Planner: ms-queuedaemon: Synchronise thinking multiple walkers

If multiple walkers want to ask the same chan, we want to serialise
them.  This is actually straightforward:  Firstly, we arrrange that
each walker finishing a thought will prompt _all_ walkers to
reconsider whether they need to continue.  Then we can simply do
nothing if we want to a chan to think that another walker is already
waiting for; since that other walker will prompt us later.

Still no actual functional change because there is still only one
walker.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

diff --git a/ms-queuedaemon b/ms-queuedaemon
index 6e1ba0e7224d1397b5792cbd0acc7375eadcb3db..d51fd5274f9281ac8ee85c752c26173804ce9474 100755 (executable)
--- a/ms-queuedaemon
+++ b/ms-queuedaemon
@@ -216,6 +216,12 @@ proc runneeded-2-requeue {} {
 #  $w/thinking_after    timeout
 # all the $w/ are generally upvar'd by walker-globals
 
+proc walkers-perhaps-queue-steps {} {
+    foreach-walker w {
+       after idle queuerun-perhaps-step $w
+    }
+}
+
 proc plan-reset {w} {
     exec ./ms-planner -w$w reset < /dev/null
 }
@@ -226,7 +232,7 @@ proc queuerun-start {w} {
     log-event "$w queuerun-start"
     plan-reset $w
     set queue_running $queue
-    after idle queuerun-perhaps-step plan
+    walkers-perhaps-queue-steps
 }
 
 proc queuerun-perhaps-step {w} {
@@ -247,7 +253,9 @@ proc queuerun-perhaps-step {w} {
     set next [lindex $queue_running 0]
     set already [we-are-thinking $next]
     if {[llength $already]} {
-       error "next $next thinking $already but also want $w"
+       # $already will wake us via walkers-perhaps-queue-steps
+       log-event "$w queuerun-perhaps-step already $already"
+       return
     }
 
     set thinking $next
@@ -304,7 +312,7 @@ proc queuerun-step-done {w why} {
         unset thinking_after
     }
     unset thinking
-    after idle queuerun-perhaps-step $w
+    walkers-perhaps-queue-steps
 }
 
 proc queue-thoughts-timedout {w} {