Fix backslash escaping in Windows parameter files

[bd-innovation]

Description

Fix backslash escaping in Windows parameter files and preserve empty arguments.

For example, C:\Program Files\SDK\ is currently written with a single backslash before the closing quote. That backslash escapes the quote, so the next argument can become part of the path.

Double backslashes before literal quotes and before an added closing quote. Leave other backslashes unchanged.

Fixes #31202

Validation

• Regression tests fail with the old implementation and pass with the fix.
• bazel test //src/test/java/com/google/devtools/build/lib/util:UtilTests --test_filter=WindowsParamFileEscaperTest passes (ten tests).
• Verified the argument-boundary failure and fix with Clang's --rsp-quoting=windows mode on macOS. No Windows/MSVC end-to-end build was run.
• Google Java Format and git diff --check pass.

Build API Changes

No.

Checklist

• I have added tests for the new use cases.
• I have updated the method documentation.

Release Notes

RELNOTES: Windows parameter files now preserve empty arguments and correctly escape backslashes before quotes and at the end of quoted arguments.

[fmeum]

Nit: Using @TestParameter on count would be more idiomatic.

[bd-innovation]

Done, thanks!

[fmeum]

@bazel-io fork 9.3.0

[meteorcloudy]

Thanks, my agent had two nit comments, I think those makes sense, can you please address them?

[meteorcloudy]

Thanks for the thorough fix and tests — I verified that the new output round-trips correctly through LLVM's Windows tokenizer (clang --rsp-quoting=windows) for backslash runs of 1–4, UNC paths, embedded quotes and trailing backslashes.

One perf consideration: escapeString is on a fairly hot path — ParameterFile.writeParameterFile calls it once per argument, and link/archive param files can contain 100k+ arguments. The previous implementation used String.replace, which is intrinsified and allocation-free when there is nothing to replace; the new one walks every character and grows an unsized StringBuilder (default capacity 16) for every argument, including the overwhelming majority that contain no \ or ".

Could we keep a fast path for those and pre-size the builder otherwise? Semantics are unchanged, since with no backslashes and no quotes the loop is a verbatim copy and the trailing-backslash handling is a no-op:

Suggested change

	public static String escapeString(String argument) {
	boolean needsSurroundingQuotes = containsWhitespace(argument);
	boolean needsSurroundingQuotes = argument.isEmpty() || containsWhitespace(argument);
	StringBuilder out = new StringBuilder();
	public static String escapeString(String argument) {
	if (!argument.isEmpty() && argument.indexOf('\\') < 0 && argument.indexOf('"') < 0) {
	return containsWhitespace(argument) ? "\"" + argument + "\"" : argument;
	}
	boolean needsSurroundingQuotes = argument.isEmpty() || containsWhitespace(argument);
	StringBuilder out = new StringBuilder(argument.length() + 2);

[bd-innovation]

Done, thanks! Addressed both comments.

[meteorcloudy]

Thanks for updating this javadoc. Could you also update the other copy of it that documents the same format for users? ParameterFileType.WINDOWS in ParameterFile.java still describes the old behavior:

/**
 * A parameter file where each parameter is correctly quoted for windows use. Double-quotes are
 * escaped, and each parameter that contains whitespace is surrounded in double-quotes.
 */
WINDOWS,

Something like:

A parameter file where each parameter is correctly quoted for Windows use. Double-quotes are escaped, backslashes immediately preceding a double-quote are doubled, and empty parameters as well as parameters containing whitespace are surrounded in double-quotes.