+	mg := modload.LoadModGraph(ctx, graphGo.String())
 
 	w := bufio.NewWriter(os.Stdout)
 	defer w.Flush()

src/cmd/go/internal/modcmd/verify.go

@@ -54,7 +54,8 @@ func runVerify(ctx context.Context, cmd *base.Command, args []string) {
 	sem := make(chan token, runtime.GOMAXPROCS(0))
 
 	// Use a slice of result channels, so that the output is deterministic.
-	mods := modload.LoadModGraph(ctx).BuildList()[1:]
+	const defaultGoVersion = ""
+	mods := modload.LoadModGraph(ctx, defaultGoVersion).BuildList()[1:]
 	errsChans := make([]<-chan []error, len(mods))
 
 	for i, mod := range mods {

src/cmd/go/internal/modget/get.go

@@ -506,7 +506,8 @@ type versionReason struct {
 func newResolver(ctx context.Context, queries []*query) *resolver {
 	// LoadModGraph also sets modload.Target, which is needed by various resolver
 	// methods.
-	mg := modload.LoadModGraph(ctx)
+	const defaultGoVersion = ""
+	mg := modload.LoadModGraph(ctx, defaultGoVersion)
 
 	buildList := mg.BuildList()
 	initialVersion := make(map[string]string, len(buildList))
@@ -1803,7 +1804,8 @@ func (r *resolver) updateBuildList(ctx context.Context, additions []module.Versi
 		return false
 	}
 
-	r.buildList = modload.LoadModGraph(ctx).BuildList()
+	const defaultGoVersion = ""
+	r.buildList = modload.LoadModGraph(ctx, defaultGoVersion).BuildList()
 	r.buildListVersion = make(map[string]string, len(r.buildList))
 	for _, m := range r.buildList {
 		r.buildListVersion[m.Path] = m.Version

src/cmd/go/internal/modload/buildlist.go

@@ -403,11 +403,33 @@ func (mg *ModuleGraph) allRootsSelected() bool {
 // LoadModGraph loads and returns the graph of module dependencies of the main module,
 // without loading any packages.
 //
+// If the goVersion string is non-empty, the returned graph is the graph
+// as interpreted by the given Go version (instead of the version indicated
+// in the go.mod file).
+//
 // Modules are loaded automatically (and lazily) in LoadPackages:
 // LoadModGraph need only be called if LoadPackages is not,
 // typically in commands that care about modules but no particular package.
-func LoadModGraph(ctx context.Context) *ModuleGraph {
-	rs, mg, err := expandGraph(ctx, LoadModFile(ctx))
+func LoadModGraph(ctx context.Context, goVersion string) *ModuleGraph {
+	rs := LoadModFile(ctx)
+
+	if goVersion != "" {
+		depth := modDepthFromGoVersion(goVersion)
+		if depth == eager && rs.depth != eager {
+			// Use newRequirements instead of convertDepth because convertDepth
+			// also updates roots; here, we want to report the unmodified roots
+			// even though they may seem inconsistent.
+			rs = newRequirements(eager, rs.rootModules, rs.direct)
+		}
+
+		mg, err := rs.Graph(ctx)
+		if err != nil {
+			base.Fatalf("go: %v", err)
+		}
+		return mg
+	}
+
+	rs, mg, err := expandGraph(ctx, rs)
 	if err != nil {
 		base.Fatalf("go: %v", err)
 	}

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

@@ -0,0 +1,101 @@
+# For this module, Go 1.17 prunes out a (transitive and otherwise-irrelevant)
+# requirement on a retracted higher version of a dependency.
+# However, when Go 1.16 reads the same requirements from the go.mod file,
+# it does not prune out that requirement, and selects the retracted version.
+#
+# The Go 1.16 module graph looks like:
+#
+# m ---- lazy v0.1.0 ---- requireincompatible v0.1.0 ---- incompatible v2.0.0+incompatible
+# |        |
+# + -------+------------- incompatible v1.0.0
+#
+# The Go 1.17 module graph is the same except that the dependencies of
+# requireincompatible are pruned out (because the module that requires
+# it — lazy v0.1.0 — specifies 'go 1.17', and it is not otherwise relevant to
+# the main module).
+
+cp go.mod go.mod.orig
+
+go mod graph
+cp stdout graph-1.17.txt
+stdout '^example\.com/m example\.com/retract/incompatible@v1\.0\.0$'
+stdout '^example\.net/lazy@v0\.1\.0 example\.com/retract/incompatible@v1\.0\.0$'
+! stdout 'example\.com/retract/incompatible@v2\.0\.0\+incompatible'
+
+go mod graph -go=1.17
+cmp stdout graph-1.17.txt
+
+cmp go.mod go.mod.orig
+
+
+# Setting -go=1.16 should report the graph as viewed by Go 1.16,
+# but should not edit the go.mod file.
+
+go mod graph -go=1.16
+cp stdout graph-1.16.txt
+stdout '^example\.com/m example\.com/retract/incompatible@v1\.0\.0$'
+stdout '^example\.net/lazy@v0\.1\.0 example.com/retract/incompatible@v1\.0\.0$'
+stdout '^example.net/requireincompatible@v0.1.0 example.com/retract/incompatible@v2\.0\.0\+incompatible$'
+
+cmp go.mod go.mod.orig
+
+
+# If we actually update the go.mod file to the requested go version,
+# we should get the same selected versions, but the roots of the graph
+# may be updated.
+#
+# TODO(#45551): The roots should not be updated.
+
+go mod edit -go=1.16
+go mod graph
+! stdout '^example\.com/m example\.com/retract/incompatible@v1\.0\.0$'
+stdout '^example\.net/lazy@v0.1.0 example.com/retract/incompatible@v1\.0\.0$'
+stdout '^example.net/requireincompatible@v0.1.0 example.com/retract/incompatible@v2\.0\.0\+incompatible$'
+	# TODO(#45551): cmp stdout graph-1.16.txt
+
+
+# Unsupported go versions should be rejected, since we don't know
+# what versions they would report.
+! go mod graph -go=1.99999999999
+stderr '^invalid value "1\.99999999999" for flag -go: maximum supported Go version is '$goversion'\nusage: go mod graph \[-go=version\]\nRun ''go help mod graph'' for details.$'
+
+
+-- go.mod --
+// Module m indirectly imports a package from
+// example.com/retract/incompatible. Its selected version of
+// that module is lower under Go 1.17 semantics than under Go 1.16.
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/requireincompatible v0.1.0 => ./requireincompatible
+)
+
+require (
+	example.com/retract/incompatible v1.0.0 // indirect
+	example.net/lazy v0.1.0
+)
+-- lazy/go.mod --
+// Module lazy requires example.com/retract/incompatible v1.0.0.
+//
+// When viewed from the outside it also has a transitive dependency
+// on v2.0.0+incompatible, but in lazy mode that transitive dependency
+// is pruned out.
+module example.net/lazy
+
+go 1.17
+
+exclude example.com/retract/incompatible v2.0.0+incompatible
+
+require (
+	example.com/retract/incompatible v1.0.0
+	example.net/requireincompatible v0.1.0
+)
+-- requireincompatible/go.mod --
+module example.net/requireincompatible
+
+go 1.15
+
+require example.com/retract/incompatible v2.0.0+incompatible