net/http/cgi: replace constant map with switch statement

The switch statement can be statically optimized by the compiler,
whereas similarly optimizing the map index expression would require
additional compiler analysis to detect the map is never mutated.

Updates #10848.

Change-Id: I2fc70d4a34dc545677b99f218b51023c7891bbbd
Reviewed-on: https://go-review.googlesource.com/c/go/+/231041
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/cgi/host.go

@@ -32,16 +32,23 @@ import (
 
 var trailingPort = regexp.MustCompile(`:([0-9]+)$`)
 
-var osDefaultInheritEnv = map[string][]string{
-	"darwin":  {"DYLD_LIBRARY_PATH"},
-	"freebsd": {"LD_LIBRARY_PATH"},
-	"hpux":    {"LD_LIBRARY_PATH", "SHLIB_PATH"},
-	"irix":    {"LD_LIBRARY_PATH", "LD_LIBRARYN32_PATH", "LD_LIBRARY64_PATH"},
-	"linux":   {"LD_LIBRARY_PATH"},
-	"openbsd": {"LD_LIBRARY_PATH"},
-	"solaris": {"LD_LIBRARY_PATH", "LD_LIBRARY_PATH_32", "LD_LIBRARY_PATH_64"},
-	"windows": {"SystemRoot", "COMSPEC", "PATHEXT", "WINDIR"},
-}
+var osDefaultInheritEnv = func() []string {
+	switch runtime.GOOS {
+	case "darwin":
+		return []string{"DYLD_LIBRARY_PATH"}
+	case "linux", "freebsd", "openbsd":
+		return []string{"LD_LIBRARY_PATH"}
+	case "hpux":
+		return []string{"LD_LIBRARY_PATH", "SHLIB_PATH"}
+	case "irix":
+		return []string{"LD_LIBRARY_PATH", "LD_LIBRARYN32_PATH", "LD_LIBRARY64_PATH"}
+	case "solaris":
+		return []string{"LD_LIBRARY_PATH", "LD_LIBRARY_PATH_32", "LD_LIBRARY_PATH_64"}
+	case "windows":
+		return []string{"SystemRoot", "COMSPEC", "PATHEXT", "WINDIR"}
+	}
+	return nil
+}()
 
 // Handler runs an executable in a subprocess with a CGI environment.
 type Handler struct {
@@ -183,7 +190,7 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		}
 	}
 
-	for _, e := range osDefaultInheritEnv[runtime.GOOS] {
+	for _, e := range osDefaultInheritEnv {
 		if v := os.Getenv(e); v != "" {
 			env = append(env, e+"="+v)
 		}