[Xen-changelog] [xen stable-4.8] oxenstored: do not commit read-only transactions

[patchbot@xxxxxxx]

commit 84ee808e363887910984b3eb443466ce42e8010f
Author:     Thomas Sanders <thomas.sanders@xxxxxxxxxx>
AuthorDate: Fri Mar 24 16:16:10 2017 +0000
Commit:     Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CommitDate: Wed Apr 5 15:10:42 2017 +0100

    oxenstored: do not commit read-only transactions
    
    The packet telling us to end the transaction has always carried an
    argument telling us whether to commit.
    
    If the transaction made no modifications to the tree, now we ignore
    that argument and do not commit: it is just a waste of effort.
    
    This makes read-only transactions immune to conflicts, and means that
    we do not need to store any of their details in the history that is
    used for assigning blame for conflicts.
    
    We count a transaction as a read-only transaction only if it contains
    no operations that modified the tree.
    
    This means that (for example) a transaction that creates a new node
    then deletes it would NOT count as read-only, even though it makes no
    change overall. A more sophisticated algorithm could judge the
    transaction based on comparison of its initial and final states, but
    this would add complexity and computational cost.
    
    Reported-by: Juergen Gross <jgross@xxxxxxxx>
    Signed-off-by: Thomas Sanders <thomas.sanders@xxxxxxxxxx>
    Reviewed-by: Jonathan Davies <jonathan.davies@xxxxxxxxxx>
---
 tools/ocaml/xenstored/process.ml     | 1 +
 tools/ocaml/xenstored/transaction.ml | 1 +
 2 files changed, 2 insertions(+)

diff --git a/tools/ocaml/xenstored/process.ml b/tools/ocaml/xenstored/process.ml
index b56e3fc..adfc7a4 100644
--- a/tools/ocaml/xenstored/process.ml
+++ b/tools/ocaml/xenstored/process.ml
@@ -393,6 +393,7 @@ let do_transaction_end con t domains cons data =
                | x :: _   -> raise (Invalid_argument x)
                | _        -> raise Invalid_Cmd_Args
                in
+       let commit = commit && not (Transaction.is_read_only t) in
        let success =
                let commit = if commit then Some (fun con trans -> 
transaction_replay con trans domains cons) else None in
                History.end_transaction t con (Transaction.get_id t) commit in
diff --git a/tools/ocaml/xenstored/transaction.ml 
b/tools/ocaml/xenstored/transaction.ml
index edd1178..8f95301 100644
--- a/tools/ocaml/xenstored/transaction.ml
+++ b/tools/ocaml/xenstored/transaction.ml
@@ -128,6 +128,7 @@ let make ?(internal=false) id store =
 let get_store t = t.store
 let get_paths t = t.paths
 
+let is_read_only t = t.paths = []
 let add_wop t ty path = t.paths <- (ty, path) :: t.paths
 let add_operation ~perm t request response =
        if !Define.maxrequests >= 0
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8

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