go.tools/godoc: fix index reading and writing

In command godoc, set IndexEnabled when the -write_index flag is set.
Previously you would need to (unintuitively) set the -http flag to
achieve this.

In package godoc, set up the FS tree before loading the index, and
then return before starting the index refresh loop. Previously the
index would be loaded and then immediately refreshed, negating the
benefits of the on-disk index.

TBR=bradfitz
R=golang-codereviews
CC=golang-codereviews
https://golang.org/cl/103370046

cmd/godoc/main.go

@@ -221,6 +221,7 @@ func main() {
 	corpus.IndexThrottle = *indexThrottle
 	if *writeIndex {
 		corpus.IndexThrottle = 1.0
+		corpus.IndexEnabled = true
 	}
 	if *writeIndex || httpMode || *urlFlag != "" {
 		if err := corpus.Init(); err != nil {

godoc/index.go

@@ -1471,10 +1471,11 @@ func (c *Corpus) UpdateIndex() {
 func (c *Corpus) RunIndexer() {
 	// initialize the index from disk if possible
 	if c.IndexFiles != "" {
+		c.initFSTree()
 		if err := c.readIndex(c.IndexFiles); err != nil {
 			log.Printf("error reading index from file %s: %v", c.IndexFiles, err)
-			return
 		}
+		return
 	}
 
 	// Repeatedly update the package directory tree and index.