godoc: fix source links for cmd/*

Fixes golang/go#12159

Change-Id: Id498443171cbdf0fe9808745a79edc863d836db1
Reviewed-on: https://go-review.googlesource.com/14001
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/godoc/godoc_test.go

@@ -266,6 +266,18 @@ func testWeb(t *testing.T, withIndex bool) {
 			},
 			needIndex: true,
 		},
+		{
+			path: "/pkg/strings/",
+			match: []string{
+				`href="/src/strings/strings.go"`,
+			},
+		},
+		{
+			path: "/cmd/compile/internal/amd64/",
+			match: []string{
+				`href="/src/cmd/compile/internal/amd64/reg.go"`,
+			},
+		},
 	}
 	for _, test := range tests {
 		if test.needIndex && !withIndex {

godoc/pres.go

@@ -119,12 +119,13 @@ func NewPresentation(c *Corpus) *Presentation {
 		p:       p,
 		c:       c,
 		pattern: "/cmd/",
-		fsRoot:  "/src/cmd",
+		fsRoot:  "/src",
 	}
 	p.pkgHandler = handlerServer{
 		p:           p,
 		c:           c,
 		pattern:     "/pkg/",
+		stripPrefix: "pkg/",
 		fsRoot:      "/src",
 		exclude:     []string{"/src/cmd"},
 	}

godoc/server.go

@@ -37,6 +37,7 @@ type handlerServer struct {
 	p           *Presentation
 	c           *Corpus  // copy of p.Corpus
 	pattern     string   // url pattern; e.g. "/pkg/"
+	stripPrefix string   // prefix to strip from import path; e.g. "pkg/"
 	fsRoot      string   // file system root to which the pattern is mapped; e.g. "/src"
 	exclude     []string // file system paths to exclude; e.g. "/src/cmd"
 }
@@ -235,7 +236,7 @@ func (h *handlerServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	relpath := pathpkg.Clean(r.URL.Path[len(h.pattern):])
+	relpath := pathpkg.Clean(r.URL.Path[len(h.stripPrefix)+1:])
 	abspath := pathpkg.Join(h.fsRoot, relpath)
 	mode := h.p.GetPageInfoMode(r)
 	if relpath == builtinPkgPath {