cmd/go: set cfg.BuildMod to "readonly" by default with no module root

modload.Init now sets the default value for -mod if it wasn't set
explicitly. This happens before go.mod is loaded, so
modload.LoadModFile sets the default value again in order to enable
automatic vendoring.

Previously, cfg.BuildMod wasn't set at all if LoadModFile wasn't
called, as is the case for commands that run outside of a module
root. This problem only affected 'go install pkg@version' since other
commands are either forbidden in module mode or run with -mod=mod
(like 'go get' and 'go mod' subcommands).

This change also suppresses "missing sum" errors when -mod=readonly is
enabled and there is no module root.

Fixes #43278
Related #40278

Change-Id: I6071cc42bc5e24d0d7e84556e5bfd8e368e0019d
Reviewed-on: https://go-review.googlesource.com/c/go/+/279490
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modload/import.go

@@ -58,7 +58,7 @@ func (e *ImportMissingError) Error() string {
 		if e.QueryErr != nil {
 			return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)
 		}
-		if cfg.BuildMod == "mod" {
+		if cfg.BuildMod == "mod" || (cfg.BuildMod == "readonly" && allowMissingModuleImports) {
 			return "cannot find module providing package " + e.Path
 		}
 
@@ -365,7 +365,7 @@ func queryImport(ctx context.Context, path string) (module.Version, error) {
 		return module.Version{}, &ImportMissingError{Path: path, isStd: true}
 	}
 
-	if cfg.BuildMod == "readonly" {
+	if cfg.BuildMod == "readonly" && !allowMissingModuleImports {
 		// In readonly mode, we can't write go.mod, so we shouldn't try to look up
 		// the module. If readonly mode was enabled explicitly, include that in
 		// the error message.
@@ -547,7 +547,7 @@ func fetch(ctx context.Context, mod module.Version, needSum bool) (dir string, i
 		mod = r
 	}
 
-	if cfg.BuildMod == "readonly" && needSum && !modfetch.HaveSum(mod) {
+	if HasModRoot() && cfg.BuildMod == "readonly" && needSum && !modfetch.HaveSum(mod) {
 		return "", false, module.VersionError(mod, &sumMissingError{})
 	}
 

src/cmd/go/internal/modload/import_test.go

@@ -58,10 +58,15 @@ var importTests = []struct {
 func TestQueryImport(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)
 	testenv.MustHaveExecPath(t, "git")
-	defer func(old bool) {
-		allowMissingModuleImports = old
-	}(allowMissingModuleImports)
-	AllowMissingModuleImports()
+
+	oldAllowMissingModuleImports := allowMissingModuleImports
+	oldRootMode := RootMode
+	defer func() {
+		allowMissingModuleImports = oldAllowMissingModuleImports
+		RootMode = oldRootMode
+	}()
+	allowMissingModuleImports = true
+	RootMode = NoRoot
 
 	ctx := context.Background()
 

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

@@ -202,6 +202,8 @@ func Init() {
 	}
 
 	// We're in module mode. Set any global variables that need to be set.
+	cfg.ModulesEnabled = true
+	setDefaultBuildMod()
 	list := filepath.SplitList(cfg.BuildContext.GOPATH)
 	if len(list) == 0 || list[0] == "" {
 		base.Fatalf("missing $GOPATH")
@@ -211,8 +213,6 @@ func Init() {
 		base.Fatalf("$GOPATH/go.mod exists but should not")
 	}
 
-	cfg.ModulesEnabled = true
-
 	if modRoot == "" {
 		// We're in module mode, but not inside a module.
 		//
@@ -348,8 +348,8 @@ func die() {
 // ensuring requirements are consistent. WriteGoMod should be called later to
 // write changes out to disk or report errors in readonly mode.
 //
-// As a side-effect, LoadModFile sets a default for cfg.BuildMod if it does not
-// already have an explicit value.
+// As a side-effect, LoadModFile may change cfg.BuildMod to "vendor" if
+// -mod wasn't set explicitly and automatic vendoring should be enabled.
 func LoadModFile(ctx context.Context) {
 	if len(buildList) > 0 {
 		return
@@ -387,7 +387,7 @@ func LoadModFile(ctx context.Context) {
 		base.Fatalf("go: %v", err)
 	}
 
-	setDefaultBuildMod()
+	setDefaultBuildMod() // possibly enable automatic vendoring
 	modFileToBuildList()
 	if cfg.BuildMod == "vendor" {
 		readVendorList()
@@ -586,8 +586,8 @@ func modFileToBuildList() {
 	buildList = list
 }
 
-// setDefaultBuildMod sets a default value for cfg.BuildMod
-// if it is currently empty.
+// setDefaultBuildMod sets a default value for cfg.BuildMod if the -mod flag
+// wasn't provided. setDefaultBuildMod may be called multiple times.
 func setDefaultBuildMod() {
 	if cfg.BuildModExplicit {
 		// Don't override an explicit '-mod=' argument.
@@ -608,7 +608,7 @@ func setDefaultBuildMod() {
 
 	if fi, err := fsys.Stat(filepath.Join(modRoot, "vendor")); err == nil && fi.IsDir() {
 		modGo := "unspecified"
-		if index.goVersionV != "" {
+		if index != nil && index.goVersionV != "" {
 			if semver.Compare(index.goVersionV, "v1.14") >= 0 {
 				// The Go version is at least 1.14, and a vendor directory exists.
 				// Set -mod=vendor by default.

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

@@ -446,7 +446,7 @@ func goModSummary(m module.Version) (*modFileSummary, error) {
 	if actual.Path == "" {
 		actual = m
 	}
-	if cfg.BuildMod == "readonly" && actual.Version != "" {
+	if HasModRoot() && cfg.BuildMod == "readonly" && actual.Version != "" {
 		key := module.Version{Path: actual.Path, Version: actual.Version + "/go.mod"}
 		if !modfetch.HaveSum(key) {
 			suggestion := fmt.Sprintf("; try 'go mod download %s' to add it", m.Path)

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

@@ -175,6 +175,11 @@ stdout '^\tmod\texample.com/cmd\tv1.0.0\t'
 go install example.com/cmd/a@v1.9.0
 go version -m $GOPATH/bin/a$GOEXE
 stdout '^\tmod\texample.com/cmd\tv1.9.0\t'
+env GO111MODULE=
+
+# 'go install pkg@version' succeeds when -mod=readonly is set explicitly.
+# Verifies #43278.
+go install -mod=readonly example.com/cmd/a@v1.0.0
 
 -- m/go.mod --
 module m