builder/amazon/chroot: don't choose partition mount point if taken

[GH-635]

CHANGELOG.md

@@ -25,6 +25,8 @@ BUG FIXES:
 
 * builder/amazon/chroot: Copying empty directories works. [GH-588]
 * builder/amazon/chroot: Chroot commands work with shell provisioners. [GH-581]
+* builder/amazon/chroot: Don't choose a mount point that is a partition of
+  an already mounted device. [GH-635]
 * builder/virtualbox: Ctrl-C interrupts during waiting for boot. [GH-618]
 * builder/vmware: VMX modifications are now case-insensitive. [GH-608]
 * builder/vmware: VMware Fusion won't ask for VM upgrade.

builder/amazon/chroot/device.go

@@ -19,6 +19,14 @@ func AvailableDevice() (string, error) {
 
 	letters := "fghijklmnop"
 	for _, letter := range letters {
+		device := fmt.Sprintf("/dev/%s%c", prefix, letter)
+
+		// If the block device itself, i.e. /dev/sf, exists, then we
+		// can't use any of the numbers either.
+		if _, err := os.Stat(device); err == nil {
+			continue
+		}
+
 		for i := 1; i < 16; i++ {
 			device := fmt.Sprintf("/dev/%s%c%d", prefix, letter, i)
 			if _, err := os.Stat(device); err != nil {