X δFtail settled

wcfs/wcfs.go

@@ -265,7 +265,10 @@ package main
 //
 //	- try to retrieve file/head/data[blk] from OS file cache;
 //	- if retrieved successfully -> store retrieved data into OS file cache
-//	  for file/@<rev>/data[blk];	XXX @rev = what? (ideally exact previous rev of blk)
+//	  for file/@<rev>/data[blk].
+//
+//	    rev = max(δFtail.by(#blk)) || zconn.at
+//
 //	- invalidate file/head/data[blk] in OS file cache.
 //
 //	This preserves previous data in OS file cache in case it will be needed
@@ -273,39 +276,49 @@ package main
 //	won't be served from OS file cache and instead will trigger a FUSE read
 //	request to wcfs.
 //
-// - for file δFtail invalidation info is maintained:
-//
-//	- tail of [](rev↑, []#blk)
-//	- {} #blk -> []rev↑ in tail	XXX needed?
-//
-//   δFtail describes changes to file we learned from ZODB invalidation.
-//   XXX and allows to quick #blk -> ... ?
+// - for every file δFtail invalidation info about head/data is maintained:
 //
-// - XXX δZtail of invalidation info is maintained.
+//	- tail: of [](rev↑, []#blk)
+//	- by:   {} #blk -> []rev↑ in tail
 //
-//	- tail of [](tid↑, []oid)
-//	- {} oid -> []tid↑ in tail
+//      δFtail.tail describes invalidations to file we learned from ZODB invalidation.
+//      δFtail.by   allows to quickly lookup information by #blk.
 //
-//	min(tid) in δZtail is min(@at) at which */head/data is currently mmapped.
+//      min(rev) in δFtail is min(@at) at which head/data is currently mmapped.
 //
 // - when we receive a FUSE read(#blk) request to a file/head/data we process it as follows:
 //
 //   1. load blkdata for head/data[blk] @zconn.at .
-//      this also gives upper bound estimate of when the block was last changed:
+//
+//      while loading this also gives upper bound estimate of when the block
+//      was last changed:
 //
 //	  rev(blk) ≤ max(_.serial for _ in (ZBlk(#blk), all BTree/Bucket that lead to ZBlk))
 //
-//	XXX it is not exact because BTree/Bucket can change (e.g. rebalance)
+//	it is not exact because BTree/Bucket can change (e.g. rebalance)
 //	but still point to the same k->ZBlk.
-//	XXX if we maintain δBTree tail we can maybe get rev(blk) as exact?
+//
+//	we also use file.δFtail to find either exact blk revision:
+//
+//	  rev(blk) = max(file.δFtail.by(#blk) -> []rev↑)
+//
+//	or another upper bound if #blk ∉ δFtail:
+//
+//	  rev(blk) ≤ min(rev) is δFtail		; #blk ∉ δFtail
+//
+//
+//	below rev'(blk) is min(of the numbers found):
+//
+//	  rev(blk) ≤ rev'(blk)		rev'(blk) = min(^^^)
+//
 //
 //   2. for all client/addr@at mmappings of file/head/data:
 //
-//	- rev(blk) ≤ at: -> do nothing
-//	- rev(blk) > at:
-//	  - if blk ∉ mmapping.pinned -> do nothing
-//	  - client.remmap(addr[blk], file/@at/data)	XXX @at -> @revprev(blk) better?
-//							XXX @at -> @prevrev(file) even more better?
+//	- rev'(blk) ≤ at: -> do nothing
+//	- rev'(blk) > at:
+//	  - if blk ∈ mmapping.pinned -> do nothing
+//	  - rev = max(δFtail.by(#blk) : _ ≤ at)	|| at
+//	  - client.remmap(addr[blk], file/@rev/data)
 //	  - mmapping.pinned += blk
 //
 //	remmapping is done synchronously via ptrace.
@@ -317,8 +330,6 @@ package main
 //
 //	is maintained.
 //
-//	XXX δZ is consulted to find out which client needs such update?
-//
 //   3. blkdata is returned to kernel.
 //
 //   Thus a client that wants latest data on pagefault will get latest data,