wcfs: Handle ZODB invalidations

Use ΔFtail.Track on every READ, and query accumulated ΔFtail upon
receiving ZODB invalidation to query it about which blocks of which
files have been changed. Then invalidate those blocks in OS file cache.

See added documentation to wcfs.go and notes.txt for details.

Now the filesystem is no longer stale: it provides view of data
that is uptodate wrt changes on ZODB storage.

Some preliminary history:

9b4a42a3    X invalidation design draftly settled
27d91d47    X δFtail settled
33e0dfce    X ΔTail draftly done
822366a7    X keeping fd to root opened prevents the filesystem from being unmounted
89ad3a79    X Don't keep ZBigFile activated during whole current transaction
245511ac    X Give pointer on from where to get nxd-fuse.ko
d1cd128c    X Hit FUSE-related deadlock
d134ee44    X FUSE lookup deadlock should be hopefully fixed
0e60e9ff    X wcfs: Don't noise ZWatcher trace logs with "select ..."
bf9a7405    X No longer rely on ZODB cache invariant for invalidations

wcfs/notes.txt

+==============================================
+ Additional notes to documentation in wcfs.go
+==============================================
+
+This file contains notes additional to usage documentation and internal
+organization overview in wcfs.go .
+
+
+Notes on OS pagecache control
+=============================
+
+The cache of snapshotted bigfile can be pre-made hot if invalidated region
+was already in pagecache of head/bigfile/file:
+
+- we can retrieve a region from pagecache of head/file with FUSE_NOTIFY_RETRIEVE.
+- we can store that retrieved data into pagecache region of @<revX>/ with FUSE_NOTIFY_STORE.
+- we can invalidate a region from pagecache of head/file with FUSE_NOTIFY_INVAL_INODE.
+
+we have to disable FUSE_AUTO_INVAL_DATA to tell the kernel we are fully
+responsible for invalidating pagecache. If we don't, the kernel will be
+clearing whole cache of head/file on e.g. its mtime change.
+
+Note: disabling FUSE_AUTO_INVAL_DATA does not fully prevent kernel from automatically
+invalidating pagecache - e.g. it will invalidate whole cache on file size changes:
+
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/fuse/inode.c?id=e0bc833d10#n233
+
+It was hoped that we could workaround it with using writeback mode (see !is_wb
+in the link above), but it turned out that in writeback mode the kernel indeed
+does not invalidate data cache on file size change, but neither it allows the
+filesystem to set the size due to external event (see https://git.kernel.org/linus/8373200b12
+"fuse: Trust kernel i_size only"). This prevents us to use writeback workaround
+as we cannot even update the file from being empty to have some data.
+
+-> we did the patch for FUSE to have proper flag for filesystem server to tell
+the kernel it is fully responsible for invalidating pagecache. The patch is
+part of Linux 5.2:
+
+  https://git.kernel.org/linus/ad2ba64dd489
+
+
+Kernel locks page on read/cache store/... - we have to be careful not to deadlock
+=================================================================================
+
+The kernel, when doing FUSE operations, locks corresponding pages. For example
+it locks a page, where it is going to read data into, before issuing FUSE read
+request. Correspondingly, on e.g. cache store, the kernel also locks page where
+data has to be stored.
+
+It is easy to deadlock if we don't take this locks into account. For example
+if we try to upload data to kernel pagecache from under serving read request,
+this can deadlock.
+
+Another case that needs to be cared about is interaction between uploadBlk and
+zwatcher: zheadMu being RWMutex, does not allow new RLocks to be taken once
+Lock request has been issued. Thus the following scenario is possible::
+
+   uploadBlk      os.Read        zwatcher
+
+                  page.Lock
+   zheadMu.Rlock
+                                 zheadMu.Lock
+   page.Lock
+                  zheadMu.Rlock
+
+
+- zwatcher is waiting for uploadBlk to release zheadMu;
+- uploadBlk is waiting for os.Read to release page;
+- os.Read is waiting for zwatcher to release zheadMu;
+- deadlock.
+
+To avoid such deadlocks zwatcher asks OS cache uploaders to pause while it is
+running, and retries taking zheadMu.Lock until all uploaders are indeed paused.

wcfs/wcfs_test.py

@@ -40,7 +40,7 @@ from errno import EINVAL, ENOTCONN
 from resource import setrlimit, getrlimit, RLIMIT_MEMLOCK
 from golang import go, chan, select, func, defer, b
 from golang import context, time
-from zodbtools.util import ashex as h
+from zodbtools.util import ashex as h, fromhex
 import pytest; xfail = pytest.mark.xfail
 from pytest import raises, fail
 from wendelin.wcfs.internal import io, mm
@@ -299,6 +299,7 @@ def timeout(parent=context.background()):   # -> ctx
 
 
 # DF represents a change in files space.
+# it corresponds to ΔF in wcfs.go .
 class DF:
     # .rev      tid
     # .byfile   {} ZBigFile -> DFile
@@ -307,6 +308,7 @@ class DF:
         dF.byfile = {}
 
 # DFile represents a change to one file.
+# it is similar to ΔFile in wcfs.go .
 class DFile:
     # .rev      tid
     # .ddata    {} blk -> data
@@ -415,6 +417,10 @@ class tDB(tWCFS):
         t.tail   = t.root._p_jar.db().storage.lastTransaction()
         t.dFtail = [] # of DF; head = dFtail[-1].rev
 
+        # fh(.wcfs/zhead) + history of zhead read from there
+        t._wc_zheadfh = open(t.wc.mountpoint + "/.wcfs/zhead")
+        t._wc_zheadv  = []
+
         # tracked opened tFiles
         t._files    = set()
 
@@ -443,6 +449,7 @@ class tDB(tWCFS):
         for tf in t._files.copy():
             tf.close()
         assert len(t._files)   == 0
+        t._wc_zheadfh.close()
 
     # open opens wcfs file corresponding to zf@at and starts to track it.
     # see returned tFile for details.
@@ -521,15 +528,18 @@ class tDB(tWCFS):
 
     # _wcsync makes sure wcfs is synchronized to latest committed transaction.
     def _wcsync(t):
-        # XXX stub: unmount/remount + close/reopen files until wcfs supports invalidations
-        files = t._files.copy()
-        for tf in files:
-            tf.close()
-        tWCFS.close(t)
-        tWCFS.__init__(t)
-        for tf in files:
-            tf.__init__(t, tf.zf, tf.at)
-        assert len(t._files) == len(files)
+        while len(t._wc_zheadv) < len(t.dFtail):
+            l = t._wc_zheadfh.readline()
+            #print('> zhead read: %r' % l)
+            l = l.rstrip('\n')
+            wchead = tAt(t, fromhex(l))
+            i = len(t._wc_zheadv)
+            if wchead != t.dFtail[i].rev:
+                raise RuntimeError("wcsync #%d: wczhead (%s) != zhead (%s)" % (i, wchead, t.dFtail[i].rev))
+            t._wc_zheadv.append(wchead)
+
+        # head/at = last txn of whole db
+        assert t.wc._read("head/at") == h(t.head)
 
 
 # tFile provides testing environment for one bigfile opened on wcfs.
@@ -763,7 +773,7 @@ def _blkDataAt(t, zf, blk, at): # -> (data, rev)
 # ---- actual tests to access data ----
 
 # exercise wcfs functionality
-# plain data access.
+# plain data access + wcfs handling of ZODB invalidations.
 @func
 def test_wcfs_basic():
     t = tDB(); zf = t.zfile
@@ -783,20 +793,20 @@ def test_wcfs_basic():
     at1 = t.commit(zf, {2:'c1'})
 
     f.assertCache([0,0,0])  # initially not cached
-    f.assertData (['','','c1']) # TODO + mtime=t.head
+    f.assertData (['','','c1'], mtime=t.head)
 
     # >>> (@at2) commit again -> we can see both latest and snapshotted states
     # NOTE blocks e(4) and f(5) will be accessed only in the end
     at2 = t.commit(zf, {2:'c2', 3:'d2', 5:'f2'})
 
     # f @head
-    #f.assertCache([1,1,0,0,0,0])                 TODO enable after wcfs supports invalidations
-    f.assertData (['','', 'c2', 'd2', 'x','x']) # TODO + mtime=t.head
+    f.assertCache([1,1,0,0,0,0])
+    f.assertData (['','', 'c2', 'd2', 'x','x'], mtime=t.head)
     f.assertCache([1,1,1,1,0,0])
 
     # f @at1
     f1 = t.open(zf, at=at1)
-    #f1.assertCache([0,0,1])       TODO enable after wcfs supports invalidations
+    f1.assertCache([0,0,1])
     f1.assertData (['','','c1']) # TODO + mtime=at1
 
 
@@ -804,22 +814,59 @@ def test_wcfs_basic():
     f2 = t.open(zf, at=at2)
     at3 = t.commit(zf, {0:'a3', 2:'c3', 5:'f3'})
 
-    #f.assertCache([0,1,0,1,0,0])   TODO enable after wcfs supports invalidations
+    f.assertCache([0,1,0,1,0,0])
+
+    # f @head is opened again -> cache must not be lost
+    f_ = t.open(zf)
+    f_.assertCache([0,1,0,1,0,0])
+    f_.close()
+    f.assertCache([0,1,0,1,0,0])
 
     # f @head
-    #f.assertCache([0,1,0,1,0,0])   TODO enable after wcfs supports invalidations
-    f.assertData (['a3','','c3','d2','x','x']) # TODO + mtime=t.head
+    f.assertCache([0,1,0,1,0,0])
+    f.assertData (['a3','','c3','d2','x','x'], mtime=t.head)
 
     # f @at2
     # NOTE f(2) is accessed but via @at/ not head/  ; f(2) in head/zf remains unaccessed
-    #f2.assertCache([0,0,1,0,0,0])  TODO enable after wcfs supports invalidations
+    f2.assertCache([0,0,1,0,0,0])
     f2.assertData (['','','c2','d2','','f2']) # TODO mtime=at2
 
     # f @at1
-    #f1.assertCache([1,1,1])        TODO enable after wcfs supports invalidations
+    f1.assertCache([1,1,1])
     f1.assertData (['','','c1']) # TODO mtime=at1
 
 
+    # >>> f close / open again -> cache must not be lost
+    # XXX a bit flaky since OS can evict whole f cache under pressure
+    f.assertCache([1,1,1,1,0,0])
+    f.close()
+    f = t.open(zf)
+    if f.cached() != [1,1,1,1,0,0]:
+        assert sum(f.cached()) > 4*1/2  # > 50%
+
+    # verify all blocks
+    f.assertData(['a3','','c3','d2','','f3'])
+    f.assertCache([1,1,1,1,1,1])
+
+
+# verify how wcfs processes ZODB invalidations when hole becomes a block with data.
+@func
+def test_wcfs_basic_hole2zblk():
+    t = tDB(); zf = t.zfile
+    defer(t.close)
+
+    f = t.open(zf)
+    t.commit(zf, {2:'c1'})  # b & a are holes
+    f.assertCache([0,0,0])
+    f.assertData(['','','c1'])
+
+    t.commit(zf, {1:'b2'})  # hole -> zblk
+    f.assertCache([1,0,1])
+    f.assertData(['','b2','c1'])
+
+# TODO ZBlk copied from blk1 -> blk2 ; for the same file and for file1 -> file2
+# TODO ZBlk moved  from blk1 -> blk2 ; for the same file and for file1 -> file2
+
 # verify that read after file size returns (0, ok)
 # (the same behaviour as on e.g. ext4 and as requested by posix)
 @func