components/nvs: handle more cases where sudden power off may happen

This commit fixes several issues with state handling in nvs::Page. It also adds extra consistency checks in nvs::PageManger initialization.
These changes were verified with a new long-running test ("test recovery from sudden poweroff"). This test works by repeatedly performing same pseudorandom sequence of calls to nvs_ APIs. Each time it repeats the sequence, it introduces a failure into one of flash operations (write or erase). So if one iteration of this test needs, say, 25000 flash operations, then this test will run 25000 iterations, each time introducing the failure point at different location.

components/nvs_flash/src/nvs_pagemanager.cpp

@@ -53,6 +53,58 @@ esp_err_t PageManager::load(uint32_t baseSector, uint32_t sectorCount)
         assert(mPageList.back().getSeqNumber(lastSeqNo) == ESP_OK);
         mSeqNumber = lastSeqNo + 1;
     }
+    
+    // if power went out after a new item for the given key was written,
+    // but before the old one was erased, we end up with a duplicate item
+    Page& lastPage = back();
+    size_t lastItemIndex = SIZE_MAX;
+    Item item;
+    size_t itemIndex = 0;
+    while (lastPage.findItem(Page::NS_ANY, ItemType::ANY, nullptr, itemIndex, item) == ESP_OK) {
+        itemIndex += item.span;
+        lastItemIndex = itemIndex;
+    }
+    
+    if (lastItemIndex != SIZE_MAX) {
+        auto last = PageManager::TPageListIterator(&lastPage);
+        for (auto it = begin(); it != last; ++it) {
+            if (it->eraseItem(item.nsIndex, item.datatype, item.key) == ESP_OK) {
+                break;
+            }
+        }
+    }
+
+    // check if power went out while page was being freed
+    for (auto it = begin(); it!= end(); ++it) {
+        if (it->state() == Page::PageState::FREEING) {
+            Page* newPage = &mPageList.back();
+            if(newPage->state() != Page::PageState::ACTIVE) {
+                auto err = activatePage();
+                if (err != ESP_OK) {
+                    return err;
+                }
+                newPage = &mPageList.back();
+            }
+            while (true) {
+                auto err = it->moveItem(*newPage);
+                if (err == ESP_ERR_NVS_NOT_FOUND) {
+                    break;
+                } else if (err != ESP_OK) {
+                    return err;
+                }
+            }
+            
+            auto err = it->erase();
+            if (err != ESP_OK) {
+                return err;
+            }
+            
+            Page* p = static_cast<Page*>(it);
+            mPageList.erase(it);
+            mFreePageList.push_back(p);
+            break;
+        }
+    }
 
     return ESP_OK;
 }