fix(bpm-bench): concurrency issues (#519)

The return value of `FetchPage` can potentially be `nullptr` (e.g.,
when the BP is busy): add necessary checks.

The reads and writes on individual pages happen concurrently: add
necessary synchronization.

Remove redundant `page_idx` addition (the addition result is not used).

Closes #518

Author: CuriousGeorgiy

tools/bpm_bench/bpm_bench.cpp

@@ -168,12 +168,17 @@ auto main(int argc, char **argv) -> int {
 
       while (!metrics.ShouldFinish()) {
         auto *page = bpm->FetchPage(page_ids[page_idx], AccessType::Scan);
+        if (page == nullptr) {
+          continue;
+        }
 
         char &ch = page->GetData()[page_idx % 1024];
+        page->WLatch();
         ch += 1;
         if (ch == 0) {
           ch = 1;
         }
+        page->WUnlatch();
 
         bpm->UnpinPage(page->GetPageId(), true, AccessType::Scan);
         page_idx = (page_idx + 1) % BUSTUB_PAGE_CNT;
@@ -197,14 +202,18 @@ auto main(int argc, char **argv) -> int {
       while (!metrics.ShouldFinish()) {
         auto page_idx = dist(gen);
         auto *page = bpm->FetchPage(page_ids[page_idx], AccessType::Get);
+        if (page == nullptr) {
+          continue;
+        }
 
+        page->RLatch();
         char ch = page->GetData()[page_idx % 1024];
+        page->RUnlatch();
         if (ch == 0) {
           throw std::runtime_error("invalid data");
         }
 
         bpm->UnpinPage(page->GetPageId(), false, AccessType::Get);
-        page_idx += 1;
         metrics.Tick();
         metrics.Report();
       }