Added allocation randomization for dynamic wear-leveling

This implements the second step of full dynamic wear-leveling, block
allocation randomization. This is the key part the uniformly distributes
wear across the filesystem, even through reboots.

The entropy actually comes from the filesystem itself, by xoring
together all of the CRCs in the metadata-pairs on the filesystem. While
this sounds like a ridiculous operation, it's easy to do when we already
scan the metadata-pairs at mount time.

This gives us a random number we can use for block allocation.
Unfortunately it's not a great general purpose random generator as the
output only changes every filesystem write. Fortunately that's exactly
when we need our allocator.

---

Additionally, the randomization created a mess for the testing
framework. Fortunately, this method of randomization is deterministic.
A very useful property for reproducing bugs.

tests/debug.py

@@ -0,0 +1,98 @@
+#!/usr/bin/env python2
+
+import struct
+import binascii
+
+TYPES = {
+    (0x1ff, 0x001): 'reg',
+    (0x1ff, 0x002): 'dir',
+    (0x1ff, 0x011): 'superblock',
+    (0x1ff, 0x012): 'root',
+    (0x1ff, 0x030): 'delete',
+    (0x1f0, 0x080): 'globals',
+    (0x1ff, 0x0c0): 'tail soft',
+    (0x1ff, 0x0c1): 'tail hard',
+    (0x1ff, 0x0f0): 'crc',
+    (0x1ff, 0x040): 'struct dir',
+    (0x1ff, 0x041): 'struct inline',
+    (0x1ff, 0x042): 'struct ctz',
+    (0x100, 0x100): 'attr',
+}
+
+def typeof(type):
+    for prefix in range(9):
+        mask = 0x1ff & ~((1 << prefix)-1)
+        if (mask, type & mask) in TYPES:
+            return TYPES[mask, type & mask] + (
+                ' [%0*x]' % (prefix/4, type & ((1 << prefix)-1))
+                if prefix else '')
+    else:
+        return '[%02x]' % type
+
+def main(*blocks):
+    # find most recent block
+    file = None
+    rev = None
+    crc = None
+    versions = []
+
+    for block in blocks:
+        try:
+            nfile = open(block, 'rb')
+            ndata = nfile.read(4)
+            ncrc = binascii.crc32(ndata)
+            nrev, = struct.unpack('<I', ndata)
+
+            assert rev != nrev
+            if not file or ((rev - nrev) & 0x80000000):
+                file = nfile
+                rev = nrev
+                crc = ncrc
+
+            versions.append((nrev, '%s (rev %d)' % (block, nrev)))
+        except IOError:
+            pass
+
+    print "--- %s ---" % ', '.join(v for _,v in sorted(versions, reverse=True))
+
+    # go through each tag, print useful information
+    print "%-4s  %-8s  %-14s  %3s  %3s  %s" % (
+        'off', 'tag', 'type', 'id', 'len', 'dump')
+
+    tag = 0
+    off = 4
+    while True:
+        try:
+            data = file.read(4)
+            crc = binascii.crc32(data, crc)
+            ntag, = struct.unpack('<I', data)
+        except struct.error:
+            break
+
+        tag ^= ntag
+        off += 4
+
+        type = (tag & 0x7fc00000) >> 22
+        id   = (tag & 0x003ff000) >> 12
+        size = (tag & 0x00000fff) >> 0
+
+        data = file.read(size)
+        if type == 0x0f0:
+            crc = binascii.crc32(data[:4], crc)
+        else:
+            crc = binascii.crc32(data, crc)
+
+        print '%04x: %08x  %-14s  %3s  %3d  %-23s  %-8s' % (
+            off, tag,
+            typeof(type) + (' bad!' if type == 0x0f0 and ~crc else ''),
+            id if id != 0x3ff else '.', size,
+            ' '.join('%02x' % ord(c) for c in data[:8]),
+            ''.join(c if c >= ' ' and c <= '~' else '.' for c in data[:8]))
+
+        off += tag & 0xfff
+        if type == 0x0f0:
+            crc = 0
+
+if __name__ == "__main__":
+    import sys
+    main(*sys.argv[1:])