Make provision for db_stress to work with a pre-existing dir

Summary: The crash_test depends on db_stress to work with pre-existing dir

Test Plan: make db_stress; Run db_stress with 'destroy_db_initially=0'

Reviewers: vamsi, dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10041

tools/db_stress.cc

@@ -62,6 +62,9 @@ static bool FLAGS_verify_before_write = false;
 // Print histogram of operation timings
 static bool FLAGS_histogram = false;
 
+// Destroys the database dir before start if this is true
+static bool FLAGS_destroy_db_initially = true;
+
 static bool FLAGS_verbose = false;
 
 // Number of bytes to buffer in memtable before compacting
@@ -472,6 +475,7 @@ class StressTest {
                        : nullptr),
         db_(nullptr),
         num_times_reopened_(0) {
+    if (FLAGS_destroy_db_initially) {
       std::vector<std::string> files;
       FLAGS_env->GetChildren(FLAGS_db, &files);
       for (unsigned int i = 0; i < files.size(); i++) {
@@ -481,6 +485,7 @@ class StressTest {
       }
       DestroyDB(FLAGS_db, Options());
     }
+  }
 
   ~StressTest() {
     delete db_;
@@ -1001,6 +1006,9 @@ int main(int argc, char** argv) {
     } else if (sscanf(argv[i], "--histogram=%d%c", &n, &junk) == 1 &&
                (n == 0 || n == 1)) {
       FLAGS_histogram = n;
+    } else if (sscanf(argv[i], "--destroy_db_initially=%d%c", &n, &junk) == 1 &&
+               (n == 0 || n == 1)) {
+      FLAGS_destroy_db_initially = n;
     } else if (sscanf(argv[i], "--verify_before_write=%d%c", &n, &junk) == 1 &&
                (n == 0 || n == 1)) {
       FLAGS_verify_before_write = n;