Use linkat via /proc/self/fd with AT_SYMLINK_FOLLOW to create a
hardlink of the test binary instead of copying it. This avoids
copying ~50MB+ binaries into each test's temp directory, making
test setup faster and reducing disk I/O.
The simpler os.Link(b.Path, ret.Path) can't be used here because
the source binary lives in the first test's TempDir, which may be
cleaned up before later tests call CopyTo. The open FD keeps the
inode alive after the path is deleted, but os.Link needs a valid
path. (See also b9f468240f which tried os.Link but is racy for
this reason.)
The /proc/self/fd approach works without elevated privileges,
unlike AT_EMPTY_PATH which requires CAP_DAC_READ_SEARCH. If the
linkat fails for any reason (e.g. cross-filesystem temp dirs), it
falls back to the existing full-copy path.
Fixes#19397
Change-Id: I4b1f97f7e63a9ae9e09dce36dfbdd1f6cff92320
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
