bitbake: runqueue: Enable dynamic task adjustment to hash equivalency

There is a compelling usecase for tasks being able to notify runqueue
that their "unihash" has changed. When this is recieved, the hashes of
all subsequent tasks should be recomputed and their new hashes checked
against existing setscene validity. Any newly available setscene tasks
should then be executed.

Making this work effectively needs several pieces. An event is added
which the cooker listen for. If a new hash becomes available it can
send an event to notify of this.

When such an event is seen, hash recomputations are made. A setscene
task can't be run until all the tasks it "covers" are stopped. The
notion of "holdoff" tasks is therefore added, these are removed from
the buildable list with the assumption that some setscene task will
run and cover them.

The workers need to be notified when taskhashes change to update their
own internal siggen data stores. A new worker command is added to do this
which will affect all newly spawned worker processes from that worker.

An example workflow which tests this code is:

Configuration:
BB_SIGNATURE_HANDLER = "OEEquivHash"
SSTATE_HASHEQUIV_SERVER = "http://localhost:8686"

$ bitbake-hashserv &
$ bitbake automake-native
$ bitbake autoconf-native automake-native -c clean
$ bitbake m4-native -c install -f
$ bitbake automake-native

with the test being whether automake-native is installed from sstate.

(Bitbake rev: 1f630fdf0260db08541d3ca9f25f852931c19905)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/bin/bitbake-worker

@@ -234,6 +234,8 @@ def fork_off_task(cfg, data, databuilder, workerdata, fn, task, taskname, taskha
                     the_data.setVar(varname, value)
 
                 bb.parse.siggen.set_taskdata(workerdata["sigdata"])
+                if "newhashes" in workerdata:
+                    bb.parse.siggen.set_taskhashes(workerdata["newhashes"])
                 ret = 0
 
                 the_data = bb_cache.loadDataFull(fn, appends)
@@ -377,6 +379,7 @@ class BitbakeWorker(object):
                 self.handle_item(b"cookerconfig", self.handle_cookercfg)
                 self.handle_item(b"extraconfigdata", self.handle_extraconfigdata)
                 self.handle_item(b"workerdata", self.handle_workerdata)
+                self.handle_item(b"newtaskhashes", self.handle_newtaskhashes)
                 self.handle_item(b"runtask", self.handle_runtask)
                 self.handle_item(b"finishnow", self.handle_finishnow)
                 self.handle_item(b"ping", self.handle_ping)
@@ -416,6 +419,9 @@ class BitbakeWorker(object):
         for mc in self.databuilder.mcdata:
             self.databuilder.mcdata[mc].setVar("PRSERV_HOST", self.workerdata["prhost"])
 
+    def handle_newtaskhashes(self, data):
+        self.workerdata["newhashes"] = pickle.loads(data)
+
     def handle_ping(self, _):
         workerlog_write("Handling ping\n")