Class Taskmaster

Validate the content of the pending_children set. Assert if an
internal error is found.

This function is used strictly for debugging the taskmaster by
checking that no invariants are violated. It is not used in
normal operation.

The pending_children set is used to detect cycles in the
dependency graph. We call a "pending child" a child that is
found in the "pending" state when checking the dependencies of
its parent node.

A pending child can occur when the Taskmaster completes a loop
through a cycle. For example, lets imagine a graph made of
three node (A, B and C) making a cycle. The evaluation starts
at node A. The taskmaster first consider whether node A's
child B is up-to-date. Then, recursively, node B needs to
check whether node C is up-to-date. This leaves us with a
dependency graph looking like:

                              Next candidate                                                               Node A (Pending) --> Node B(Pending) --> Node C (NoState)
        ^                                     |
        |                                     |
        +-------------------------------------+

Now, when the Taskmaster examines the Node C's child Node A,
it finds that Node A is in the "pending" state. Therefore,
Node A is a pending child of node C.

Pending children indicate that the Taskmaster has potentially
loop back through a cycle. We say potentially because it could
also occur when a DAG is evaluated in parallel. For example,
consider the following graph:


Node A (Pending) --> Node B(Pending) --> Node C (Pending) --> ...
        |                                     ^
        |                                     |
        +----------> Node D (NoState) --------+
                          /
          Next candidate /

The Taskmaster first evaluates the nodes A, B, and C and
starts building some children of node C. Assuming, that the
maximum parallel level has not been reached, the Taskmaster
will examine Node D. It will find that Node C is a pending
child of Node D.

In summary, evaluating a graph with a cycle will always
involve a pending child at one point. A pending child might
indicate either a cycle or a diamond-shaped DAG. Only a
fraction of the nodes ends-up being a "pending child" of
another node. This keeps the pending_children set small in
practice.

We can differentiate between the two cases if we wait until
the end of the build. At this point, all the pending children
nodes due to a diamond-shaped DAG will have been properly
built (or will have failed to build). But, the pending
children involved in a cycle will still be in the pending
state.

The taskmaster removes nodes from the pending_children set as
soon as a pending_children node moves out of the pending
state. This also helps to keep the pending_children set small.

Finds the next node that is ready to be built.

This is *the* main guts of the DAG walk.  We loop through the
list of candidates, looking for something that has no un-built
children (i.e., that is a leaf Node or has dependencies that are
all leaf Nodes or up-to-date).  Candidate Nodes are re-scanned
(both the target Node itself and its sources, which are always
scanned in the context of a given target) to discover implicit
dependencies.  A Node that must wait for some children to be
built will be put back on the candidates list after the children
have finished building.  A Node that has been put back on the
candidates list in this way may have itself (or its sources)
re-scanned, in order to handle generated header files (e.g.) and
the implicit dependencies therein.

Note that this method does not do any signature calculation or
up-to-date check itself.  All of that is handled by the Task
class.  This is purely concerned with the dependency graph walk.