merge upstream caches via os.scandir + os.link#137
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Walking each upstream cache in Python with os.walk and copying every entry via shutil.copy is slow on deep dependency graphs and duplicates upstream content on every downstream action, blowing up exec-root size and saturating disk write bandwidth. This change reduces the wall time of some targets with deeper nested dependencies by 60%. But that varies greatly depending on the target. Replace the loop with a recursive os.scandir + os.link traversal. - os.scandir walks at C speed, much faster than os.walk on deep trees and avoids stat call per entry. - Hardlinks share inodes, so each downstream cache adds essentially zero unique disk blocks rather than re-copying transitive content.
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Walking each upstream cache in Python with os.walk and copying every entry via shutil.copy is slow on deep dependency graphs and duplicates upstream content on every downstream action, blowing up exec-root size and saturating disk write bandwidth.
This change reduces the wall time of some targets with deeper nested dependencies by 60%. But that varies greatly depending on the target.
Replace the loop with a recursive os.scandir + os.link traversal.