playground: serve 404 for non-existent pages

The server.handleEdit handler is being registered to handle the "/"
pattern. Its code checks if the request URL path has a "/p/" prefix,
and otherwise assumes it's the index page. There's no check if it's
some other unsupported URL. As a result, any URL that isn't handled
elsewhere serves the index page. For example:

	https://play.golang.org/foobarbaz

This change fixes that so non-existent pages return a 404 Not Found
error instead.

Also use context.Background() instead of a nil context in a test,
per the context package instructions.

Change-Id: I4c43492397a6f71bffc1e6a657ff2a523a245f94
Reviewed-on: https://go-review.googlesource.com/129795
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

edit.go

diff --git a/edit.go b/edit.go
index b69c1c0..81923e4 100644
--- a/edit.go
+++ b/edit.go
@@ -32,6 +32,12 @@
 		return
 	}
 
+	// Serve 404 for /foo.
+	if r.URL.Path != "/" && !strings.HasPrefix(r.URL.Path, "/p/") {
+		http.NotFound(w, r)
+		return
+	}
+
 	snip := &snippet{Body: []byte(hello)}
 	if strings.HasPrefix(r.URL.Path, "/p/") {
 		if !allowShare(r) {

server_test.go

diff --git a/server_test.go b/server_test.go
index 0c17407..8621742 100644
--- a/server_test.go
+++ b/server_test.go
@@ -5,6 +5,7 @@
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"io/ioutil"
 	"net/http"
@@ -42,8 +43,8 @@
 	id := "bar"
 	barBody := []byte("Snippy McSnipface")
 	snip := &snippet{Body: barBody}
-	if err := s.db.PutSnippet(nil, id, snip); err != nil {
-		t.Fatalf("s.dbPutSnippet(nil, %+v, %+v): %v", id, snip, err)
+	if err := s.db.PutSnippet(context.Background(), id, snip); err != nil {
+		t.Fatalf("s.dbPutSnippet(context.Background(), %+v, %+v): %v", id, snip, err)
 	}
 
 	testCases := []struct {
@@ -54,6 +55,7 @@
 		respBody   []byte
 	}{
 		{"foo.play.golang.org to play.golang.org", "https://foo.play.golang.org", http.StatusFound, map[string]string{"Location": "https://play.golang.org"}, nil},
+		{"Non-existent page", "https://play.golang.org/foo", http.StatusNotFound, nil, nil},
 		{"Unknown snippet", "https://play.golang.org/p/foo", http.StatusNotFound, nil, nil},
 		{"Existing snippet", "https://play.golang.org/p/" + id, http.StatusOK, nil, nil},
 		{"Plaintext snippet", "https://play.golang.org/p/" + id + ".go", http.StatusOK, nil, barBody},