Make the benchmark fixture reusable, since building it is the expensive part

I wrote in PERFORMANCE.md that twenty thousand files are an argument for building the fixture once and keeping it, and then deleted it after the run - twice. The script now makes keeping it the default: create leaves a folder alone when it already holds at least the requested number of files, prints what it found, and still re-grants storage because a reinstall clears app-ops. --fresh rebuilds. Verified by running --count 300 twice and watching the second call reuse.

tools/benchmark-fixture.sh

@@ -23,6 +23,10 @@ count=1000
 # benchmark uses by default, and 5 000 or 20 000 is the same run with a bigger folder:
 #
 #   ./tools/benchmark-fixture.sh --count 20000
+#
+# Creating is idempotent: a folder that already holds at least `--count` files is left alone, because
+# building twenty thousand files through a phone's storage layer takes about half an hour and the fixture
+# is worth keeping between runs. `--fresh` rebuilds it anyway.
 if [ "${1:-}" = "--count" ]; then
     count=${2:?"--count needs a number"}
     case "$count" in
@@ -31,22 +35,35 @@ if [ "${1:-}" = "--count" ]; then
     shift 2
 fi
 
+fresh=0
+if [ "${1:-}" = "--fresh" ]; then
+    fresh=1
+    shift
+fi
+
 case "${1:-create}" in
     --remove)
         adb shell "rm -rf $directory"
         echo "benchmark-fixture: removed $directory"
         ;;
     create)
-        # One shell loop with builtin redirection: no fork per file, so a thousand files take a moment
-        # rather than a minute.
+        existing=$(adb shell "ls $directory 2>/dev/null | wc -l" | tr -d '\r ')
+        if [ "$fresh" -eq 0 ] && [ "${existing:-0}" -ge "$count" ]; then
+            echo "benchmark-fixture: reusing $directory ($existing files, $count asked for)"
+        else
+            # One shell loop with builtin redirection: no fork per file, so a thousand files take a
+            # moment rather than a minute.
             adb shell "rm -rf $directory; mkdir -p $directory; \
                 i=0; while [ \$i -lt $count ]; do : > $directory/file-\$i.txt; i=\$((i+1)); done"
+            echo "benchmark-fixture: $count files in $directory"
+        fi
         # The benchmark build is not the debug build, so it starts with no storage grant; granting it
         # here is what lets the run be unattended. No `--uid`: that flag takes a uid, not a package
         # name, and passing a package there grants nothing while still reporting that something was
-        # set — which is exactly the kind of half-truth that costs an afternoon.
+        # set — which is exactly the kind of half-truth that costs an afternoon. Re-granted even when the
+        # fixture is reused, because a reinstall of the app clears its app-ops.
         adb shell "appops set $package MANAGE_EXTERNAL_STORAGE allow"
-        echo "benchmark-fixture: $count files in $directory, storage granted to $package"
+        echo "benchmark-fixture: storage granted to $package"
         ;;
     *)
         echo "usage: $0 [--remove]" >&2