Implement Missing Command Line Quoting on Windows

Author: gmittert

Foundation/Process.swift

@@ -102,6 +102,62 @@ private func processIsEqual(_ a : UnsafeRawPointer?, _ b : UnsafeRawPointer?) ->
     return true
 }
 
+#if os(Windows)
+private func quoteWindowsCommandArg(arg: String) -> String {
+    // Windows escaping, adapted from Daniel Colascione's "Everyone quotes
+    // command line arguments the wrong way" - Microsoft Developer Blog
+    if !arg.contains(where: {" \t\n\"".contains($0)}) {
+        return arg;
+    }
+
+    // To escape the command line, we surround the argument with quotes. However
+    // the complication comes due to how the Windows command line parser treats
+    // backslashes (\) and quotes (")
+    //
+    // - \ is normally treated as a literal backslash
+    //     - e.g. foo\bar\baz => foo\bar\baz
+    // - However, the sequence \" is treated as a literal "
+    //     - e.g. foo\"bar => foo"bar
+    //
+    // But then what if we are given a path that ends with a \? Surrounding
+    // foo\bar\ with " would be "foo\bar\" which would be an unterminated string
+    // since it ends on a literal quote. To allow this case the parser treats:
+    //
+    // - \\" as \ followed by the " metachar
+    // - \\\" as \ followed by a literal "
+    // - In general:
+    //     - 2n \ followed by " => n \ followed by the " metachar
+    //     - 2n+1 \ followed by " => n \ followed by a literal "
+    var quoted = "\"";
+    var unquoted = arg.unicodeScalars
+
+    while !unquoted.isEmpty {
+        guard let firstNonBackslash = unquoted.firstIndex(where: { $0 != "\\" }) else {
+            // String ends with a backslash e.g. foo\bar\, escape all the backslashes
+            // then add the metachar " below
+            let backslashCount = unquoted.count
+            quoted.append(String(repeating: "\\", count: backslashCount * 2))
+            break
+        }
+        let backslashCount = unquoted.distance(from: unquoted.startIndex, to: firstNonBackslash)
+        if (unquoted[firstNonBackslash] == "\"") {
+            // This is  a string of \ followed by a " e.g. foo\"bar. Escape the
+            // backslashes and the quote
+            quoted.append(String(repeating: "\\", count: backslashCount * 2 + 1))
+            quoted.append(String(unquoted[firstNonBackslash]))
+        } else {
+            // These are just literal backslashes
+            quoted.append(String(repeating: "\\", count: backslashCount))
+            quoted.append(String(unquoted[firstNonBackslash]))
+        }
+        // Drop the backslashes and the following character
+        unquoted.removeFirst(backslashCount + 1)
+    }
+    quoted.append("\"");
+    return quoted;
+}
+#endif
+
 open class Process: NSObject {
     private static func setup() {
         struct Once {
@@ -318,12 +374,11 @@ open class Process: NSObject {
         }
 
 #if os(Windows)
-        // TODO(compnerd) quote the commandline correctly
-        // https://blogs.msdn.microsoft.com/twistylittlepassagesallalike/2011/04/23/everyone-quotes-command-line-arguments-the-wrong-way/
         var command: [String] = [launchPath]
         if let arguments = self.arguments {
           command.append(contentsOf: arguments)
         }
+        command = command.map(quoteWindowsCommandArg)
 
         var siStartupInfo: STARTUPINFOW = STARTUPINFOW()
         siStartupInfo.cb = DWORD(MemoryLayout<STARTUPINFOW>.size)