wcfs: Adjust ΔFtail/ΔBtail to allow point-queries with at=tail

This is needed because when e.g. wcfs is just started the coverage of
ΔFtail is (head,head] i.e. empty, and if user wants to setup a watch
with at=head, it becomes watch with at=tail. Then that at is used in a
query and if point-queries with at=tail are disallowed it panics with
"at out of bounds".

This fixes crashes in test_wcfs_watch_setup (see 339f1884 "wcfs: tests:
Always start tDB with ZBigFile pre-created before WCFS startup") and in
test_wcfs_crash_old_data (see 97ce5105 "wcfs: tests: Add test do
demonstrate "at out of bounds" crash on readPinWatchers ->
ΔFtail.BlkRevAt")

For the reference zodb.ΔTail already allows point queries with at=tail:

https://lab.nexedi.com/kirr/neo/blob/1193c44e/go/zodb/δtail.go#L202-206
https://lab.nexedi.com/kirr/neo/blob/1193c44e/go/zodb/δtail.go#L225-228

wcfs/internal/xbtree/δbtail.go

-// Copyright (C) 2018-2021  Nexedi SA and Contributors.
+// Copyright (C) 2018-2022  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -935,7 +935,7 @@ func (δTtail *_ΔTtail) _forgetPast(revCut zodb.Tid) {
 //	revExact:   false
 //
 // key must be tracked
-// at  must ∈ (tail, head]
+// at  must ∈ {tail} ∪ (tail, head]
 func (δBtail *ΔBtail) GetAt(root zodb.Oid, key Key, at zodb.Tid) (value Value, rev zodb.Tid, valueExact, revExact bool, err error) {
 	defer xerr.Contextf(&err, "ΔBtail: root<%s>: get %d @%s", root, key, at)
 
@@ -956,7 +956,7 @@ func (δBtail *ΔBtail) GetAt(root zodb.Oid, key Key, at zodb.Tid) (value Value,
 
 	tail := δBtail.Tail()
 	head := δBtail.Head()
-	if !(tail < at && at <= head) {
+	if !(tail <= at && at <= head) {
 		panicf("at out of bounds: at: @%s,  (tail, head] = (@%s, @%s]", at, tail, head)
 	}
 

wcfs/internal/xbtree/δbtail_test.go

-// Copyright (C) 2020-2021  Nexedi SA and Contributors.
+// Copyright (C) 2020-2022  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -1167,6 +1167,11 @@ func TestΔBtailGetAt(t_ *testing.T) {
 	}
 
 	//          @at  key  value rev  valueExact  revExact
+	assertGetAt(at1, 1,   ø,    at1, false,      false)
+	assertGetAt(at1, 2,   ø,    at1, false,      false)
+	assertGetAt(at1, 3,   "c",  at1, true,       false)
+	assertGetAt(at1, 4,   ø,    at1, true,       false)
+
 	assertGetAt(at2, 1,   ø,    at1, false,      false)
 	assertGetAt(at2, 2,   ø,    at1, false,      false)
 	assertGetAt(at2, 3,   "c",  at1, true,       false)

wcfs/internal/zdata/δftail.go

-// Copyright (C) 2019-2021  Nexedi SA and Contributors.
+// Copyright (C) 2019-2022  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -1061,8 +1061,8 @@ func flipsign(x int64) int64 {
 //
 // if exact=False - what is returned is only an upper bound for last block revision.
 //
-// zfile must be any checkout from (tail, head]
-// at must ∈ (tail, head]
+// zfile must be any checkout from {tail} ∪ (tail, head]
+// at must ∈ {tail} ∪ (tail, head]
 // blk must be tracked
 func (δFtail *ΔFtail) BlkRevAt(ctx context.Context, zfile *ZBigFile, blk int64, at zodb.Tid) (_ zodb.Tid, exact bool, err error) {
 	foid := zfile.POid()
@@ -1070,17 +1070,17 @@ func (δFtail *ΔFtail) BlkRevAt(ctx context.Context, zfile *ZBigFile, blk int64
 
 	//fmt.Printf("\nblkrev #%d @%s\n", blk, at)
 
-	// assert at ∈ (tail, head]
+	// assert at ∈ [tail, head]
 	tail := δFtail.Tail()
 	head := δFtail.Head()
-	if !(tail < at && at <= head) {
+	if !(tail <= at && at <= head) {
 		panicf("at out of bounds: at: @%s,  (tail, head] = (@%s, @%s]", at, tail, head)
 	}
 
-	// assert zfile.at ∈ (tail, head]
+	// assert zfile.at ∈ [tail, head]
 	zconn := zfile.PJar()
 	zconnAt := zconn.At()
-	if !(tail < zconnAt && zconnAt <= head) {
+	if !(tail <= zconnAt && zconnAt <= head) {
 		panicf("zconn.at out of bounds: zconn.at: @%s,  (tail, head] = (@%s, @%s]", zconnAt, tail, head)
 	}
 
@@ -1119,7 +1119,7 @@ func (δFtail *ΔFtail) BlkRevAt(ctx context.Context, zfile *ZBigFile, blk int64
 	//fmt.Printf("  epoch: @%s  root: %s\n", epoch, root)
 
 	if root == xbtree.VDEL {
-		return epoch, true, nil
+		return epoch, (epoch > tail), nil
 	}
 
 	zblk, tabRev, zblkExact, tabRevExact, err := δFtail.δBtail.GetAt(root, blk, at)
@@ -1135,7 +1135,7 @@ func (δFtail *ΔFtail) BlkRevAt(ctx context.Context, zfile *ZBigFile, blk int64
 	}
 
 	// if δBtail does not have entry that covers root[blk] - get it
-	// through any zconn with .at ∈ (tail, head].
+	// through any zconn with .at ∈ [tail, head].
 	if !zblkExact {
 		xblktab, err := zconn.Get(ctx, root)
 		if err != nil {

wcfs/internal/zdata/δftail_test.go

-// Copyright (C) 2019-2021  Nexedi SA and Contributors.
+// Copyright (C) 2019-2022  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -571,9 +571,16 @@ func testΔFtail(t_ *testing.T, testq chan ΔFTestEntry) {
 			return blkv[i] < blkv[j]
 		})
 
-		for j := 0; j < len(vδf); j++ {
-			at := vδf[j].Rev
-			blkRev := blkRevAt[at]
+		for j := -1; j < len(vδf); j++ {
+			var at     zodb.Tid
+			var blkRev map[int64]zodb.Tid
+			if j == -1 {
+				at = δFtail.Tail()
+				// blkRev remains ø
+			} else {
+				at = vδf[j].Rev
+				blkRev = blkRevAt[at]
+			}
 			for _, blk := range blkv {
 				rev, exact, err := δFtail.BlkRevAt(ctx, zfile, blk, at);  X(err)
 				revOK, ok := blkRev[blk]

wcfs/wcfs_test.py

@@ -1836,7 +1836,7 @@ def test_wcfs_crash_old_data():
     wl = t.openwatch()
     wl.watch(zf, at1, {})
 
-    # wcfs is crashing on readPinWatcher -> ΔFtail.BlkRevAt with
+    # wcfs was crashing on readPinWatcher -> ΔFtail.BlkRevAt with
     #   "at out of bounds: at: @at1,  (tail,head] = (@at1,@at1]
     # because BlkRevAt(at=tail) query was disallowed.
     f.assertBlk(0, 'a')          # [0] becomes tracked