X Adjust ΔTail to what wcfs expects

go/zodb/δtail.go

-// Copyright (C) 2018-2020  Nexedi SA and Contributors.
+// Copyright (C) 2018-2021  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -202,46 +202,35 @@ func (δtail *ΔTail) ForgetPast(revCut Tid) {
 // XXX -> RevAt ?
 // LastRevOf tries to return what was the last revision that changed id as of at database state.
 //
-// Depending on current information in δtail it returns either exact result, or
-// an upper-bound estimate for the last id revision, assuming id was changed ≤ at:
+// it must be called with the following condition:
 //
-// 1) if δtail does not cover at, at is returned:
+//	tail ≤ at ≤ head
 //
-//	# at ∉ [min(rev ∈ δtail), max(rev ∈ δtail)]
-//	LastRevOf(id, at) = at
+// Depending on current information in δtail it returns either exact result, or
+// an upper-bound estimate for the last id revision:
 //
-// 2) if δtail has an entry corresponding to id change, it gives exactly the
+// 1) if δtail has an entry corresponding to id change, it gives exactly the
 // last revision that changed id:
 //
-//	# at ∈ [min(rev ∈ δtail), max(rev ∈ δtail)]
 //	# ∃ rev ∈ δtail: rev changed id && rev ≤ at
-//	LastRevOf(id, at) = max(rev: rev changed id && rev ≤ at)
+//	LastRevOf(id, at) = max(rev: rev changed id && rev ≤ at), true
 //
-// 3) if δtail does not contain appropriate record with id - it returns δtail's
+// 2) if δtail does not contain appropriate record with id - it returns δtail's
 // lower bound as the estimate for the upper bound of the last id revision:
 //
-//	# at ∈ [min(rev ∈ δtail), max(rev ∈ δtail)]
 //	# ∄ rev ∈ δtail: rev changed id && rev ≤ at
-//	LastRevOf(id, at) = min(rev ∈ δtail)
+//	LastRevOf(id, at) = δtail.tail, false
 //
 // On return exact indicates whether returned revision is exactly the last
 // revision of id, or only an upper-bound estimate of it.
 func (δtail *ΔTail) LastRevOf(id Oid, at Tid) (_ Tid, exact bool) {
-	// check if we have no coverage at all
-	l := len(δtail.tailv)
-	if l == 0 {
-		return at, false
-	}
-	revMin := δtail.tailv[0].Rev
-	revMax := δtail.tailv[l-1].Rev
-	if !(revMin <= at && at <= revMax) {
-		return at, false
-	}
+	if !(δtail.tail <= at && at <= δtail.head) {
+		panic(fmt.Sprintf("at out of bounds: at: @%s,  (tail, head] = (@%s, @%s]", at, δtail.tail, δtail.head))
+        }
 
-	// we have the coverage
 	rev, ok := δtail.lastRevOf[id]
 	if !ok {
-		return δtail.tailv[0].Rev, false
+		return δtail.tail, false
 	}
 
 	if rev <= at {
@@ -250,7 +239,7 @@ func (δtail *ΔTail) LastRevOf(id Oid, at Tid) (_ Tid, exact bool) {
 
 	// what's in index is after at - scan tailv back to find appropriate entry
 	// XXX linear scan - see .lastRevOf comment.
-	for i := l - 1; i >= 0; i-- {
+	for i := len(δtail.tailv) - 1; i >= 0; i-- {
 		δ := δtail.tailv[i]
 		if δ.Rev > at {
 			continue
@@ -264,5 +253,5 @@ func (δtail *ΔTail) LastRevOf(id Oid, at Tid) (_ Tid, exact bool) {
 	}
 
 	// nothing found
-	return δtail.tailv[0].Rev, false
+	return δtail.tail, false
 }

go/zodb/δtail_test.go

-// Copyright (C) 2018-2019  Nexedi SA and Contributors.
+// Copyright (C) 2018-2021  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -174,14 +174,13 @@ func TestΔTail(t *testing.T) {
 	δtail = NewΔTail(3)
 
 	δCheck(3,3)
-	δCheckLastUP(4, 12, 12)	// δtail = ø
+	δCheckLastUP(3, 3, 3)	// δtail = ø
 
 	δAppend(R(10, 3,5))
 	δCheck(3,10, R(10, 3,5))
 
-	δCheckLastUP(3,  2,  2)	// at < δtail
-	δCheckLastUP(3, 12, 12)	// at > δtail
-	δCheckLastUP(4, 10, 10)	// id ∉ δtail
+	δCheckLastUP(3,  9, 3)  // id ∈ δtail, but has no entry with rev ≤ at
+	δCheckLastUP(4, 10, 3)	// id ∉ δtail
 
 	δAppend(R(11, 7))
 	δCheck(3,11, R(10, 3,5), R(11, 7))
@@ -192,7 +191,7 @@ func TestΔTail(t *testing.T) {
 	δAppend(R(14, 3,8))
 	δCheck(3,14, R(10, 3,5), R(11, 7), R(12, 7), R(14, 3,8))
 
-	δCheckLastUP(8, 12, 10) // id ∈ δtail, but has no entry with rev ≤ at
+	δCheckLastUP(8, 12, 3)  // id ∈ δtail, but has no entry with rev ≤ at
 
 	δtail.ForgetPast(9)
 	δCheck(9,14, R(10, 3,5), R(11, 7), R(12, 7), R(14, 3,8))