cmd/doc: fix -http on Windows

On Windows, GOMODCACHE almost never starts with a slash, and
"go doc -http" constructs a GOPROXY URL by doing "file://" + GOMODCACHE,
resulting in an invalid file URI.

For example, if GOMODCACHE is "C:\foo", then the file URI should be
"file:///C:/foo", but it becomes "file://C:/foo" instead, where "C:" is
understood as a host name, not a drive letter.

Fixes #74137.

Change-Id: I23e776e0f649a0062e01d1a4a6ea8268ba467331
Reviewed-on: https://go-review.googlesource.com/c/go/+/684575
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>

src/cmd/go/alldocs.go

@@ -405,6 +405,8 @@
 //
 //	go doc
 //		Show documentation for current package.
+//	go doc -http
+//		Serve HTML documentation over HTTP for the current package.
 //	go doc Foo
 //		Show documentation for Foo in the current package.
 //		(Foo starts with a capital letter so it cannot match
@@ -447,6 +449,8 @@
 //			Treat a command (package main) like a regular package.
 //			Otherwise package main's exported symbols are hidden
 //			when showing the package's top-level documentation.
+//	  	-http
+//			Serve HTML docs over HTTP.
 //		-short
 //			One-line representation for each symbol.
 //		-src

src/cmd/go/internal/doc/doc.go

@@ -75,6 +75,8 @@ different cases. If this occurs, documentation for all matches is printed.
 Examples:
 	go doc
 		Show documentation for current package.
+	go doc -http
+		Serve HTML documentation over HTTP for the current package.
 	go doc Foo
 		Show documentation for Foo in the current package.
 		(Foo starts with a capital letter so it cannot match
@@ -116,6 +118,8 @@ Flags:
 		Treat a command (package main) like a regular package.
 		Otherwise package main's exported symbols are hidden
 		when showing the package's top-level documentation.
+  	-http
+		Serve HTML docs over HTTP.
 	-short
 		One-line representation for each symbol.
 	-src

src/cmd/internal/doc/main.go

@@ -227,8 +227,16 @@ func doPkgsite(urlPath string) error {
 	fields := strings.Fields(vars)
 	if err == nil && len(fields) == 2 {
 		goproxy, gomodcache := fields[0], fields[1]
-		goproxy = "file://" + filepath.Join(gomodcache, "cache", "download") + "," + goproxy
+		gomodcache = filepath.Join(gomodcache, "cache", "download")
-		env = append(env, "GOPROXY="+goproxy)
+		// Convert absolute path to file URL. pkgsite will not accept
+		// Windows absolute paths because they look like a host:path remote.
+		// TODO(golang.org/issue/32456): use url.FromFilePath when implemented.
+		if strings.HasPrefix(gomodcache, "/") {
+			gomodcache = "file://" + gomodcache
+		} else {
+			gomodcache = "file:///" + filepath.ToSlash(gomodcache)
+		}
+		env = append(env, "GOPROXY="+gomodcache+","+goproxy)
 	}
 
 	const version = "v0.0.0-20250608123103-82c52f1754cd"