[release-branch.go1.21] cmd/go/internal/toolchain: apply the -modcacherw flag when downloading a module to determine what toolchain it needs

Fixes #64497.
Updates #64282.

Change-Id: I3f211c599ee70cb58254d0bc07eeb3c135124e58
Reviewed-on: https://go-review.googlesource.com/c/go/+/555436
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
(cherry picked from commit cc38c68ae09fa591697a4239a7dedd2efe386995)
Reviewed-on: https://go-review.googlesource.com/c/go/+/559198
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

2 files changed, 149 insertions, 44 deletions

diff --git a/src/cmd/go/internal/toolchain/select.go b/src/cmd/go/internal/toolchain/select.go
index a44f393bc0..3446a48d2d 100644
--- a/src/cmd/go/internal/toolchain/select.go
+++ b/src/cmd/go/internal/toolchain/select.go
@@ -8,6 +8,7 @@ package toolchain
 import (
 	"context"
 	"errors"
+	"flag"
 	"fmt"
 	"go/build"
 	"io/fs"
@@ -25,6 +26,7 @@ import (
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/run"
+	"cmd/go/internal/work"
 
 	"golang.org/x/mod/module"
 )
@@ -485,74 +487,132 @@ func goInstallVersion() bool {
 	// Note: We assume there are no flags between 'go' and 'install' or 'run'.
 	// During testing there are some debugging flags that are accepted
 	// in that position, but in production go binaries there are not.
-	if len(os.Args) < 3 || (os.Args[1] != "install" && os.Args[1] != "run") {
+	if len(os.Args) < 3 {
 		return false
 	}
 
-	// Check for pkg@version.
-	var arg string
+	var cmdFlags *flag.FlagSet
 	switch os.Args[1] {
 	default:
+		// Command doesn't support a pkg@version as the main module.
 		return false
 	case "install":
-		// We would like to let 'go install -newflag pkg@version' work even
-		// across a toolchain switch. To make that work, assume the pkg@version
-		// is the last argument and skip the flag parsing.
-		arg = os.Args[len(os.Args)-1]
+		cmdFlags = &work.CmdInstall.Flag
 	case "run":
-		// For run, the pkg@version can be anywhere on the command line,
-		// because it is preceded by run flags and followed by arguments to the
-		// program being run. To handle that precisely, we have to interpret the
-		// flags a little bit, to know whether each flag takes an optional argument.
-		// We can still allow unknown flags as long as they have an explicit =value.
-		args := os.Args[2:]
-		for i := 0; i < len(args); i++ {
-			a := args[i]
-			if !strings.HasPrefix(a, "-") {
-				arg = a
-				break
-			}
-			if a == "-" {
-				// non-flag but also non-pkg@version
+		cmdFlags = &run.CmdRun.Flag
+	}
+
+	// The modcachrw flag is unique, in that it affects how we fetch the
+	// requested module to even figure out what toolchain it needs.
+	// We need to actually set it before we check the toolchain version.
+	// (See https://go.dev/issue/64282.)
+	modcacherwFlag := cmdFlags.Lookup("modcacherw")
+	if modcacherwFlag == nil {
+		base.Fatalf("internal error: modcacherw flag not registered for command")
+	}
+	modcacherwVal, ok := modcacherwFlag.Value.(interface {
+		IsBoolFlag() bool
+		flag.Value
+	})
+	if !ok || !modcacherwVal.IsBoolFlag() {
+		base.Fatalf("internal error: modcacherw is not a boolean flag")
+	}
+
+	// Make a best effort to parse the command's args to find the pkg@version
+	// argument and the -modcacherw flag.
+	var (
+		pkgArg         string
+		modcacherwSeen bool
+	)
+	for args := os.Args[2:]; len(args) > 0; {
+		a := args[0]
+		args = args[1:]
+		if a == "--" {
+			if len(args) == 0 {
 				return false
 			}
-			if a == "--" {
-				if i+1 >= len(args) {
-					return false
-				}
-				arg = args[i+1]
-				break
+			pkgArg = args[0]
+			break
+		}
+
+		a, ok := strings.CutPrefix(a, "-")
+		if !ok {
+			// Not a flag argument. Must be a package.
+			pkgArg = a
+			break
+		}
+		a = strings.TrimPrefix(a, "-") // Treat --flag as -flag.
+
+		name, val, hasEq := strings.Cut(a, "=")
+
+		if name == "modcacherw" {
+			if !hasEq {
+				val = "true"
 			}
-			a = strings.TrimPrefix(a, "-")
-			a = strings.TrimPrefix(a, "-")
-			if strings.HasPrefix(a, "-") {
-				// non-flag but also non-pkg@version
+			if err := modcacherwVal.Set(val); err != nil {
 				return false
 			}
-			if strings.Contains(a, "=") {
-				// already has value
-				continue
-			}
-			f := run.CmdRun.Flag.Lookup(a)
-			if f == nil {
-				// Unknown flag. Give up. The command is going to fail in flag parsing.
+			modcacherwSeen = true
+			continue
+		}
+
+		if hasEq {
+			// Already has a value; don't bother parsing it.
+			continue
+		}
+
+		f := run.CmdRun.Flag.Lookup(a)
+		if f == nil {
+			// We don't know whether this flag is a boolean.
+			if os.Args[1] == "run" {
+				// We don't know where to find the pkg@version argument.
+				// For run, the pkg@version can be anywhere on the command line,
+				// because it is preceded by run flags and followed by arguments to the
+				// program being run. Since we don't know whether this flag takes
+				// an argument, we can't reliably identify the end of the run flags.
+				// Just give up and let the user clarify using the "=" form..
 				return false
 			}
-			if bf, ok := f.Value.(interface{ IsBoolFlag() bool }); ok && bf.IsBoolFlag() {
-				// Does not take value.
-				continue
+
+			// We would like to let 'go install -newflag pkg@version' work even
+			// across a toolchain switch. To make that work, assume by default that
+			// the pkg@version is the last argument and skip the remaining args unless
+			// we spot a plausible "-modcacherw" flag.
+			for len(args) > 0 {
+				a := args[0]
+				name, _, _ := strings.Cut(a, "=")
+				if name == "-modcacherw" || name == "--modcacherw" {
+					break
+				}
+				if len(args) == 1 && !strings.HasPrefix(a, "-") {
+					pkgArg = a
+				}
+				args = args[1:]
 			}
-			i++ // Does take a value; skip it.
+			continue
+		}
+
+		if bf, ok := f.Value.(interface{ IsBoolFlag() bool }); !ok || !bf.IsBoolFlag() {
+			// The next arg is the value for this flag. Skip it.
+			args = args[1:]
+			continue
 		}
 	}
-	if !strings.Contains(arg, "@") || build.IsLocalImport(arg) || filepath.IsAbs(arg) {
+
+	if !strings.Contains(pkgArg, "@") || build.IsLocalImport(pkgArg) || filepath.IsAbs(pkgArg) {
 		return false
 	}
-	path, version, _ := strings.Cut(arg, "@")
+	path, version, _ := strings.Cut(pkgArg, "@")
 	if path == "" || version == "" || gover.IsToolchain(path) {
 		return false
 	}
 
+	if !modcacherwSeen && base.InGOFLAGS("-modcacherw") {
+		fs := flag.NewFlagSet("goInstallVersion", flag.ExitOnError)
+		fs.Var(modcacherwVal, "modcacherw", modcacherwFlag.Usage)
+		base.SetFromGOFLAGS(fs)
+	}
+
 	// It would be correct to simply return true here, bypassing use
 	// of the current go.mod or go.work, and let "go run" or "go install"
 	// do the rest, including a toolchain switch.
diff --git a/src/cmd/go/testdata/script/install_modcacherw_issue64282.txt b/src/cmd/go/testdata/script/install_modcacherw_issue64282.txt
new file mode 100644
index 0000000000..3e1e6e562a
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_modcacherw_issue64282.txt
@@ -0,0 +1,45 @@
+# Regression test for https://go.dev/issue/64282.
+#
+# 'go install' and 'go run' with pkg@version arguments should make
+# a best effort to parse flags relevant to downloading modules
+# (currently only -modcacherw) before actually downloading the module
+# to identify which toolchain version to use.
+#
+# However, the best-effort flag parsing should not interfere with
+# actual flag parsing if we don't switch toolchains. In particular,
+# unrecognized flags should still be diagnosed after the module for
+# the requested package has been downloaded and checked for toolchain
+# upgrades.
+
+
+! go install -cake=delicious -modcacherw example.com/printversion@v0.1.0
+stderr '^flag provided but not defined: -cake$'
+	# Because the -modcacherw flag was set, we should be able to modify the contents
+	# of a directory within the module cache.
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/printversion@v0.1.0/extraneous_file.go
+go clean -modcache
+
+
+! go install -unknownflag -tags -modcacherw example.com/printversion@v0.1.0
+stderr '^flag provided but not defined: -unknownflag$'
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/printversion@v0.1.0/extraneous_file.go
+go clean -modcache
+
+
+# Also try it with a 'go install' that succeeds.
+# (But skip in short mode, because linking a binary is expensive.)
+[!short] go install -modcacherw example.com/printversion@v0.1.0
+[!short] cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/printversion@v0.1.0/extraneous_file.go
+[!short] go clean -modcache
+
+
+# The flag should also be applied if given in GOFLAGS
+# instead of on the command line.
+env GOFLAGS=-modcacherw
+! go install -cake=delicious example.com/printversion@v0.1.0
+stderr '^flag provided but not defined: -cake$'
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/printversion@v0.1.0/extraneous_file.go
+
+
+-- $WORK/extraneous.txt --
+This is not a Go source file.