cmd/golangorg: fix package link URLs

Doc comment text was linking to / instead of /pkg,
which was creating dead links. Fix that.

All other doc links were using /pkg/cmd/ instead of /cmd/.
And we were serving everything in cmd in both places.
Fix the links, and redirect /pkg/cmd/ to /cmd/.

Change-Id: Ib5c205b3faf53aff0f0bb6d475fa942a6718e616
Reviewed-on: https://go-review.googlesource.com/c/website/+/596436
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

cmd/golangorg/testdata/godev.txt

@@ -123,3 +123,14 @@ body contains <i>This content is part of the <a href="/wiki/">Go Wiki</a>.</i>
 GET https://go.dev/wiki/Comments
 body contains Go Wiki: Comments
 body contains <i>This content is part of the <a href="/wiki/">Go Wiki</a>.</i>
+
+GET https://go.dev/pkg/cmd/go/?m=old
+redirect == /cmd/go/?m=old
+
+GET https://go.dev/pkg/slices/?m=old
+body !contains href="/cmp
+body contains href="/pkg/cmp/?m=old#Compare
+
+GET https://go.dev/cmd/link/internal/ld/?m=old
+body !contains href="/pkg/cmd
+body contains href="/cmd/link/internal/loader/?m=old#Loader

internal/pkgdoc/doc.go

@@ -520,10 +520,15 @@ func (p *Page) ModeQuery() string {
 }
 
 func maybeRedirect(w http.ResponseWriter, r *http.Request) (redirected bool) {
+	r.URL.Host = ""
+	r.URL.Scheme = ""
 	canonical := path.Clean(r.URL.Path)
 	if !strings.HasSuffix(canonical, "/") {
 		canonical += "/"
 	}
+	if strings.HasPrefix(canonical, "/pkg/cmd/") {
+		canonical = canonical[len("/pkg"):]
+	}
 	if r.URL.Path != canonical {
 		url := *r.URL
 		url.Path = canonical

internal/pkgdoc/funcs.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/doc"
+	"go/doc/comment"
 	"go/format"
 	"go/printer"
 	"go/token"
@@ -154,8 +155,23 @@ func firstIdent(x []byte) string {
 }
 
 // Comment formats the given documentation comment as HTML.
-func (p *Page) Comment(comment string) template.HTML {
-	return template.HTML(p.PDoc.HTML(comment))
+func (p *Page) Comment(text string) template.HTML {
+	pr := p.PDoc.Printer()
+	pr.DocLinkURL = func(link *comment.DocLink) string {
+		url := link.DefaultURL("/pkg/")
+		if strings.HasPrefix(url, "/pkg/cmd/") {
+			url = url[len("/pkg"):]
+		}
+		if p.OldDocs {
+			if base, frag, ok := strings.Cut(url, "#"); ok {
+				url = base + "?m=old#" + frag
+			} else {
+				url += "?m=old"
+			}
+		}
+		return url
+	}
+	return template.HTML(pr.HTML(p.PDoc.Parser().Parse(text)))
 }
 
 // sanitize sanitizes the argument src by replacing newlines with

internal/texthtml/ast.go

@@ -11,6 +11,7 @@ import (
 	"go/doc"
 	"go/token"
 	"strconv"
+	"strings"
 	"unicode"
 	"unicode/utf8"
 )
@@ -24,13 +25,17 @@ type goLink struct {
 }
 
 func (l *goLink) tags() (start, end string) {
+	prefix := "/pkg/"
+	if strings.HasPrefix(l.path, "cmd/") {
+		prefix = "/"
+	}
 	switch {
 	case l.path != "" && l.name == "":
 		// package path
-		return `<a href="/pkg/` + l.path + `/` + l.docSuffix() + `">`, `</a>`
+		return `<a href="` + prefix + l.path + `/` + l.docSuffix() + `">`, `</a>`
 	case l.path != "" && l.name != "":
 		// qualified identifier
-		return `<a href="/pkg/` + l.path + `/` + l.docSuffix() + `#` + l.name + `">`, `</a>`
+		return `<a href="` + prefix + l.path + `/` + l.docSuffix() + `#` + l.name + `">`, `</a>`
 	case l.path == "" && l.name != "":
 		// local identifier
 		if l.isVal {