bigfile/zodb: Don't write ZBlk to DB if it was not changed

For ZBlk1 we already compare ZData content about whether it was changed
compared to data already stored to DB, and do not store it twice if data
is the same.

However ZBlk itself is always marked as changed, if corresponding memory
page was dirtied. This results in transactions like

Trans #33915309 tid=03b6944919befeee time=2016-04-17 22:01:06.034237 offset=140320105842
    status=' ' user='...' description='...'
  # ... other parts, but no ZData here
  data #00002 oid=000000000026fc4c size=79 class=wendelin.bigfile.file_zodb.ZBlk1

where ZBlk1 is committed the same without necessity.

NOTE we cannot avoid committing ZBlk in all cases, because it is used to signal
    other DB clients that a ZBlk needs to be invalidated and this way associated
    fileh pages are invalidated too.

    This cannot work via ZData, because ZData don't have back-pointer to
    ZBlk1 or to corresponding zfile.

bigfile/file_zodb.py

@@ -113,6 +113,12 @@ class ZBlkBase(Persistent):
 
     # client requests us to set blkdata to be later saved to DB
     # (DB <- )  .blkdata <- memory-page
+    #
+    # return: blkchanged=(True|False) - whether blk should be considered changed
+    #         after setting its data.
+    #
+    #         False - when we know the data set was the same
+    #         True  - data was not the same or we don't know
     def setblkdata(self, buf):
         raise NotImplementedError()
 
@@ -182,6 +188,10 @@ class ZBlk0(ZBlkBase):
         # trim trailing \0
         self._v_blkdata = blkdata.rstrip(b'\0') # FIXME copy
 
+        # buf always considered to be "changed", as we do not keep old data to
+        # compare.
+        return True
+
 
     # DB (through pickle) requests us to emit state to save
     # DB <- ._v_blkdata  (<- memory-page)
@@ -296,6 +306,7 @@ class ZBlk1(ZBlkBase):
     def setblkdata(self, buf):
         chunktab  = self.chunktab
         CHUNKSIZE = self.CHUNKSIZE
+        blkchanged= False
 
         # first make sure chunktab was previously written with the same CHUNKSIZE
         # (for simplicity we don't allow several chunk sizes to mix)
@@ -320,6 +331,7 @@ class ZBlk1(ZBlkBase):
                 if chunk is not None:
                     del chunktab[start]
                     chunk._p_deactivate()
+                    blkchanged = True
 
             # some !0 data -> compare and store if changed
             else:
@@ -328,6 +340,7 @@ class ZBlk1(ZBlkBase):
 
                 if chunk.data != data:
                     chunk.data = data
+                    blkchanged = True
                     # data changed and is queued to be committed to db.
                     # ZData will care about this chunk deactivation after DB
                     # asks for its data - see ZData.__getstate__().
@@ -337,6 +350,8 @@ class ZBlk1(ZBlkBase):
                     # more needed
                     chunk._p_deactivate()
 
+        return blkchanged
+
 
     # DB (through pickle) requests us to emit state to save
     # DB <- .chunktab  (<- memory-page)
@@ -481,7 +496,8 @@ class ZBigFile(LivePersistent):
            type(zblk) is not zblk_type_write:
             zblk = self.blktab[blk] = zblk_type_write()
 
-        zblk.setblkdata(buf)
+        blkchanged = zblk.setblkdata(buf)
+        if blkchanged:
             zblk._p_changed = True      # if zblk was already in DB: _p_state -> CHANGED
         zblk.bindzfile(self, blk)