[release-branch.go1.13] cmd/go: do not append to the global cfg.OrigEnv slice

Appending to a global slice is only safe if its length is already
equal to its capacity. That property is not guaranteed for slices in
general, and empirically does not hold for this one.

This is a minimal fix to make it easier to backport.
A more robust cleanup of the base.EnvForDir function will be sent in a
subsequent CL.

Fixes #38082
Updates #38077

Change-Id: I731d5bbd0e516642c2cf43e713eeea15402604e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/225577
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
(cherry picked from commit bfb1342)
Reviewed-on: https://go-review.googlesource.com/c/go/+/225660

Author: Bryan C. Mills

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

@@ -22,6 +22,7 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
 	"cmd/go/internal/work"
 )
 
@@ -438,7 +439,7 @@ func (g *Generator) exec(words []string) {
 	cmd.Stderr = os.Stderr
 	// Run the command in the package directory.
 	cmd.Dir = g.dir
-	cmd.Env = append(cfg.OrigEnv, g.env...)
+	cmd.Env = str.StringList(cfg.OrigEnv, g.env)
 	err := cmd.Run()
 	if err != nil {
 		g.errorf("running %q: %s", words[0], err)

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

@@ -1142,7 +1142,7 @@ func (c *runCache) builderRunTest(b *work.Builder, a *work.Action) error {
 
 	cmd := exec.Command(args[0], args[1:]...)
 	cmd.Dir = a.Package.Dir
-	cmd.Env = base.EnvForDir(cmd.Dir, cfg.OrigEnv)
+	cmd.Env = base.EnvForDir(cmd.Dir, cfg.OrigEnv[:len(cfg.OrigEnv):len(cfg.OrigEnv)])
 	cmd.Stdout = stdout
 	cmd.Stderr = stdout