caddycmd: Implement fix for performing cleanup on Windows

Without this, the cleanup operation would fail with an error message like this:

upgrade: download succeeded, but unable to clean up backup binary: remove C:\caddy\caddy.exe.tmp: Access is denied.

cmd/packagesfuncs.go

@@ -162,7 +162,7 @@ func upgradeBuild(pluginPkgs map[string]struct{}, fl Flags) (int, error) {
 
 	// clean up the backup file
 	if !fl.Bool("skip-cleanup") {
-		if err = os.Remove(backupExecPath); err != nil {
+		if err = removeCaddyBinary(backupExecPath); err != nil {
 			return caddy.ExitCodeFailedStartup, fmt.Errorf("download succeeded, but unable to clean up backup binary: %v", err)
 		}
 	} else {

cmd/removebinary_nonwindows.go

@@ -0,0 +1,30 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !windows
+// +build !windows
+
+package caddycmd
+
+import (
+	"os"
+)
+
+// removeCaddyBinary removes the Caddy binary at the given path.
+//
+// On any non-Windows OS, this simply calls os.Remove, since they should
+// probably not exhibit any issue with processes deleting themselves.
+func removeCaddyBinary(path string) error {
+	return os.Remove(path)
+}

cmd/removebinary_windows.go

@@ -0,0 +1,39 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build windows
+// +build windows
+
+package caddycmd
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+// removeCaddyBinary removes the Caddy binary at the given path.
+//
+// On Windows, this uses a syscall to indirectly remove the file,
+// because otherwise we get an "Access is denied." error when trying
+// to delete the binary while Caddy is still running and performing
+// the upgrade. "cmd.exe /C" executes a command specified by the
+// following arguments, i.e. "del" which will run as a separate process,
+// which avoids the "Access is denied." error.
+func removeCaddyBinary(path string) error {
+	var sI syscall.StartupInfo
+	var pI syscall.ProcessInformation
+	argv := syscall.StringToUTF16Ptr(filepath.Join(os.Getenv("windir"), "system32", "cmd.exe") + " /C del " + path)
+	return syscall.CreateProcess(nil, argv, nil, nil, true, 0, nil, nil, &sI, &pI)
+}