After a recent 'repo sync', if I cros_workon start cryptohome, emerging it fails with:
cryptohome-9999:  * Copying sources from /mnt/host/source/src/platform2
cryptohome-9999: rsync: mkdir "/build/link/tmp/portage/chromeos-base/cryptohome-9999/work/cryptohome-9999/platform2" failed: No such file or directory (2)
cryptohome-9999: rsync error: error in file IO (code 11) at main.c(657) [Receiver=3.1.2]
cryptohome-9999:  * ERROR: chromeos-base/cryptohome-9999::chromiumos failed (unpack phase):
cryptohome-9999:  *   rsync -a --exclude= --exclude=*.pyc /mnt/host/source/src/platform2/// /build/link/tmp/portage/chromeos-base/cryptohome-9999/work/cryptohome-9999/platform2///
cryptohome-9999:  * 
cryptohome-9999:  * Call stack:
cryptohome-9999:  *     ebuild.sh, line   93:  Called src_unpack
cryptohome-9999:  *   environment, line 4068:  Called platform_src_unpack
cryptohome-9999:  *   environment, line 3633:  Called cros-workon_src_unpack
cryptohome-9999:  *   environment, line 1138:  Called local_copy '/mnt/host/source/src/platform2' '/build/link/tmp/portage/chromeos-base/cryptohome-9999/work/cryptohome-9999/platform2'
cryptohome-9999:  *   environment, line 2927:  Called local_copy_cp '/mnt/host/source/src/platform2' '/build/link/tmp/portage/chromeos-base/cryptohome-9999/work/cryptohome-9999/platform2'
cryptohome-9999:  *   environment, line 2941:  Called die
cryptohome-9999:  * The specific snippet of code:
cryptohome-9999:  *               rsync -a --safe-links "${blacklist[@]}" "${src}/${sl}/" "${dst}/${sl}/" || die "rsync -a ${blacklist[*]} ${src}/${sl}/ ${dst}/${sl}/";
cryptohome-9999:  * 
cryptohome-9999:  * If you need support, post the output of `emerge --info '=chromeos-base/cryptohome-9999::chromiumos'`,
cryptohome-9999:  * the complete build log and the output of `emerge -pqv '=chromeos-base/cryptohome-9999::chromiumos'`.
cryptohome-9999:  * The complete build log is located at '/build/link/tmp/portage/logs/chromeos-base:cryptohome-9999:20171018-014351.log'.
cryptohome-9999:  * For convenience, a symlink to the build log is located at '/build/link/tmp/portage/chromeos-base/cryptohome-9999/temp/build.log'.
cryptohome-9999:  * The ebuild environment file is located at '/build/link/tmp/portage/chromeos-base/cryptohome-9999/temp/environment'.
cryptohome-9999:  * Working directory: '/build/link/tmp/portage/chromeos-base/cryptohome-9999/work'
cryptohome-9999:  * S: '/build/link/tmp/portage/chromeos-base/cryptohome-9999/work/cryptohome-9999'


If I don't work on it, emerging works fine.
Same for other packages:
mkdir "/build/link/tmp/portage/chromeos-base/<package>-9999/work<package>-9999/<etc>" failed: No such file or directory

I checked. There is an empty 
/build/$BOARD/tmp/portage/chromeos-base/<package>-9999/work
but there is no
/build/$BOARD/tmp/portage/chromeos-base/<package>-9999/work/<package>-9999/

Looks like either some change dropped '-p' from mkdir here, or we add an extra <package>-9999 to the path, which we were not adding  before..