Package SCons :: Module Taskmaster :: Class Taskmaster
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Class Taskmaster

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object --+
         |
        Taskmaster

The Taskmaster for walking the dependency DAG.
Instance Methods [hide private]
 
__init__(self, targets=[], tasker=None, order=None, trace=None)
x.__init__(...) initializes x; see help(type(x)) for signature
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find_next_candidate(self)
Returns the next candidate Node for (potential) evaluation.
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no_next_candidate(self)
Stops Taskmaster processing by not returning a next candidate.
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_validate_pending_children(self)
Validate the content of the pending_children set. Assert if an internal error is found.
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trace_message(self, message) source code
 
trace_node(self, node) source code
 
_find_next_ready_node(self)
Finds the next node that is ready to be built.
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next_task(self)
Returns the next task to be executed.
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will_not_build(self, nodes, node_func=<function <lambda> at 0x9529b1c>)
Perform clean-up about nodes that will never be built. Invokes a user defined function on all of these nodes (including all of their parents).
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stop(self)
Stops the current build completely.
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cleanup(self)
Check for dependency cycles.
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Inherited from object: __delattr__, __format__, __getattribute__, __hash__, __new__, __reduce__, __reduce_ex__, __repr__, __setattr__, __sizeof__, __str__, __subclasshook__

Properties [hide private]

Inherited from object: __class__

Method Details [hide private]

__init__(self, targets=[], tasker=None, order=None, trace=None)
(Constructor)

source code 
x.__init__(...) initializes x; see help(type(x)) for signature
Overrides: object.__init__
(inherited documentation)

find_next_candidate(self)

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Returns the next candidate Node for (potential) evaluation.

The candidate list (really a stack) initially consists of all of the top-level (command line) targets provided when the Taskmaster was initialized. While we walk the DAG, visiting Nodes, all the children that haven't finished processing get pushed on to the candidate list. Each child can then be popped and examined in turn for whether their children are all up-to-date, in which case a Task will be created for their actual evaluation and potential building.

Here is where we also allow candidate Nodes to alter the list of Nodes that should be examined. This is used, for example, when invoking SCons in a source directory. A source directory Node can return its corresponding build directory Node, essentially saying, "Hey, you really need to build this thing over here instead."

no_next_candidate(self)

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Stops Taskmaster processing by not returning a next candidate.

Note that we have to clean-up the Taskmaster candidate list because the cycle detection depends on the fact all nodes have been processed somehow.

_validate_pending_children(self)

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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.

_find_next_ready_node(self)

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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.

next_task(self)

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Returns the next task to be executed.

This simply asks for the next Node to be evaluated, and then wraps it in the specific Task subclass with which we were initialized.