Windows: Use absolute and shortened path for GOROOT environment variable

go/tools/builders/BUILD.bazel

@@ -142,7 +142,10 @@ go_tool_binary(
         "flags.go",
         "replicate.go",
         "stdlib.go",
-    ],
+    ] + select({
+        "@bazel_tools//src/conditions:windows": ["path_windows.go"],
+        "//conditions:default": ["path.go"],
+    }),
     visibility = ["//visibility:public"],
 )
 

go/tools/builders/link.go

@@ -155,7 +155,7 @@ func buildImportcfgFile(archives []archive, packageList, installSuffix, dir stri
 		if line == "" {
 			continue
 		}
-		fmt.Fprintf(buf, "packagefile %s=%s/%s.a\n", line, prefix, filepath.FromSlash(line))
+		fmt.Fprintf(buf, "packagefile %s=%s.a\n", line, filepath.Join(prefix, filepath.FromSlash(line)))
 	}
 	if err := scanner.Err(); err != nil {
 		return "", err

go/tools/builders/path.go

@@ -0,0 +1,7 @@
+// +build !windows
+
+package main
+
+func processPath(path string) (string, error) {
+	return path, nil
+}

go/tools/builders/path_windows.go

@@ -0,0 +1,25 @@
+// +build windows
+
+package main
+
+import (
+	"runtime"
+	"syscall"
+)
+
+func processPath(path string) (string, error) {
+	if runtime.GOOS != "windows" {
+		return path, nil
+	}
+
+	var buf [258]uint16
+	up, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return path, err
+	}
+	_, err = syscall.GetShortPathName(up, &buf[0], 258)
+	if err != nil {
+		return path, err
+	}
+	return syscall.UTF16ToString(buf[:]), nil
+}

go/tools/builders/stdlib.go

@@ -55,6 +55,11 @@ func run(args []string) error {
 		return err
 	}
 
+	output, err = processPath(output)
+	if err != nil {
+		return err
+	}
+
 	// Now switch to the newly created GOROOT
 	os.Setenv("GOROOT", output)