xar: fix Linux build on staging-next

  #   # …

  #   rm -r ../pkgs/by-name/xa/xar/patches

  #   git format-patch --zero-commit --output-directory ../pkgs/by-name/xa/xar/patches main

  patches = lib.filesystem.listFilesRecursive ./patches;

  patches =

    # Avoid Darwin rebuilds on staging-next

    lib.filter (

      p: stdenv.hostPlatform.isDarwin -> baseNameOf p != "0020-Fall-back-to-readlink-on-Linux.patch"

    ) (lib.filesystem.listFilesRecursive ./patches);

  # We do not use or modify files outside of the xar subdirectory.

  patchFlags = [ "-p2" ];

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001

From: Randy Eckenrode <randy@largeandhighquality.com>

Date: Wed, 30 Oct 2024 20:19:20 -0400

Subject: [PATCH 20/20] Fall back to readlink on Linux

---

 xar/lib/archive.c | 21 +++++++++++++++++++++

 1 file changed, 21 insertions(+)

diff --git a/xar/lib/archive.c b/xar/lib/archive.c

index b7f9cbf..3a79c74 100644

--- a/xar/lib/archive.c

+++ b/xar/lib/archive.c

@@ -507,10 +507,31 @@ xar_t xar_fdopen_digest_verify(int fd, int32_t flags, void *expected_toc_digest,

 	// If there are hardlinks, the path we pick is the most recently opened by

 	// the filesystem; which is effectively random.

 	char path_buff[PATH_MAX];

+#if defined(__APPLE__)

 	if (fcntl(fd, F_GETPATH, path_buff) < 0) {

 		close(fd);

 		return NULL;

 	}

+#elif defined(__linux__)

+	// Linux has to get the path to the file via `/proc`.

+	char proc_buff[PATH_MAX];

+	if (snprintf(proc_buff, PATH_MAX, "/proc/self/fd/%i", fd) < 0) {

+		close(fd);

+		return NULL;

+	}

+	if (readlink(proc_buff, &path_buff, PATH_MAX) < 0) {

+		close(fd);

+		return NULL;

+	}

+	// The filename is the file’s when the fd was created. Check to make sure it still exists.

+	struct stat stat_buff;

+	if (stat(path_buff, &stat_buff) < 0) {

+		close(fd);

+		return NULL;

+	}

+#else

+#error "Unknown platform. Please update with an implementation of `F_GETPATH`."

+#endif

 	// Don't trust the position of the descriptor we were given, reset it back to 0

 	if (lseek(fd, 0, SEEK_SET) != 0) {

-- 

2.47.0