[OSV SCALIBR] Performance optimization in the trace package to redu…

…ce extractor calls when tracing origin layer. Previously, looping through the inventory would result in an `Extract` call. Now, the inventory results are cached via a local map.

PiperOrigin-RevId: 710624564

artifact/image/layerscanning/trace/trace.go

@@ -27,6 +27,12 @@ import (
 	scalibrfs "github.com/google/osv-scalibr/fs"
 )
 
+// locationAndIndex is a struct to represent a location and the index of the layer it was found in.
+type locationAndIndex struct {
+	location string
+	index    int
+}
+
 // PopulateLayerDetails populates the LayerDetails field of the inventory with the origin details
 // obtained by tracing the inventory in the image.
 //
@@ -46,17 +52,19 @@ import (
 // Note that a precondition of this algorithm is that the chain layers are ordered by order of
 // creation.
 func PopulateLayerDetails(ctx context.Context, inventory []*extractor.Inventory, chainLayers []scalibrImage.ChainLayer, config *filesystem.Config) {
-	layerDetailsList := []*extractor.LayerDetails{}
+	chainLayerDetailsList := []*extractor.LayerDetails{}
 
+	// Create list of layer details struct to be referenced by inventory.
 	for i, chainLayer := range chainLayers {
-		layerDetailsList = append(layerDetailsList, &extractor.LayerDetails{
+		chainLayerDetailsList = append(chainLayerDetailsList, &extractor.LayerDetails{
 			Index:       i,
 			DiffID:      chainLayer.Layer().DiffID(),
 			Command:     chainLayer.Layer().Command(),
 			InBaseImage: false,
 		})
 	}
 
+	// Helper function to update the extractor config.
 	updateExtractorConfig := func(filesToExtract []string, extractor filesystem.Extractor, chainFS scalibrfs.FS) {
 		config.Extractors = []filesystem.Extractor{extractor}
 		config.FilesToExtract = filesToExtract
@@ -67,11 +75,13 @@ func PopulateLayerDetails(ctx context.Context, inventory []*extractor.Inventory,
 		}
 	}
 
-	for _, inv := range inventory {
-		lastChainLayer := chainLayers[len(chainLayers)-1]
-		layerIndex := lastChainLayer.Index()
-		layerDetails := layerDetailsList[layerIndex]
+	// locationIndexToInventory is used as an inventory cache to avoid re-extracting the same
+	// inventory from a file multiple times.
+	locationIndexToInventory := map[locationAndIndex][]*extractor.Inventory{}
+	lastLayerIndex := len(chainLayers) - 1
 
+	for _, inv := range inventory {
+		layerDetails := chainLayerDetailsList[lastLayerIndex]
 		invExtractor, isFilesystemExtractor := inv.Extractor.(filesystem.Extractor)
 
 		// Only filesystem extractors are supported for layer scanning. Also, if the inventory has no
@@ -90,12 +100,26 @@ func PopulateLayerDetails(ctx context.Context, inventory []*extractor.Inventory,
 		for i := len(chainLayers) - 2; i >= 0; i-- {
 			oldChainLayer := chainLayers[i]
 
-			// Update the extractor config to use the files from the current layer.
-			updateExtractorConfig(inv.Locations, invExtractor, oldChainLayer.FS())
+			invLocationAndIndex := locationAndIndex{
+				location: inv.Locations[0],
+				index:    i,
+			}
+
+			var oldInventory []*extractor.Inventory
+			if cachedInventory, ok := locationIndexToInventory[invLocationAndIndex]; ok {
+				oldInventory = cachedInventory
+			} else {
+				// Update the extractor config to use the files from the current layer.
+				updateExtractorConfig(inv.Locations, invExtractor, oldChainLayer.FS())
 
-			oldInventory, _, err := filesystem.Run(ctx, config)
-			if err != nil {
-				break
+				var err error
+				oldInventory, _, err = filesystem.Run(ctx, config)
+				if err != nil {
+					break
+				}
+
+				// Cache the inventory for future use.
+				locationIndexToInventory[invLocationAndIndex] = oldInventory
 			}
 
 			foundPackage := false
@@ -108,7 +132,7 @@ func PopulateLayerDetails(ctx context.Context, inventory []*extractor.Inventory,
 
 			// If the inventory is not present in the old layer, then it was introduced in layer i+1.
 			if !foundPackage {
-				layerDetails = layerDetailsList[i+1]
+				layerDetails = chainLayerDetailsList[i+1]
 				foundOrigin = true
 				break
 			}
@@ -117,7 +141,7 @@ func PopulateLayerDetails(ctx context.Context, inventory []*extractor.Inventory,
 		// If the inventory is present in every layer, then it means it was introduced in the first
 		// layer.
 		if !foundOrigin {
-			layerDetails = layerDetailsList[0]
+			layerDetails = chainLayerDetailsList[0]
 		}
 		inv.LayerDetails = layerDetails
 	}