windows: convert TestCommandLineRecomposition to a fuzz test and fix discrepancies

Notably, this fixes the escaping of the first argument when it
contains quoted spaces, and fixes a panic in DecomposeCommandLine
when it contains more than 8192 arguments.

Fixes golang/go#58817.
For golang/go#17149.
For golang/go#63236.

Change-Id: Ib72913b8182998adc1420d73ee0f9dc017dfbf32
Reviewed-on: https://go-review.googlesource.com/c/sys/+/530275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

windows/exec_windows.go

@@ -22,7 +22,7 @@ import (
 //     but only if there is space or tab inside s.
 func EscapeArg(s string) string {
 	if len(s) == 0 {
-		return "\"\""
+		return `""`
 	}
 	n := len(s)
 	hasSpace := false
@@ -35,7 +35,7 @@ func EscapeArg(s string) string {
 		}
 	}
 	if hasSpace {
-		n += 2
+		n += 2 // Reserve space for quotes.
 	}
 	if n == len(s) {
 		return s
@@ -82,20 +82,68 @@ func EscapeArg(s string) string {
 // in CreateProcess's CommandLine argument, CreateService/ChangeServiceConfig's BinaryPathName argument,
 // or any program that uses CommandLineToArgv.
 func ComposeCommandLine(args []string) string {
-	var commandLine string
-	for i := range args {
-		if i > 0 {
-			commandLine += " "
-		}
-		commandLine += EscapeArg(args[i])
+	if len(args) == 0 {
+		return ""
 	}
-	return commandLine
+
+	// Per https://learn.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-commandlinetoargvw:
+	// “This function accepts command lines that contain a program name; the
+	// program name can be enclosed in quotation marks or not.”
+	//
+	// Unfortunately, it provides no means of escaping interior quotation marks
+	// within that program name, and we have no way to report them here.
+	prog := args[0]
+	mustQuote := len(prog) == 0
+	for i := 0; i < len(prog); i++ {
+		c := prog[i]
+		if c <= ' ' || (c == '"' && i == 0) {
+			// Force quotes for not only the ASCII space and tab as described in the
+			// MSDN article, but also ASCII control characters.
+			// The documentation for CommandLineToArgvW doesn't say what happens when
+			// the first argument is not a valid program name, but it empirically
+			// seems to drop unquoted control characters.
+			mustQuote = true
+			break
+		}
+	}
+	var commandLine []byte
+	if mustQuote {
+		commandLine = make([]byte, 0, len(prog)+2)
+		commandLine = append(commandLine, '"')
+		for i := 0; i < len(prog); i++ {
+			c := prog[i]
+			if c == '"' {
+				// This quote would interfere with our surrounding quotes.
+				// We have no way to report an error, so just strip out
+				// the offending character instead.
+				continue
+			}
+			commandLine = append(commandLine, c)
+		}
+		commandLine = append(commandLine, '"')
+	} else {
+		if len(args) == 1 {
+			// args[0] is a valid command line representing itself.
+			// No need to allocate a new slice or string for it.
+			return prog
+		}
+		commandLine = []byte(prog)
+	}
+
+	for _, arg := range args[1:] {
+		commandLine = append(commandLine, ' ')
+		// TODO(bcmills): since we're already appending to a slice, it would be nice
+		// to avoid the intermediate allocations of EscapeArg.
+		// Perhaps we can factor out an appendEscapedArg function.
+		commandLine = append(commandLine, EscapeArg(arg)...)
+	}
+	return string(commandLine)
 }
 
 // DecomposeCommandLine breaks apart its argument command line into unescaped parts using CommandLineToArgv,
 // as gathered from GetCommandLine, QUERY_SERVICE_CONFIG's BinaryPathName argument, or elsewhere that
 // command lines are passed around.
-// DecomposeCommandLine returns error if commandLine contains NUL.
+// DecomposeCommandLine returns an error if commandLine contains NUL.
 func DecomposeCommandLine(commandLine string) ([]string, error) {
 	if len(commandLine) == 0 {
 		return []string{}, nil
@@ -105,14 +153,18 @@ func DecomposeCommandLine(commandLine string) ([]string, error) {
 		return nil, errorspkg.New("string with NUL passed to DecomposeCommandLine")
 	}
 	var argc int32
-	argv, err := CommandLineToArgv(&utf16CommandLine[0], &argc)
+	argv8192, err := CommandLineToArgv(&utf16CommandLine[0], &argc)
 	if err != nil {
 		return nil, err
 	}
-	defer LocalFree(Handle(unsafe.Pointer(argv)))
+	defer LocalFree(Handle(unsafe.Pointer(argv8192)))
+
 	var args []string
-	for _, v := range (*argv)[:argc] {
-		args = append(args, UTF16ToString((*v)[:]))
+	// Note: CommandLineToArgv hard-codes an incorrect return type
+	// (see https://go.dev/issue/63236).
+	// We use an unsafe.Pointer conversion here to work around it.
+	for _, p := range unsafe.Slice((**uint16)(unsafe.Pointer(argv8192)), argc) {
+		args = append(args, UTF16PtrToString(p))
 	}
 	return args, nil
 }