gopls/internal/server: add "View package documentation" code action

This CL adds a "View package documentation" code action to any
selection in a Go source file. Its command has two effects:

1) to start a web server associated with the current LSP
   server, with coupled lifetimes, and register a handler
   to serve the package documentation; and

2) to direct the client to open a browser page
   showing the package documentation for the selection.

The server is a minimal imitation of pkg.go.dev,
working off the cache.Package representation. This means
we can display doc markup even for unsaved editor buffers,
and show type-derived information such as method sets
of each type.

Clicking through to the source--which on pkg.go.dev goes
to cs.opensource.google--causes the client editor to
navigate directly to the source file, using the magic
of showDocument requests.

The web server is secure in that all its endpoint URLs contain
an unguessable secret, so a local process belonging to
a different user can't access source code by port scanning.

The CL includes a test that
(a) the webserver content reflects edits (even unsaved)
    in the source buffers, and
(b) that "clicking" through the source link causes
    the server to navigate the client editor to the
    correct source location.

A couple of tests that asserted "no code actions" needed
to be tweaked, as now there is always at least one
code action in any Go source file.

Change-Id: I2fe1f97e4e2b0b15cff6a4feb66501fce349b97d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/571215
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

gopls/doc/commands.md

@@ -222,6 +222,30 @@ Args:
 }
 ```
 
+### **View package documentation.**
+Identifier: `gopls.doc`
+
+Opens the Go package documentation page for the current
+package in a browser.
+
+Args:
+
+```
+{
+	"uri": string,
+	"range": {
+		"start": {
+			"line": uint32,
+			"character": uint32,
+		},
+		"end": {
+			"line": uint32,
+			"character": uint32,
+		},
+	},
+}
+```
+
 ### **Run go mod edit -go=version**
 Identifier: `gopls.edit_go_directive`
 

gopls/internal/golang/codeaction.go

@@ -93,7 +93,8 @@ func CodeActions(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle,
 	// Code actions requiring type information.
 	if want[protocol.RefactorRewrite] ||
 		want[protocol.RefactorInline] ||
-		want[protocol.GoTest] {
+		want[protocol.GoTest] ||
+		want[protocol.GoDoc] {
 		pkg, pgf, err := NarrowestPackageForFile(ctx, snapshot, fh.URI())
 		if err != nil {
 			return nil, err
@@ -121,6 +122,19 @@ func CodeActions(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle,
 			}
 			actions = append(actions, fixes...)
 		}
+
+		if want[protocol.GoDoc] {
+			loc := protocol.Location{URI: pgf.URI, Range: rng}
+			cmd, err := command.NewDocCommand("View package documentation", loc)
+			if err != nil {
+				return nil, err
+			}
+			actions = append(actions, protocol.CodeAction{
+				Title:   cmd.Title,
+				Kind:    protocol.GoDoc,
+				Command: &cmd,
+			})
+		}
 	}
 	return actions, nil
 }