Fix CLOSE_RANGE_CLOEXEC build error on Alpine

[Copilot]

Alpine Linux builds fail because musl libc headers don't define CLOSE_RANGE_CLOEXEC, despite the syscall being available:

error: 'CLOSE_RANGE_CLOEXEC' was not declared in this scope
  135 |   if (syscall(SYS_close_range, 3, ~0U, CLOSE_RANGE_CLOEXEC) == 0) {

Changes

• src/unix/pty.cc: Modified the conditional compilation to check for both SYS_close_range and CLOSE_RANGE_CLOEXEC before using the close_range() syscall

  #if defined(SYS_close_range) && defined(CLOSE_RANGE_CLOEXEC)

When CLOSE_RANGE_CLOEXEC is not defined (as on Alpine/musl), the code gracefully skips the close_range() optimization and falls back to the existing loop-based approach for setting close-on-exec flags.

Testing

• ✅ Code compiles successfully on Ubuntu 24.04
• ✅ All 20 tests pass
• ✅ Linting passes without errors

The changes are minimal and surgical, ensuring compatibility with Alpine Linux while maintaining the optimization on systems where it's available.

Original prompt

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This section details on the original issue you should resolve

<issue_title>CLOEXEC does not work on Alpine</issue_title>
<issue_description>In the VS Code repo, when I bump node-pty, the Alpine build reports the following error

2026-01-13T21:31:14.7685867Z npm error ../src/unix/pty.cc: In function 'void pty_close_inherited_fds()':
2026-01-13T21:31:14.7686207Z npm error ../src/unix/pty.cc:135:40: error: 'CLOSE_RANGE_CLOEXEC' was not declared in this scope
2026-01-13T21:31:14.7686533Z npm error   135 |   if (syscall(SYS_close_range, 3, ~0U, CLOSE_RANGE_CLOEXEC) == 0) {
2026-01-13T21:31:14.7686816Z npm error       |                                        ^~~~~~~~~~~~~~~~~~~
2026-01-13T21:31:14.7687112Z npm error make: *** [pty.target.mk:115: Release/obj.target/pty/src/unix/pty.o] Error 1
2026-01-13T21:31:14.7687392Z npm error gyp ERR! build error 
2026-01-13T21:31:14.7687660Z npm error gyp ERR! stack Error: `make` failed with exit code: 2

Look in pty.cc and see whether we have to use a different constant or implementation for Alpine.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes CLOEXEC does not work on Alpine #872

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[rzhao271]

@copilot do not declare a value for the constant. Instead, add to the #if so that if the constant is not defined, skip that block.

[Copilot]

@copilot do not declare a value for the constant. Instead, add to the #if so that if the constant is not defined, skip that block.

Changed the conditional to check for both SYS_close_range and CLOSE_RANGE_CLOEXEC. When the constant is not defined, the close_range() block is skipped and the fallback loop is used instead. (fd2eea6)