godoc: update to use new goroot finding logic

The logic to determine whether a filesystem root was in GOROOT or GOPATH
still relied on runtime.GOROOT(), whereas cmd/godoc was updated to copy
the goroot finding logic from standard library.

Hence, godoc is unable to determine if a filesystem is in GOROOT or not
when the binary is outside runtime.GOROOT(). So we expose a new variable
and set that from cmd/godoc to avoid copying the logic again for the 3rd time.

Fixes golang/go#27162

Change-Id: I160dcdbdd262e671f09f7bf01c329be5eac280ad
Reviewed-on: https://go-review.googlesource.com/130796
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

cmd/godoc/main.go

@@ -183,6 +183,9 @@ func main() {
 		usage()
 	}
 
+	// Setting the resolved goroot.
+	vfs.GOROOT = *goroot
+
 	var fsGate chan bool
 	fsGate = make(chan bool, 20)
 

godoc/vfs/os.go

@@ -14,6 +14,14 @@ import (
 	"runtime"
 )
 
+// GOROOT returns the GOROOT path under which the godoc binary is running.
+// It is needed to check whether a filesystem root is under GOROOT or not.
+// This is set from cmd/godoc/main.go
+
+// We expose a new variable because otherwise we need to copy the findGOROOT logic again
+// from cmd/godoc which is already copied twice from the standard library.
+var GOROOT = runtime.GOROOT()
+
 // OS returns an implementation of FileSystem reading from the
 // tree rooted at root.  Recording a root is convenient everywhere
 // but necessary on Windows, because the slash-separated path
@@ -22,7 +30,7 @@ import (
 func OS(root string) FileSystem {
 	var t RootType
 	switch {
-	case root == runtime.GOROOT():
+	case root == GOROOT:
 		t = RootTypeGoRoot
 	case isGoPath(root):
 		t = RootTypeGoPath

godoc/vfs/zipfs/zipfs.go

@@ -25,7 +25,6 @@ import (
 	"os"
 	"path"
 	"path/filepath"
-	"runtime"
 	"sort"
 	"strings"
 	"time"
@@ -87,7 +86,7 @@ func (fs *zipFS) String() string {
 func (fs *zipFS) RootType(abspath string) vfs.RootType {
 	var t vfs.RootType
 	switch {
-	case abspath == runtime.GOROOT():
+	case abspath == vfs.GOROOT:
 		t = vfs.RootTypeGoRoot
 	case isGoPath(abspath):
 		t = vfs.RootTypeGoPath