Stop reading the mount table for every single-file source (#5257)

Signed-off-by: Darragh McGurk <120114728+dmcg310@users.noreply.github.com>

syft/internal/fileresolver/file_indexer.go

@@ -29,11 +29,15 @@ func newFileIndexer(path, base string, visitors ...PathIndexVisitor) *fileIndexe
 		base:  base,
 		tree:  filetree.New(),
 		index: filetree.NewIndex(),
+		// Unlike the directory indexer there is no skipPathsByMountTypeAndName here: it exists
+		// to keep a directory walk out of /proc, /sys and similar, and to do so it reads the
+		// whole mount table on construction. A file source indexes exactly one file and its
+		// parent, so there is nothing to walk into, and the mount table read is paid for
+		// every file source with nothing in return.
 		pathIndexVisitors: append(
 			[]PathIndexVisitor{
 				requireFileInfo,
 				disallowByFileType,
-				skipPathsByMountTypeAndName(path),
 			},
 			visitors...,
 		),

syft/internal/fileresolver/file_indexer_bench_test.go

@@ -0,0 +1,51 @@
+package fileresolver
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/moby/sys/mountinfo"
+)
+
+// BenchmarkNewFromFile is the cost of building a resolver for a single-file source, which is
+// paid once per file by callers that catalog files individually. The default case is
+// NewFromFile as shipped. The with-mount-skipper case attaches skipPathsByMountTypeAndName
+// the way the file indexer used to, so the difference is what reading the mount table for
+// every file source cost. That cost scales with the mount table, so its size is reported as
+// the mounts metric: expect a bigger gap on a container host than on a laptop.
+//
+//	go test ./syft/internal/fileresolver/ -run '^$' -bench NewFromFile -benchmem -count 6 | tee bench.txt
+//	benchstat -col /variant bench.txt
+func BenchmarkNewFromFile(b *testing.B) {
+	path := filepath.Join(b.TempDir(), "file.txt")
+	if err := os.WriteFile(path, []byte("hello\n"), 0o644); err != nil {
+		b.Fatal(err)
+	}
+	mounts, err := mountinfo.GetMounts(nil)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	b.Run("variant=default", func(b *testing.B) {
+		for b.Loop() {
+			if _, err := NewFromFile(path); err != nil {
+				b.Fatal(err)
+			}
+		}
+		b.ReportMetric(float64(len(mounts)), "mounts")
+	})
+
+	b.Run("variant=with-mount-skipper", func(b *testing.B) {
+		for b.Loop() {
+			r, err := newFromFileWithoutIndex(path, skipPathsByMountTypeAndName(path))
+			if err != nil {
+				b.Fatal(err)
+			}
+			if err := r.buildIndex(); err != nil {
+				b.Fatal(err)
+			}
+		}
+		b.ReportMetric(float64(len(mounts)), "mounts")
+	})
+}