cmd/go/internal/vgo: track directly-used vs indirectly-used modules

A cleanup pass in mvs.BuildList discards modules that are not reachable
in the requirement graph as satisfied for this build. For example, suppose:

	A -> B1, C1
	B1 -> D1
	B2 -> nothing
	C1 -> nothing
	D1 -> nothing
	D2 -> nothing

The effective build list is A, B1, C1, D1 (no cleanup possible).

Suppose that we update from B1 to B2. The effective build list
becomes A, B2, C1, D1, and since there is no path through those
module versions from A to D, the cleanup pass drops D.

This cleanup, which is not in https://research.swtch.com/vgo-mvs,
aims to avoid user confusion by not listing irrelevant modules in
the output of commands like "vgo list -m all".

Unfortunately, the cleanup is not sound in general, because
there is no guarantee all of A's needs are listed as direct requirements.
For example, maybe A imports D. In that case, dropping D and then
building A will re-add the latest version of D (D2 instead of D1).
The most common time this happens is after an upgrade.

The fix is to make sure that go.mod does list all of the modules
required directly by A, and to make sure that the go.mod
minimizer (Algorithm R in the blog post) does not remove
direct requirements in the name of simplifying go.mod.

The way this is done is to annotate the requirements NOT used
directly by A with a known comment, "// indirect".

For example suppose A imports rsc.io/quote. Then the go.mod
looks like it always has:

	module m

	require rsc.io/quote v1.5.2

But now suppose we upgrade our packages to their latest versions.
Then go.mod becomes:

	module m

	require (
        		golang.org/x/text v0.3.0 // indirect
        		rsc.io/quote v1.5.2
        		rsc.io/sampler v1.99.99 // indirect
        	)

The "// indirect" comments indicate that this requirement is used
only to upgrade something needed outside module m, not to satisfy
any packages in module m itself.

Vgo adds and removes these comments automatically.
If we add a direct import of golang.org/x/text to some package in m,
then the first time we build that package vgo strips the "// indirect"
on the golang.org/x/text requirement line. If we then remove that
package, the requirement remains listed as direct (the conservative
choice) until the next "vgo mod -sync", which considers all packages
in m and can mark the requirement indirect again.
Algorithm R is modified to be given a set of import paths that must
be preserved in the final output (all the ones not marked // indirect).

Maintenance of this extra information makes the cleanup pass safe.

Seeing all directly-imported modules in go.mod
and distinguishing between directly- and indirectly-imported modules in go.mod
are two of the most commonly-requested features,
so it's extra nice that the fix for the cleanup-induced bug
makes go.mod closer to what users expect.

Fixes golang/go#24042.
Fixes golang/go#25371.
Fixes golang/go#25969.

Change-Id: I4ed0729b867723fe90e836c2325f740b55b2b27b
Reviewed-on: https://go-review.googlesource.com/121304
Reviewed-by: Bryan C. Mills <bcmills@google.com>

vendor/cmd/go/internal/list/list.go

@@ -199,6 +199,7 @@ applied to a Go struct, but now a Module struct:
         Time     *time.Time   // time version was created
         Update   *Module      // available update, if any (with -u)
         Main     bool         // is this the main module?
+        Indirect bool         // is this module only an indirect dependency of main module?
         Dir      string       // directory holding files for this module, if any
         Error    *ModuleError // error loading module
     }

vendor/cmd/go/internal/modcmd/mod.go

@@ -84,9 +84,20 @@ Go types:
 
 	type GoMod struct {
 		Module Module
-		Require []Module
+		Require []Require
 		Exclude []Module
-		Replace []struct{ Old, New Module }
+		Replace []Replace
+	}
+	
+	type Require struct {
+		Path string
+		Version string
+		Indirect bool
+	}
+	
+	type Replace string {
+		Old Module
+		New Module
 	}
 
 Note that this only describes the go.mod file itself, not other modules
@@ -432,11 +443,17 @@ func flagDropReplace(arg string) {
 // fileJSON is the -json output data structure.
 type fileJSON struct {
 	Module  module.Version
-	Require []module.Version
+	Require []requireJSON
 	Exclude []module.Version
 	Replace []replaceJSON
 }
 
+type requireJSON struct {
+	Path     string
+	Version  string `json:",omitempty"`
+	Indirect bool   `json:",omitempty"`
+}
+
 type replaceJSON struct {
 	Old module.Version
 	New module.Version
@@ -449,7 +466,7 @@ func modPrintJSON() {
 	var f fileJSON
 	f.Module = modFile.Module.Mod
 	for _, r := range modFile.Require {
-		f.Require = append(f.Require, r.Mod)
+		f.Require = append(f.Require, requireJSON{Path: r.Mod.Path, Version: r.Mod.Version, Indirect: r.Indirect})
 	}
 	for _, x := range modFile.Exclude {
 		f.Exclude = append(f.Exclude, x.Mod)

vendor/cmd/go/internal/modconv/convert.go

@@ -72,7 +72,7 @@ func ConvertLegacyConfig(f *modfile.File, file string, data []byte) error {
 	}
 	sort.Strings(paths)
 	for _, path := range paths {
-		f.AddNewRequire(path, need[path])
+		f.AddNewRequire(path, need[path], false)
 	}
 
 	for _, r := range mf.Replace {

vendor/cmd/go/internal/modfile/rule.go

@@ -36,8 +36,9 @@ type Module struct {
 
 // A Require is a single require statement.
 type Require struct {
-	Mod    module.Version
-	Syntax *Line
+	Mod      module.Version
+	Indirect bool // has "// indirect" comment
+	Syntax   *Line
 }
 
 // An Exclude is a single exclude statement.
@@ -182,8 +183,9 @@ func (f *File) add(errs *bytes.Buffer, line *Line, verb string, args []string, f
 		}
 		if verb == "require" {
 			f.Require = append(f.Require, &Require{
-				Mod:    module.Version{Path: s, Version: v},
-				Syntax: line,
+				Mod:      module.Version{Path: s, Version: v},
+				Syntax:   line,
+				Indirect: isIndirect(line),
 			})
 		} else {
 			f.Exclude = append(f.Exclude, &Exclude{
@@ -253,6 +255,54 @@ func (f *File) add(errs *bytes.Buffer, line *Line, verb string, args []string, f
 	}
 }
 
+// isIndirect reports whether line has a "// indirect" comment,
+// meaning it is in go.mod only for its effect on indirect dependencies,
+// so that it can be dropped entirely once the effective version of the
+// indirect dependency reaches the given minimum version.
+func isIndirect(line *Line) bool {
+	if len(line.Suffix) == 0 {
+		return false
+	}
+	f := strings.Fields(line.Suffix[0].Token)
+	return (len(f) == 2 && f[1] == "indirect" || len(f) > 2 && f[1] == "indirect;") && f[0] == "//"
+}
+
+// setIndirect sets line to have (or not have) a "// indirect" comment.
+func setIndirect(line *Line, indirect bool) {
+	if isIndirect(line) == indirect {
+		return
+	}
+	if indirect {
+		// Adding comment.
+		if len(line.Suffix) == 0 {
+			// New comment.
+			line.Suffix = []Comment{{Token: "// indirect", Suffix: true}}
+			return
+		}
+		// Insert at beginning of existing comment.
+		com := &line.Suffix[0]
+		space := " "
+		if len(com.Token) > 2 && com.Token[2] == ' ' || com.Token[2] == '\t' {
+			space = ""
+		}
+		com.Token = "// indirect;" + space + com.Token[2:]
+		return
+	}
+
+	// Removing comment.
+	f := strings.Fields(line.Suffix[0].Token)
+	if len(f) == 2 {
+		// Remove whole comment.
+		line.Suffix = nil
+		return
+	}
+
+	// Remove comment prefix.
+	com := &line.Suffix[0]
+	i := strings.Index(com.Token, "indirect;")
+	com.Token = "//" + com.Token[i+len("indirect;"):]
+}
+
 // IsDirectoryPath reports whether the given path should be interpreted
 // as a directory path. Just like on the go command line, relative paths
 // and rooted paths are directory paths; the rest are module paths.
@@ -403,24 +453,29 @@ func (f *File) AddRequire(path, vers string) error {
 	}
 
 	if need {
-		f.AddNewRequire(path, vers)
+		f.AddNewRequire(path, vers, false)
 	}
 	return nil
 }
 
-func (f *File) AddNewRequire(path, vers string) {
-	f.Require = append(f.Require, &Require{module.Version{Path: path, Version: vers}, f.Syntax.addLine(nil, "require", AutoQuote(path), vers)})
+func (f *File) AddNewRequire(path, vers string, indirect bool) {
+	line := f.Syntax.addLine(nil, "require", AutoQuote(path), vers)
+	setIndirect(line, indirect)
+	f.Require = append(f.Require, &Require{module.Version{Path: path, Version: vers}, indirect, line})
 }
 
-func (f *File) SetRequire(req []module.Version) {
+func (f *File) SetRequire(req []*Require) {
 	need := make(map[string]string)
-	for _, m := range req {
-		need[m.Path] = m.Version
+	indirect := make(map[string]bool)
+	for _, r := range req {
+		need[r.Mod.Path] = r.Mod.Version
+		indirect[r.Mod.Path] = r.Indirect
 	}
 
 	for _, r := range f.Require {
 		if v, ok := need[r.Mod.Path]; ok {
 			r.Mod.Version = v
+			r.Indirect = indirect[r.Mod.Path]
 		}
 	}
 
@@ -434,6 +489,7 @@ func (f *File) SetRequire(req []module.Version) {
 					if p, err := parseString(&line.Token[0]); err == nil && need[p] != "" {
 						line.Token[1] = need[p]
 						delete(need, p)
+						setIndirect(line, indirect[p])
 						newLines = append(newLines, line)
 					}
 				}
@@ -448,6 +504,7 @@ func (f *File) SetRequire(req []module.Version) {
 				if p, err := parseString(&stmt.Token[1]); err == nil && need[p] != "" {
 					stmt.Token[2] = need[p]
 					delete(need, p)
+					setIndirect(stmt, indirect[p])
 				} else {
 					continue // drop stmt
 				}
@@ -458,7 +515,7 @@ func (f *File) SetRequire(req []module.Version) {
 	f.Syntax.Stmt = newStmts
 
 	for path, vers := range need {
-		f.AddNewRequire(path, vers)
+		f.AddNewRequire(path, vers, indirect[path])
 	}
 	f.SortBlocks()
 }

vendor/cmd/go/internal/modinfo/info.go

@@ -10,14 +10,15 @@ import "time"
 // and the fields are documented in the help text in ../list/list.go
 
 type ModulePublic struct {
-	Path    string        `json:",omitempty"` // module path
-	Version string        `json:",omitempty"` // module version
-	Replace *ModulePublic `json:",omitempty"` // replaced by this module
-	Time    *time.Time    `json:",omitempty"` // time version was created
-	Update  *ModulePublic `json:",omitempty"` // available update (with -u)
-	Main    bool          `json:",omitempty"` // is this the main module?
-	Dir     string        `json:",omitempty"` // directory holding local copy of files, if any
-	Error   *ModuleError  `json:",omitempty"` // error loading module
+	Path     string        `json:",omitempty"` // module path
+	Version  string        `json:",omitempty"` // module version
+	Replace  *ModulePublic `json:",omitempty"` // replaced by this module
+	Time     *time.Time    `json:",omitempty"` // time version was created
+	Update   *ModulePublic `json:",omitempty"` // available update (with -u)
+	Main     bool          `json:",omitempty"` // is this the main module?
+	Indirect bool          `json:",omitempty"` // module is only indirectly needed by main module
+	Dir      string        `json:",omitempty"` // directory holding local copy of files, if any
+	Error    *ModuleError  `json:",omitempty"` // error loading module
 }
 
 type ModuleError struct {

vendor/cmd/go/internal/module/module.go

@@ -26,7 +26,7 @@ type Version struct {
 	// and uses Version = "".
 	// Second, during MVS calculations the version "none" is used
 	// to represent the decision to take no version of a given module.
-	Version string
+	Version string `json:",omitempty"`
 }
 
 // Check checks that a given module path, version pair is valid.

vendor/cmd/go/internal/mvs/mvs.go

@@ -142,8 +142,9 @@ func buildList(target module.Version, reqs Reqs, upgrade func(module.Version) mo
 }
 
 // Req returns the minimal requirement list for the target module
-// that results in the given build list.
-func Req(target module.Version, list []module.Version, reqs Reqs) ([]module.Version, error) {
+// 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.
@@ -197,7 +198,14 @@ func Req(target module.Version, list []module.Version, reqs Reqs) ([]module.Vers
 			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 {

vendor/cmd/go/internal/mvs/mvs_test.go

@@ -166,6 +166,13 @@ D2:
 build A: A B1 C1 D1
 upgrade* A: A B2 C2
 
+name: simplify
+A: B1 C1
+B1: C2
+C1: D1
+C2:
+build A: A B1 C2
+
 name: up1
 A: B1 C1
 B1:
@@ -259,6 +266,22 @@ C2:
 D2:
 build A: A B1
 upgrade* A: A B2
+
+# Requirement minimization.
+
+name: req1
+A: B1 C1 D1 E1 F1
+B1: C1 E1 F1
+req A: B1 D1
+req A C: B1 C1 D1
+
+name: req2
+A: G1 H1
+G1: H1
+H1: G1
+req A: G1
+req A G: G1
+req A H: H1
 `
 
 func Test(t *testing.T) {
@@ -341,19 +364,19 @@ func Test(t *testing.T) {
 			continue
 		case "upgradereq":
 			if len(kf) < 2 {
-				t.Fatalf("upgrade takes at least one arguments: %q", line)
+				t.Fatalf("upgrade takes at least one argument: %q", line)
 			}
 			fns = append(fns, func(t *testing.T) {
 				list, err := Upgrade(m(kf[1]), reqs, ms(kf[2:])...)
 				if err == nil {
-					list, err = Req(m(kf[1]), list, reqs)
+					list, err = Req(m(kf[1]), list, nil, reqs)
 				}
 				checkList(t, key, list, err, val)
 			})
 			continue
 		case "upgrade":
 			if len(kf) < 2 {
-				t.Fatalf("upgrade takes at least one arguments: %q", line)
+				t.Fatalf("upgrade takes at least one argument: %q", line)
 			}
 			fns = append(fns, func(t *testing.T) {
 				list, err := Upgrade(m(kf[1]), reqs, ms(kf[2:])...)
@@ -362,13 +385,26 @@ func Test(t *testing.T) {
 			continue
 		case "downgrade":
 			if len(kf) < 2 {
-				t.Fatalf("downgrade takes at least one arguments: %q", line)
+				t.Fatalf("downgrade takes at least one argument: %q", line)
 			}
 			fns = append(fns, func(t *testing.T) {
 				list, err := Downgrade(m(kf[1]), reqs, ms(kf[1:])...)
 				checkList(t, key, list, err, val)
 			})
 			continue
+		case "req":
+			if len(kf) < 2 {
+				t.Fatalf("req takes at least one argument: %q", line)
+			}
+			fns = append(fns, func(t *testing.T) {
+				list, err := BuildList(m(kf[1]), reqs)
+				if err != nil {
+					t.Fatal(err)
+				}
+				list, err = Req(m(kf[1]), list, kf[2:], reqs)
+				checkList(t, key, list, err, val)
+			})
+			continue
 		}
 		if len(kf) == 1 && 'A' <= key[0] && key[0] <= 'Z' {
 			var rs []module.Version