internal/localdatasource: prevent panic for std modules

Previously when using the frontend with the local file system data source,
a panic would occur when trying to view docs for stdlib modules. Now the
panic is replaced by returning error 'not found' for missing stdlib
modules.

For golang/go#44704

Change-Id: I584e1663823c494d090237f80878a679ce8ce82f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/297829
Reviewed-by: Jonathan Amsterdam <[email protected]>
Run-TryBot: Julie Qiu <[email protected]>
Trust: Julie Qiu <[email protected]>

Author: gabriel-vasile

internal/localdatasource/datasource.go

@@ -98,7 +98,7 @@ func getFullPath(modulePath string) string {
 }
 
 // GetUnit returns information about a unit. Both the module path and package
-// path must both be known.
+// path must be known.
 func (ds *DataSource) GetUnit(ctx context.Context, pathInfo *internal.UnitMeta, fields internal.FieldSet) (_ *internal.Unit, err error) {
 	defer derrors.Wrap(&err, "GetUnit(%q, %q)", pathInfo.Path, pathInfo.ModulePath)
 
@@ -135,6 +135,9 @@ func (ds *DataSource) GetUnitMeta(ctx context.Context, path, requestedModulePath
 	ds.mu.Lock()
 	module := ds.loadedModules[requestedModulePath]
 	ds.mu.Unlock()
+	if module == nil {
+		return nil, fmt.Errorf("%s not loaded: %w", requestedModulePath, derrors.NotFound)
+	}
 
 	um := &internal.UnitMeta{
 		Path: path,

internal/localdatasource/datasource_test.go

@@ -18,6 +18,7 @@ import (
 	"golang.org/x/pkgsite/internal/fetch"
 	"golang.org/x/pkgsite/internal/godoc/dochtml"
 	"golang.org/x/pkgsite/internal/licenses"
+	"golang.org/x/pkgsite/internal/stdlib"
 	"golang.org/x/pkgsite/internal/testing/testhelper"
 )
 
@@ -175,6 +176,11 @@ func TestGetUnitMeta(t *testing.T) {
 			modulePath: internal.UnknownModulePath,
 			wantErr:    derrors.NotFound,
 		},
+		{
+			path:       "net/http",
+			modulePath: stdlib.ModulePath,
+			wantErr:    derrors.NotFound,
+		},
 	} {
 		t.Run(test.path, func(t *testing.T) {
 			got, err := ds.GetUnitMeta(ctx, test.path, test.modulePath, fetch.LocalVersion)