cmd/go: resolve std-vendored dependencies as std packages except in 'go get' and 'go mod'

In CL 251159, I removed a hard-coded special case changing the
rewriting behavior for std dependencies in GOROOT/src/vendor and
GOROOT/src/cmd/vendor. Unfortunately, that caused packages in 'std' to
be reported as stale when run within GOROOT/src.

This change restores the special-case behavior, but plumbs it through
the PackageOpts explicitly instead of comparing strings stored in
global variables.

Fixes #44725

Change-Id: If084fe74972ce1704715ca79b0b7e092dd90c88b
Reviewed-on: https://go-review.googlesource.com/c/go/+/297869
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>

src/cmd/go/internal/modcmd/tidy.go

@@ -63,6 +63,7 @@ func runTidy(ctx context.Context, cmd *base.Command, args []string) {
 
 	modload.LoadPackages(ctx, modload.PackageOpts{
 		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
 		ResolveMissingImports:    true,
 		LoadTests:                true,
 		AllowErrors:              tidyE,

src/cmd/go/internal/modcmd/vendor.go

@@ -65,6 +65,7 @@ func runVendor(ctx context.Context, cmd *base.Command, args []string) {
 
 	loadOpts := modload.PackageOpts{
 		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
 		ResolveMissingImports:    true,
 		UseVendorAll:             true,
 		AllowErrors:              vendorE,

src/cmd/go/internal/modcmd/why.go

@@ -69,6 +69,7 @@ func runWhy(ctx context.Context, cmd *base.Command, args []string) {
 
 	loadOpts := modload.PackageOpts{
 		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
 		LoadTests:                !*whyVendor,
 		SilenceErrors:            true,
 		UseVendorAll:             *whyVendor,

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

@@ -1121,6 +1121,7 @@ func (r *resolver) findAndUpgradeImports(ctx context.Context, queries []*query)
 func (r *resolver) loadPackages(ctx context.Context, patterns []string, findPackage func(ctx context.Context, path string, m module.Version) (versionOk bool)) {
 	opts := modload.PackageOpts{
 		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
 		LoadTests:                *getT,
 		SilenceErrors:            true, // May be fixed by subsequent upgrades or downgrades.
 	}
@@ -1459,6 +1460,7 @@ func (r *resolver) checkPackagesAndRetractions(ctx context.Context, pkgPatterns
 		// LoadPackages will print errors (since it has more context) but will not
 		// exit, since we need to load retractions later.
 		pkgOpts := modload.PackageOpts{
+			VendorModulesInGOROOTSrc: true,
 			LoadTests:                *getT,
 			ResolveMissingImports:    false,
 			AllowErrors:              true,

src/cmd/go/internal/modload/load.go

@@ -134,6 +134,11 @@ type PackageOpts struct {
 	// If nil, treated as equivalent to imports.Tags().
 	Tags map[string]bool
 
+	// VendorModulesInGOROOTSrc indicates that if we are within a module in
+	// GOROOT/src, packages in the module's vendor directory should be resolved as
+	// actual module dependencies (instead of standard-library packages).
+	VendorModulesInGOROOTSrc bool
+
 	// ResolveMissingImports indicates that we should attempt to add module
 	// dependencies as needed to resolve imports of packages that are not found.
 	//
@@ -1170,13 +1175,13 @@ func (ld *loader) stdVendor(parentPath, path string) string {
 	}
 
 	if str.HasPathPrefix(parentPath, "cmd") {
-		if Target.Path != "cmd" {
+		if !ld.VendorModulesInGOROOTSrc || Target.Path != "cmd" {
 			vendorPath := pathpkg.Join("cmd", "vendor", path)
 			if _, err := os.Stat(filepath.Join(cfg.GOROOTsrc, filepath.FromSlash(vendorPath))); err == nil {
 				return vendorPath
 			}
 		}
-	} else if Target.Path != "std" || str.HasPathPrefix(parentPath, "vendor") {
+	} else if !ld.VendorModulesInGOROOTSrc || Target.Path != "std" || str.HasPathPrefix(parentPath, "vendor") {
 		// If we are outside of the 'std' module, resolve imports from within 'std'
 		// to the vendor directory.
 		//

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

@@ -0,0 +1,31 @@
+# https://golang.org/issue/44725: packages in std should not be reported as stale,
+# regardless of whether they are listed from within or outside GOROOT/src.
+
+# Control case: net should not be stale at the start of the test,
+# and should depend on vendor/golang.org/… instead of golang.org/….
+
+! stale net
+
+go list -deps net
+stdout '^vendor/golang.org/x/net'
+! stdout '^golang.org/x/net'
+
+# Net should also not be stale when viewed from within GOROOT/src,
+# and should still report the same package dependencies.
+
+cd $GOROOT/src
+! stale net
+
+go list -deps net
+stdout '^vendor/golang.org/x/net'
+! stdout '^golang.org/x/net'
+
+
+# However, 'go mod' and 'go get' subcommands should report the original module
+# dependencies, not the vendored packages.
+
+[!net] stop
+
+env GOPROXY=
+go mod why -m golang.org/x/net
+stdout '^# golang.org/x/net\nnet\ngolang.org/x/net'

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

@@ -48,18 +48,20 @@ stdout ^vendor/golang.org/x/crypto/internal/subtle
 ! stdout ^golang\.org/x
 
 # Within the std module, the dependencies of the non-vendored packages within
-# std should appear to come from modules, but they should be loaded from the
-# vendor directory (just like ordinary vendored module dependencies).
+# std should appear to be packages beginning with 'vendor/', not 'golang.org/…'
+# module dependencies.
 
 go list all
-stdout ^golang.org/x/
+! stdout ^golang.org/x/
 ! stdout ^std/
 ! stdout ^cmd/
-! stdout ^vendor/
+stdout ^vendor/
 
 go list -deps -f '{{if not .Standard}}{{.ImportPath}}{{end}}' std
-! stdout ^vendor/golang.org/x/net/http2/hpack
-stdout ^golang.org/x/net/http2/hpack
+! stdout .
+
+# However, the 'golang.org/…' module dependencies should resolve to those same
+# directories.
 
 go list -f '{{.Dir}}' golang.org/x/net/http2/hpack
 stdout $GOROOT[/\\]src[/\\]vendor

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

@@ -36,11 +36,11 @@ stderr 'use of vendored package'
 
 
 # When run within the 'std' module, 'go list -test' should report vendored
-# transitive dependencies at their original module paths.
+# transitive dependencies at their vendored paths.
 cd $GOROOT/src
 go list -test -f '{{range .Deps}}{{.}}{{"\n"}}{{end}}' net/http
-stdout ^golang.org/x/net/http2/hpack
-! stdout ^vendor/golang.org/x/net/http2/hpack
+! stdout ^golang.org/x/net/http2/hpack
+stdout ^vendor/golang.org/x/net/http2/hpack
 
 -- go.mod --
 module m