clientupdate/distsign: add ability to validate a binary that is already located on disk

Our build system caches files locally and only updates them when something
changes. Since I need to integrate some distsign stuff into the build system
to validate our Windows 7 MSIs, I want to be able to check the cached copy
of a package before downloading a fresh copy from pkgs.

If the signature changes, then obviously the local copy is outdated and we
return an error, at which point we call Download to refresh the package.

Updates https://github.com/tailscale/corp/issues/14334

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

clientupdate/distsign/distsign_test.go

@@ -119,6 +119,124 @@ func TestDownload(t *testing.T) {
 	}
 }
 
+func TestValidateLocalBinary(t *testing.T) {
+	srv := newTestServer(t)
+	c := srv.client(t)
+
+	tests := []struct {
+		desc    string
+		before  func(*testing.T)
+		src     string
+		wantErr bool
+	}{
+		{
+			desc:    "missing file",
+			before:  func(*testing.T) {},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "success",
+			before: func(*testing.T) {
+				srv.addSigned("hello", []byte("world"))
+			},
+			src: "hello",
+		},
+		{
+			desc: "contents changed",
+			before: func(*testing.T) {
+				srv.addSigned("hello", []byte("new world"))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "no signature",
+			before: func(*testing.T) {
+				srv.add("hello", []byte("world"))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "bad signature",
+			before: func(*testing.T) {
+				srv.add("hello", []byte("world"))
+				srv.add("hello.sig", []byte("potato"))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "signed with untrusted key",
+			before: func(t *testing.T) {
+				srv.add("hello", []byte("world"))
+				srv.add("hello.sig", newSigningKeyPair(t).sign([]byte("world")))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "signed with root key",
+			before: func(t *testing.T) {
+				srv.add("hello", []byte("world"))
+				srv.add("hello.sig", ed25519.Sign(srv.roots[0].k, []byte("world")))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+		{
+			desc: "bad signing key signature",
+			before: func(t *testing.T) {
+				srv.add("distsign.pub.sig", []byte("potato"))
+				srv.addSigned("hello", []byte("world"))
+			},
+			src:     "hello",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			srv.reset()
+
+			// First just do a successful Download.
+			want := []byte("world")
+			srv.addSigned("hello", want)
+			dst := filepath.Join(t.TempDir(), tt.src)
+			t.Cleanup(func() {
+				os.Remove(dst)
+			})
+			err := c.Download(context.Background(), tt.src, dst)
+			if err != nil {
+				t.Fatalf("unexpected error from Download(%q): %v", tt.src, err)
+			}
+			got, err := os.ReadFile(dst)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !bytes.Equal(want, got) {
+				t.Errorf("Download(%q): got %q, want %q", tt.src, got, want)
+			}
+
+			// Now we reset srv with the test case and validate against the local dst.
+			srv.reset()
+			tt.before(t)
+
+			err = c.ValidateLocalBinary(tt.src, dst)
+			if err != nil {
+				if tt.wantErr {
+					return
+				}
+				t.Fatalf("unexpected error from ValidateLocalBinary(%q): %v", tt.src, err)
+			}
+			if tt.wantErr {
+				t.Fatalf("ValidateLocalBinary(%q) succeeded, expected an error", tt.src)
+			}
+		})
+	}
+}
+
 func TestRotateRoot(t *testing.T) {
 	srv := newTestServer(t)
 	c1 := srv.client(t)