cmd/go: place GOROOT/bin at the beginning of PATH in 'go generate' and 'go test'

This causes tests and generators that execute 'go' as a subprocess to
use the same go command as the parent 'go test' or 'go generate'
command.

For #51473.

Change-Id: I003cf1d05d1c93a26c6a7fdfad25e86c11765f59
Reviewed-on: https://go-review.googlesource.com/c/go/+/404134
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/base/env.go

@@ -5,14 +5,18 @@
 package base
 
 import (
+	"cmd/go/internal/cfg"
 	"fmt"
+	"os"
 	"path/filepath"
+	"runtime"
 )
 
 // AppendPWD returns the result of appending PWD=dir to the environment base.
 //
 // The resulting environment makes os.Getwd more efficient for a subprocess
-// running in dir.
+// running in dir, and also improves the accuracy of paths relative to dir
+// if one or more elements of dir is a symlink.
 func AppendPWD(base []string, dir string) []string {
 	// POSIX requires PWD to be absolute.
 	// Internally we only use absolute paths, so dir should already be absolute.
@@ -21,3 +25,22 @@ func AppendPWD(base []string, dir string) []string {
 	}
 	return append(base, "PWD="+dir)
 }
+
+// AppendPATH returns the result of appending PATH=$GOROOT/bin:$PATH
+// (or the platform equivalent) to the environment base.
+func AppendPATH(base []string) []string {
+	if cfg.GOROOTbin == "" {
+		return base
+	}
+
+	pathVar := "PATH"
+	if runtime.GOOS == "plan9" {
+		pathVar = "path"
+	}
+
+	path := os.Getenv(pathVar)
+	if path == "" {
+		return append(base, pathVar+"="+cfg.GOROOTbin)
+	}
+	return append(base, pathVar+"="+cfg.GOROOTbin+string(os.PathListSeparator)+path)
+}

src/cmd/go/internal/generate/generate.go

@@ -328,7 +328,7 @@ func isGoGenerate(buf []byte) bool {
 // setEnv sets the extra environment variables used when executing a
 // single go:generate command.
 func (g *Generator) setEnv() {
-	g.env = []string{
+	env := []string{
 		"GOROOT=" + cfg.GOROOT,
 		"GOARCH=" + cfg.BuildContext.GOARCH,
 		"GOOS=" + cfg.BuildContext.GOOS,
@@ -337,7 +337,9 @@ func (g *Generator) setEnv() {
 		"GOPACKAGE=" + g.pkg,
 		"DOLLAR=" + "$",
 	}
-	g.env = base.AppendPWD(g.env, g.dir)
+	env = base.AppendPATH(env)
+	env = base.AppendPWD(env, g.dir)
+	g.env = env
 }
 
 // split breaks the line into words, evaluating quoted
@@ -446,7 +448,20 @@ func (g *Generator) setShorthand(words []string) {
 // exec runs the command specified by the argument. The first word is
 // the command name itself.
 func (g *Generator) exec(words []string) {
-	cmd := exec.Command(words[0], words[1:]...)
+	path := words[0]
+	if path != "" && !strings.Contains(path, string(os.PathSeparator)) {
+		// If a generator says '//go:generate go run <blah>' it almost certainly
+		// intends to use the same 'go' as 'go generate' itself.
+		// Prefer to resolve the binary from GOROOT/bin, and for consistency
+		// prefer to resolve any other commands there too.
+		gorootBinPath, err := exec.LookPath(filepath.Join(cfg.GOROOTbin, path))
+		if err == nil {
+			path = gorootBinPath
+		}
+	}
+	cmd := exec.Command(path, words[1:]...)
+	cmd.Args[0] = words[0] // Overwrite with the original in case it was rewritten above.
+
 	// Standard in and out of generator should be the usual.
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr

src/cmd/go/internal/test/test.go

@@ -1354,7 +1354,12 @@ func (c *runCache) builderRunTest(b *work.Builder, ctx context.Context, a *work.
 
 	cmd := exec.Command(args[0], args[1:]...)
 	cmd.Dir = a.Package.Dir
-	cmd.Env = base.AppendPWD(cfg.OrigEnv[:len(cfg.OrigEnv):len(cfg.OrigEnv)], cmd.Dir)
+
+	env := cfg.OrigEnv[:len(cfg.OrigEnv):len(cfg.OrigEnv)]
+	env = base.AppendPATH(env)
+	env = base.AppendPWD(env, cmd.Dir)
+	cmd.Env = env
+
 	cmd.Stdout = stdout
 	cmd.Stderr = stdout
 

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

@@ -0,0 +1,38 @@
+# https://go.dev/issue/51473: to avoid the need for generators to rely on
+# runtime.GOROOT, 'go generate' should run the test with its own GOROOT/bin
+# at the beginning of $PATH.
+
+[short] skip
+
+[!plan9] env PATH=
+[plan9] env path=
+go generate .
+
+[!plan9] env PATH=$WORK${/}bin
+[plan9] env path=$WORK${/}bin
+go generate .
+
+-- go.mod --
+module example
+
+go 1.19
+-- main.go --
+//go:generate go run .
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+func main() {
+	_, err := exec.LookPath("go")
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+}
+-- $WORK/bin/README.txt --
+This directory contains no executables.

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

@@ -0,0 +1,41 @@
+# https://go.dev/issue/51473: to avoid the need for tests to rely on
+# runtime.GOROOT, 'go test' should run the test with its own GOROOT/bin
+# at the beginning of $PATH.
+
+[short] skip
+
+[!plan9] env PATH=
+[plan9] env path=
+go test .
+
+[!plan9] env PATH=$WORK${/}bin
+[plan9] env path=$WORK${/}bin
+go test .
+
+-- go.mod --
+module example
+
+go 1.19
+-- example_test.go --
+package example
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+func TestGoCommandExists(t *testing.T) {
+	got, err := exec.LookPath("go")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := filepath.Join(os.Getenv("GOROOT"), "bin", "go" + os.Getenv("GOEXE"))
+	if got != want {
+		t.Fatalf(`exec.LookPath("go") = %q; want %q`, got, want)
+	}
+}
+-- $WORK/bin/README.txt --
+This directory contains no executables.