solver: fix issue with double merged edges#29
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"Merged edges" is an optimization in the solve graph where two different active LLB edges can be combined into one after cache key computation finds that they generated equivalent cache chain. In that case, one edge is released and is set to point to another. The reference count for the second one is increased. An issue was discovered where an edge that was already pointing to another edge became a target to the third one. The current implementation did not handle the case where an edge that already had a different target itself became a target edge as well. This resulted in an inconsistent graph state where edges that were thought to be released could get scheduled again. Instead of setting the same edge value to two different maps, the new logic is to chain the edges into a linked list that should handle multiple levels of targets. This slightly increases the number of lookups, but "merged edges" case is very rare anyway, and a couple of extra pointer dereferences do not affect build speed. Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
mikejholly
approved these changes
Oct 20, 2023
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"Merged edges" is an optimization in the solve graph where two different active LLB edges can be combined into one after cache key computation finds that they generated equivalent cache chain.
In that case, one edge is released and is set to point to another. The reference count for the second one is increased.
An issue was discovered where an edge that was already pointing to another edge became a target to the third one. The current implementation did not handle the case where an edge that already had a different target itself became a target edge as well. This resulted in an inconsistent graph state where edges that were thought to be released could get scheduled again.
Instead of setting the same edge value to two different maps, the new logic is to chain the edges into a linked list that should handle multiple levels of targets. This slightly increases the number of lookups, but "merged edges" case is very rare anyway, and a couple of extra pointer dereferences do not affect build speed.