Fix/expand prefix extractor testing in crash test (#9461)

Summary:
Changes in #9453 could trigger
```
stderr:
Error: prefixpercent is non-zero while prefix_size is not positive!
```

Pull Request resolved: #9461

Test Plan: run `make blackbox_crashtest` for a long time

Reviewed By: ajkr

Differential Revision: D33830751

Pulled By: pdillinger

fbshipit-source-id: be88377dcaa47e4bb7adb0347762639eff8f1476

tools/db_crashtest.py

@@ -87,7 +87,7 @@
     "partition_filters": lambda: random.randint(0, 1),
     "partition_pinning": lambda: random.randint(0, 3),
     "pause_background_one_in": 1000000,
-    "prefix_size" : -1,
+    "prefix_size" : lambda: random.choice([-1, 1, 5, 7, 8]),
     "prefixpercent": 5,
     "progress_reports": 0,
     "readpercent": 45,
@@ -236,8 +236,6 @@ def is_direct_io_supported(dbname):
     "max_background_compactions": 1,
     "max_bytes_for_level_base": 67108864,
     "memtablerep": "skip_list",
-    "prefixpercent": 0,
-    "readpercent": 50,
     "target_file_size_base": 16777216,
     "target_file_size_multiplier": 1,
     "test_batches_snapshots": 0,
@@ -393,6 +391,9 @@ def finalize_and_sanitize(src_params):
     if (dest_params.get("prefix_size") == -1 and
         dest_params.get("memtable_whole_key_filtering") == 0):
         dest_params["memtable_prefix_bloom_size_ratio"] = 0
+    if dest_params.get("prefix_size") == -1:
+        dest_params["readpercent"] += dest_params.get("prefixpercent", 20)
+        dest_params["prefixpercent"] = 0
     return dest_params
 
 def gen_cmd_params(args):