Ben, I don't understand your patch for this at all. The error message you showed is "umount: /proc: device is busy." How can umount -n avoid it trying to umount the wrong /proc?
Also, portability is a concern. busybox umount does not support -n, for example. When I run debootstrap, it ends successfully with $chroot/proc not being mounted. Is there something about it being run by pbuilder that triggers this bug? -- see shy jo
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