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Better document the loop on "after" deps

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Add also a description of that loop in the general header
This commit is contained in:
Pierre Labastie 2021-12-19 17:24:52 +01:00
parent e0bbc6e6ac
commit a78cfc961e
1 changed files with 39 additions and 1 deletions
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BLFS/libs/func_dependencies
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@ -29,7 +29,9 @@
# package A depends on B with an "after" qualifier, and a package C depends #
# on A with a "before" qualifier, C may need B to be able to use A. So the #
# only safe way to consider "after" qualifiers is to consider that they are #
# "before" deps for any parent of the packages considered. #
# "before" deps for any parent of the packages considered. There is an #
# exception to that rule: if B depends on C (possibly through a chain of #
# several dependencies), then C should still be built before B. #
# We'll therefore have a 3 pass procedure. First build the set of nodes #
# reachable from the root set. Second, remove dangling edges (those pointing #
# to packages outside the node set), and move "after" edges to "before" edges #
@ -73,6 +75,42 @@
# - The weight of the edge leading to that node (for printing) #
# - The qualifier (for printing) #
# #
# Pass 2: graph transformation #
# ============================ #
# We now have three loops over nodes of the graph #
# Loop 1: Remove dead edges #
# ------------------------- #
# Since some nodes have not been created because the edge leading to them #
# had too high a weight, the edges leading to them have to be suppressed. #
# For each existing node file, we make a list of lines to remove by #
# testing whether the destination exists. We then remove the lines. #
# Another approach would be to make a temporary file and output only #
# lines that should stay, then rename the file. This would save a loop. #
# All in all it is an N*e process, where N is the number of nodes and e #
# the average number of edges originating from a node. #
# Loop 2: Treat "after" edges #
# --------------------------- #
# If a node is the origin of edges qualified as "after", we want the #
# nodes which are the destination of those edges to be built after #
# the origin node, but before any node that depend on the origin #
# node. For that, the general rule is to change: #
# P---b--->A---a--->D #
# to: #
# P---b--->Agroupxx---b--->A #
# | #
# ---b--->D #
# But there is a problem if D depends on P, possibly through a chain, #
# because we create a cycle which shouldn't exist. If this is the case, #
# we leave A as a dependency of P: #
# P---b--->A #
# #
# Agroupxx---b--->A #
# | #
# ---b--->D #
# Doing so, it may happen that Agroupxx has no parent. We then add #
# Agroupxx as a dependency of root. The problem with this algorithm is #
# the search for paths from D to A, which may be exponential in the #
# number of nodes in the graph. #
# TODO: document other passes #
# TODO: needs also to document the .tree files #
# TODO: The following is obsolete #