Merge branch '14-allow-more-than-26-disks-per-instance' into 'master'

Resolve "Allow more than 26 disks per instance"

Closes #14

See merge request !8

driver/src/driver.sh

@@ -176,10 +176,27 @@ domount() {
         exitWithFailure "$errorMessage"
     fi
 
-    # Convert numeric disk slot index to device letter, so
-    # 0 becomes a, 1 becomes b, etc.
-    diskLetter=$(printf \\$(printf '%03o' $((97 + $diskSlot))))
-    device="/dev/xvd${diskLetter}1"
+    # Convert a numerical disk index (zero-based) to an actual device path like
+    # /dev/xvda1 (index 0), /dev/xvdz1 (25) or /dev/xvdbc1 (2 * 26 + 2 = 54).
+    # We use at most two letters in the variable part, so we can have 26
+    # single-letter ones and 26^2 double-letter ones, 702 in total. As the disk
+    # index is zero-based, 701 is the highest allowed index.
+    if [[ $diskSlot -gt 701 ]]
+    then
+        errorMessage="disk_slot is larger than 701, and thus not supported."
+        exitWithFailure "$errorMessage"
+    fi
+    if [[ $diskSlot -le 25 ]]
+    then
+        letter=$(printf \\$(printf '%03o' $((97 + $diskSlot))))
+        device="/dev/xvd${letter}1"
+    else
+        # Minus 1, because the quotient $diskIndex / 26 is one-based,
+        # not zero-based.
+        firstLetter=$(printf \\$(printf '%03o' $((97 + $diskSlot / 26 - 1))))
+        secondLetter=$(printf \\$(printf '%03o' $((97 + $diskSlot % 26))))
+        device="/dev/xvd${firstLetter}${secondLetter}1"
+    fi
     debug "device: $device"
 
     # Wait for the device file to appear. It might take the VPS kernel some