cmd/go: only add a 'go' directive to the main module when the go.mod file will be written

Then, write the 'go.mod' file with that version before further
processing. That way, if the command errors out due to a change in
behavior, the reason for the change in behavior will be visible in the
file diffs.

If the 'go.mod' file cannot be written (due to -mod=readonly or
-mod=vendor), assume Go 1.11 instead of the current Go release.
(cmd/go has added 'go' directives automatically, including in 'go mod
init', since Go 1.12.)

For #44976

Change-Id: If9d4af557366f134f40ce4c5638688ba3bab8380
Reviewed-on: https://go-review.googlesource.com/c/go/+/302051
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>
Reviewed-by: Michael Matloob <matloob@golang.org>

doc/go1.17.html

@@ -55,6 +55,18 @@ Do not send CLs removing the interior tags from such phrases.
   <code>environment</code> for details.
 </p>
 
+<h4 id="missing-go-directive"><code>go.mod</code> files missing <code>go</code> directives</h4>
+
+<p><!-- golang.org/issue/44976 -->
+  If the main module's <code>go.mod</code> file does not contain
+  a <a href="/doc/modules/gomod-ref#go"><code>go</code> directive</a> and
+  the <code>go</code> command cannot update the <code>go.mod</code> file, the
+  <code>go</code> command now assumes <code>go 1.11</code> instead of the
+  current release. (<code>go</code> <code>mod</code> <code>init</code> has added
+  <code>go</code> directives automatically <a href="/doc/go1.12#modules">since
+  Go 1.12</a>.)
+</p>
+
 <h2 id="runtime">Runtime</h2>
 
 <p>

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

@@ -147,12 +147,14 @@ func moduleInfo(ctx context.Context, m module.Version, fromBuildList, listRetrac
 			Version: m.Version,
 			Main:    true,
 		}
+		if v, ok := rawGoVersion.Load(Target); ok {
+			info.GoVersion = v.(string)
+		} else {
+			panic("internal error: GoVersion not set for main module")
+		}
 		if HasModRoot() {
 			info.Dir = ModRoot()
 			info.GoMod = ModFilePath()
-			if modFile.Go != nil {
-				info.GoVersion = modFile.Go.Version
-			}
 		}
 		return info
 	}

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

@@ -35,11 +35,30 @@ import (
 	"golang.org/x/mod/semver"
 )
 
+// Variables set by other packages.
+//
+// TODO(#40775): See if these can be plumbed as explicit parameters.
+var (
+	// RootMode determines whether a module root is needed.
+	RootMode Root
+
+	// ForceUseModules may be set to force modules to be enabled when
+	// GO111MODULE=auto or to report an error when GO111MODULE=off.
+	ForceUseModules bool
+
+	allowMissingModuleImports bool
+)
+
+// Variables set in Init.
 var (
 	initialized bool
+	modRoot     string
+	gopath      string
+)
 
-	modRoot string
-	Target  module.Version
+// Variables set in initTarget (during {Load,Create}ModFile).
+var (
+	Target module.Version
 
 	// targetPrefix is the path prefix for packages in Target, without a trailing
 	// slash. For most modules, targetPrefix is just Target.Path, but the
@@ -49,17 +68,6 @@ var (
 	// targetInGorootSrc caches whether modRoot is within GOROOT/src.
 	// The "std" module is special within GOROOT/src, but not otherwise.
 	targetInGorootSrc bool
-
-	gopath string
-
-	// RootMode determines whether a module root is needed.
-	RootMode Root
-
-	// ForceUseModules may be set to force modules to be enabled when
-	// GO111MODULE=auto or to report an error when GO111MODULE=off.
-	ForceUseModules bool
-
-	allowMissingModuleImports bool
 )
 
 type Root int
@@ -362,6 +370,7 @@ func LoadModFile(ctx context.Context) {
 		Target = module.Version{Path: "command-line-arguments"}
 		targetPrefix = "command-line-arguments"
 		buildList = []module.Version{Target}
+		rawGoVersion.Store(Target, latestGoVersion())
 		return
 	}
 
@@ -377,24 +386,29 @@ func LoadModFile(ctx context.Context) {
 		// Errors returned by modfile.Parse begin with file:line.
 		base.Fatalf("go: errors parsing go.mod:\n%s\n", err)
 	}
-	modFile = f
-	index = indexModFile(data, f, fixed)
-
 	if f.Module == nil {
 		// No module declaration. Must add module path.
 		base.Fatalf("go: no module declaration in go.mod. To specify the module path:\n\tgo mod edit -module=example.com/mod")
 	}
 
+	modFile = f
+	initTarget(f.Module.Mod)
+	index = indexModFile(data, f, fixed)
+
 	if err := checkModulePathLax(f.Module.Mod.Path); err != nil {
 		base.Fatalf("go: %v", err)
 	}
 
 	setDefaultBuildMod() // possibly enable automatic vendoring
-	modFileToBuildList()
+	buildList = modFileToBuildList(modFile)
 	if cfg.BuildMod == "vendor" {
 		readVendorList()
 		checkVendorConsistency()
 	}
+	if index.goVersionV == "" && cfg.BuildMod == "mod" {
+		addGoStmt()
+		WriteGoMod()
+	}
 }
 
 // CreateModFile initializes a new module by creating a go.mod file.
@@ -427,6 +441,7 @@ func CreateModFile(ctx context.Context, modPath string) {
 	fmt.Fprintf(os.Stderr, "go: creating new go.mod: module %s\n", modPath)
 	modFile = new(modfile.File)
 	modFile.AddModuleStmt(modPath)
+	initTarget(modFile.Module.Mod)
 	addGoStmt() // Add the go directive before converted module requirements.
 
 	convertedFrom, err := convertLegacyConfig(modPath)
@@ -437,7 +452,7 @@ func CreateModFile(ctx context.Context, modPath string) {
 		base.Fatalf("go: %v", err)
 	}
 
-	modFileToBuildList()
+	buildList = modFileToBuildList(modFile)
 	WriteGoMod()
 
 	// Suggest running 'go mod tidy' unless the project is empty. Even if we
@@ -563,19 +578,31 @@ func AllowMissingModuleImports() {
 	allowMissingModuleImports = true
 }
 
-// modFileToBuildList initializes buildList from the modFile.
-func modFileToBuildList() {
-	Target = modFile.Module.Mod
-	targetPrefix = Target.Path
+// initTarget sets Target and associated variables according to modFile,
+func initTarget(m module.Version) {
+	Target = m
+	targetPrefix = m.Path
+
 	if rel := search.InDir(base.Cwd, cfg.GOROOTsrc); rel != "" {
 		targetInGorootSrc = true
-		if Target.Path == "std" {
+		if m.Path == "std" {
+			// The "std" module in GOROOT/src is the Go standard library. Unlike other
+			// modules, the packages in the "std" module have no import-path prefix.
+			//
+			// Modules named "std" outside of GOROOT/src do not receive this special
+			// treatment, so it is possible to run 'go test .' in other GOROOTs to
+			// test individual packages using a combination of the modified package
+			// and the ordinary standard library.
+			// (See https://golang.org/issue/30756.)
 			targetPrefix = ""
 		}
 	}
+}
 
+// modFileToBuildList returns the list of non-excluded requirements from f.
+func modFileToBuildList(f *modfile.File) []module.Version {
 	list := []module.Version{Target}
-	for _, r := range modFile.Require {
+	for _, r := range f.Require {
 		if index != nil && index.exclude[r.Mod] {
 			if cfg.BuildMod == "mod" {
 				fmt.Fprintf(os.Stderr, "go: dropping requirement on excluded version %s %s\n", r.Mod.Path, r.Mod.Version)
@@ -586,7 +613,7 @@ func modFileToBuildList() {
 			list = append(list, r.Mod)
 		}
 	}
-	buildList = list
+	return list
 }
 
 // setDefaultBuildMod sets a default value for cfg.BuildMod if the -mod flag
@@ -650,20 +677,29 @@ func convertLegacyConfig(modPath string) (from string, err error) {
 	return "", nil
 }
 
-// addGoStmt adds a go directive to the go.mod file if it does not already include one.
-// The 'go' version added, if any, is the latest version supported by this toolchain.
+// addGoStmt adds a go directive to the go.mod file if it does not already
+// include one. The 'go' version added, if any, is the latest version supported
+// by this toolchain.
 func addGoStmt() {
 	if modFile.Go != nil && modFile.Go.Version != "" {
 		return
 	}
+	v := latestGoVersion()
+	if err := modFile.AddGoStmt(v); err != nil {
+		base.Fatalf("go: internal error: %v", err)
+	}
+	rawGoVersion.Store(Target, v)
+}
+
+// latestGoVersion returns the latest version of the Go language supported by
+// this toolchain.
+func latestGoVersion() string {
 	tags := build.Default.ReleaseTags
 	version := tags[len(tags)-1]
 	if !strings.HasPrefix(version, "go") || !modfile.GoVersionRE.MatchString(version[2:]) {
 		base.Fatalf("go: unrecognized default version %q", version)
 	}
-	if err := modFile.AddGoStmt(version[2:]); err != nil {
-		base.Fatalf("go: internal error: %v", err)
-	}
+	return version[2:]
 }
 
 var altConfigs = []string{
@@ -880,10 +916,6 @@ func WriteGoMod() {
 		return
 	}
 
-	if cfg.BuildMod != "readonly" {
-		addGoStmt()
-	}
-
 	if loaded != nil {
 		reqs := MinReqs()
 		min, err := reqs.Required(Target)
@@ -1010,7 +1042,12 @@ func keepSums(keepBuildListZips bool) map[module.Version]bool {
 	}
 	buildList, err := mvs.BuildList(Target, reqs)
 	if err != nil {
-		panic(fmt.Sprintf("unexpected error reloading build list: %v", err))
+		// This call to mvs.BuildList should not fail if we have already read the
+		// complete build list. However, the initial “build list” initialized by
+		// modFileToBuildList is not complete: it contains only the explicit
+		// dependencies of the main module. So this call can fair if this is the
+		// first time we have actually loaded the real build list.
+		base.Fatalf("go: %v", err)
 	}
 
 	actualMods := make(map[string]module.Version)

src/cmd/go/internal/modload/modfile.go

@@ -257,10 +257,13 @@ func indexModFile(data []byte, modFile *modfile.File, needsFix bool) *modFileInd
 	}
 
 	i.goVersionV = ""
-	if modFile.Go != nil {
+	if modFile.Go == nil {
+		rawGoVersion.Store(Target, "")
+	} else {
 		// We're going to use the semver package to compare Go versions, so go ahead
 		// and add the "v" prefix it expects once instead of every time.
 		i.goVersionV = "v" + modFile.Go.Version
+		rawGoVersion.Store(Target, modFile.Go.Version)
 	}
 
 	i.require = make(map[module.Version]requireMeta, len(modFile.Require))

src/cmd/go/internal/work/gc.go

@@ -63,8 +63,18 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 
 	pkgpath := pkgPath(a)
 	gcargs := []string{"-p", pkgpath}
-	if p.Module != nil && p.Module.GoVersion != "" && allowedVersion(p.Module.GoVersion) {
-		gcargs = append(gcargs, "-lang=go"+p.Module.GoVersion)
+	if p.Module != nil {
+		v := p.Module.GoVersion
+		if v == "" {
+			// We started adding a 'go' directive to the go.mod file unconditionally
+			// as of Go 1.12, so any module that still lacks such a directive must
+			// either have been authored before then, or have a hand-edited go.mod
+			// file that hasn't been updated by cmd/go since that edit.
+			v = "1.11"
+		}
+		if allowedVersion(v) {
+			gcargs = append(gcargs, "-lang=go"+v)
+		}
 	}
 	if p.Standard {
 		gcargs = append(gcargs, "-std")

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

@@ -107,3 +107,4 @@ import _ "m"
 -- go.mod --
 module m
 
+go 1.16

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

@@ -0,0 +1,97 @@
+cp go.mod go.mod.orig
+
+# With -mod=readonly, we should not update the go version in use.
+#
+# We started adding the go version automatically in Go 1.12, so a module without
+# one encountered in the wild (such as in the module cache) should assume Go
+# 1.11 semantics.
+
+# For Go 1.11 modules, 'all' should include dependencies of tests.
+# (They are pruned out as of Go 1.16.)
+
+go list -mod=readonly all
+stdout '^example.com/dep$'
+stdout '^example.com/testdep$'
+cp stdout list-1.txt
+cmp go.mod go.mod.orig
+
+# For Go 1.11 modules, automatic vendoring should not take effect.
+# (That behavior was added in Go 1.14.)
+
+go list all  # should default to -mod=readonly, not -mod=vendor.
+cmp stdout list-1.txt
+
+# When we set -mod=mod, the go version should be updated immediately,
+# narrowing the "all" pattern reported by that command.
+
+go list -mod=mod all
+! stdout '^example.com/testdep$'
+cp stdout list-2.txt
+cmpenv go.mod go.mod.want
+
+go list -mod=mod all
+cmp stdout list-2.txt
+
+# The updated version should have been written back to go.mod, so
+# automatic vendoring should come into effect (and fail).
+! go list all
+stderr '^go: inconsistent vendoring'
+
+cp go.mod.orig go.mod
+
+# In readonly or vendor mode (not -mod=mod), the inferred Go version is 1.11.
+# For Go 1.11 modules, Go 1.13 features should not be enabled.
+
+! go build -mod=readonly .
+stderr '^# example\.com/m\n\.[/\\]m\.go:5:11: underscores in numeric literals requires go1\.13 or later \(-lang was set to go1\.11; check go\.mod\)$'
+cmp go.mod go.mod.orig
+
+
+-- go.mod --
+module example.com/m
+
+require example.com/dep v0.1.0
+
+replace (
+	example.com/dep v0.1.0 => ./dep
+	example.com/testdep v0.1.0 => ./testdep
+)
+-- go.mod.want --
+module example.com/m
+
+go $goversion
+
+require example.com/dep v0.1.0
+
+replace (
+	example.com/dep v0.1.0 => ./dep
+	example.com/testdep v0.1.0 => ./testdep
+)
+-- vendor/example.com/dep/dep.go --
+package dep
+import _ "example.com/bananas"
+-- vendor/modules.txt --
+HAHAHA this is broken.
+
+-- m.go --
+package m
+
+import _ "example.com/dep"
+
+const x = 1_000
+
+-- dep/go.mod --
+module example.com/dep
+
+require example.com/testdep v0.1.0
+-- dep/dep.go --
+package dep
+-- dep/dep_test.go --
+package dep_test
+
+import _ "example.com/testdep"
+
+-- testdep/go.mod --
+module example.com/testdep
+-- testdep/testdep.go --
+package testdep

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

@@ -169,6 +169,8 @@ go build -n -o ignore ./stdonly/stdonly.go
 # 'go build' should succeed for standard-library packages.
 go build -n fmt
 
+# 'go build' should use the latest version of the Go language.
+go build ./newgo/newgo.go
 
 # 'go doc' without arguments implicitly operates on the current directory, and should fail.
 # TODO(golang.org/issue/32027): currently, it succeeds.
@@ -331,3 +333,15 @@ func Test(t *testing.T) {
 	fmt.Println("stdonly was tested")
 }
 
+-- newgo/newgo.go --
+// Package newgo requires Go 1.14 or newer.
+package newgo
+
+import "io"
+
+const C = 299_792_458
+
+type ReadWriteCloser interface {
+	io.ReadCloser
+	io.WriteCloser
+}

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

@@ -60,6 +60,8 @@ go list -test
 -- a/go.mod.empty --
 module example.com/user/a
 
+go 1.11
+
 -- a/a.go --
 package a