bigfile/zodb: Fix thinko in ZBlk1.setblkdata()

ZBlk1.setblkdata() has logic to detect CHUNKSIZE change, and if so
recreate whole chunktab from scratch for simplicity. There was a thinko
however - len(chunk.data) == CHUNKSIZE is ok and actually very often
happens when data does not have zeroes.

Because of this off-by-1 comparison mistake, ZData objects were
constantly created and thrown out instead of being reused which led to
fast ZODB growth.

Fix it.

/reported-by @Tyagov

bigfile/file_zodb.py

@@ -287,7 +287,7 @@ class ZBlk1(ZBlkBase):
         # first make sure chunktab was previously written with the same CHUNKSIZE
         # (for simplicity we don't allow several chunk sizes to mix)
         for start, chunk in chunktab.items():
-            if (start % CHUNKSIZE) or len(chunk.data) >= CHUNKSIZE:
+            if (start % CHUNKSIZE) or len(chunk.data) > CHUNKSIZE:
                 chunktab.clear()
                 break
 

bigfile/tests/test_filezodb.py

@@ -761,3 +761,50 @@ def test_bigfile_filezodb_fmt_change():
         file_zodb.ZBlk_fmt_write = fmt_write_save
 
     dbclose(root)
+
+
+# test that ZData are reused for changed chunks in ZBlk1 format
+def test_bigfile_zblk1_zdata_reuse():
+    # set ZBlk_fmt_write to ZBlk1 for this test
+    fmt_write_save = file_zodb.ZBlk_fmt_write
+    file_zodb.ZBlk_fmt_write = 'ZBlk1'
+    try:
+        _test_bigfile_zblk1_zdata_reuse()
+    finally:
+        file_zodb.ZBlk_fmt_write = fmt_write_save
+
+def _test_bigfile_zblk1_zdata_reuse():
+    root = dbopen()
+    root['zfile6'] = f = ZBigFile(blksize)
+    transaction.commit()
+
+    fh  = f.fileh_open()    # TODO + ram
+    vma = fh.mmap(0, 1)
+    b   = Blk(vma, 0)
+
+    # initially empty and zero
+    assert len(f.blktab) == 0
+    assert (b == 0).all()
+
+    # set all to 1 and save ZData instances
+    b[:] = 1
+    assert (b == 1).all()
+    transaction.commit()
+    assert len(f.blktab) == 1
+    zblk0_v1 = f.blktab[0]
+    assert len(zblk0_v1.chunktab) == blksize / file_zodb.ZBlk1.CHUNKSIZE
+    zdata_v1 = zblk0_v1.chunktab.values()
+
+    # set all to 2 and verify ZBlk/ZData instances were reused
+    b[:] = 2
+    assert (b == 2).all()
+    transaction.commit()
+    assert len(f.blktab) == 1
+    zblk0_v2 = f.blktab[0]
+    assert zblk0_v2 is zblk0_v1
+    assert len(zblk0_v2.chunktab) == blksize / file_zodb.ZBlk1.CHUNKSIZE
+    zdata_v2 = zblk0_v2.chunktab.values()
+
+    assert len(zdata_v1) == len(zdata_v2)
+    for i in range(len(zdata_v1)):
+        assert zdata_v1[i] is zdata_v2[i]