Batch up the db operations associated with adding new tracks

This is ostensibly yet another 'prepare for multithreaded updates'
commit, however it does actually save us another 60(!!) odd milliseconds
per track.

1 files changed, 75 insertions, 108 deletions

diff --git a/src/tangara/database/database.cpp b/src/tangara/database/database.cpp
index c543b941..aec661d9 100644
--- a/src/tangara/database/database.cpp
+++ b/src/tangara/database/database.cpp
@@ -352,11 +352,19 @@ auto Database::updateIndexes() -> void {
         // We couldn't read the tags for this track. Either they were
         // malformed, or perhaps the file is missing. Either way, tombstone
         // this record.
-        ESP_LOGW(kTag, "entombing missing #%lx", track->id);
+        ESP_LOGI(kTag, "entombing missing #%lx", track->id);
+
+        // Remove the indexes first, so that interrupted operations don't leave
+        // dangling index records.
         dbRemoveIndexes(track);
+
+        // Do the rest of the tombstoning as one atomic write.
+        leveldb::WriteBatch batch;
         track->is_tombstoned = true;
-        dbPutTrackData(*track);
-        db_->Delete(leveldb::WriteOptions{}, EncodePathKey(track->filepath));
+        batch.Put(EncodeDataKey(track->id), EncodeDataValue(*track));
+        batch.Delete(EncodePathKey(track->filepath));
+
+        db_->Write(leveldb::WriteOptions(), &batch);
         continue;
       }
 
@@ -370,12 +378,20 @@ auto Database::updateIndexes() -> void {
         // database.
         ESP_LOGI(kTag, "updating hash (%llx -> %llx)", track->tags_hash,
                  new_hash);
+
+        // Again, we remove the old index records first so has to avoid
+        // dangling references.
         dbRemoveIndexes(track);
 
+        // Atomically correct the hash + create the new index records.
+        leveldb::WriteBatch batch;
         track->tags_hash = new_hash;
-        dbIngestTagHashes(*tags, track->individual_tag_hashes);
-        dbPutTrackData(*track);
-        dbPutHash(new_hash, track->id);
+        dbIngestTagHashes(*tags, track->individual_tag_hashes, batch);
+
+        dbCreateIndexesForTrack(*track, *tags, batch);
+        batch.Put(EncodeDataKey(track->id), EncodeDataValue(*track));
+        batch.Put(EncodeHashKey(new_hash), EncodeHashValue(track->id));
+        db_->Write(leveldb::WriteOptions(), &batch);
       }
     }
   }
@@ -404,72 +420,56 @@ auto Database::updateIndexes() -> void {
       return;
     }
 
-    // Check for any existing record with the same hash.
+    // Check for any existing track with the same hash.
     uint64_t hash = tags->Hash();
-    std::string key = EncodeHashKey(hash);
-    std::optional<TrackId> existing_hash;
+    std::optional<TrackId> existing_id;
     std::string raw_entry;
-    if (db_->Get(leveldb::ReadOptions(), key, &raw_entry).ok()) {
-      existing_hash = ParseHashValue(raw_entry);
+    if (db_->Get(leveldb::ReadOptions(), EncodeHashKey(hash), &raw_entry)
+            .ok()) {
+      existing_id = ParseHashValue(raw_entry);
     }
 
-    std::pair<uint16_t, uint16_t> modified{info.fdate, info.ftime};
-    if (!existing_hash) {
-      // We've never met this track before! Or we have, but the entry is
-      // malformed. Either way, record this as a new track.
-      TrackId id = dbMintNewTrackId();
-      ESP_LOGD(kTag, "recording new 0x%lx", id);
+    std::shared_ptr<TrackData> data;
+    if (existing_id) {
+      // Do we have any existing data for this track? This could be the case if
+      // this is a tombstoned entry. In such as case, we want to reuse the
+      // previous TrackData so that any extra metadata is preserved.
+      data = dbGetTrackData(*existing_id);
+      if (!data) {
+        data = std::make_shared<TrackData>();
+        data->id = *existing_id;
+      } else if (data->filepath != path) {
+        ESP_LOGW(kTag, "hash collision: %s, %s, %s",
+                 tags->title().value_or("no title").c_str(),
+                 tags->artist().value_or("no artist").c_str(),
+                 tags->album().value_or("no album").c_str());
+        // Don't commit anything if there's a hash collision, since we're
+        // likely to make a big mess.
+        return;
+      }
+    } else {
       num_new_tracks++;
-
-      auto data = std::make_shared<TrackData>();
-      data->id = id;
-      data->filepath = path;
-      data->tags_hash = hash;
-      data->modified_at = modified;
-      dbIngestTagHashes(*tags, data->individual_tag_hashes);
-
-      dbPutTrackData(*data);
-      dbPutHash(hash, id);
-      auto t = std::make_shared<Track>(data, tags);
-      dbCreateIndexesForTrack(*t);
-      db_->Put(leveldb::WriteOptions{}, EncodePathKey(path),
-               TrackIdToBytes(id));
-      return;
+      data = std::make_shared<TrackData>();
+      data->id = dbMintNewTrackId();
     }
 
-    std::shared_ptr<TrackData> existing_data = dbGetTrackData(*existing_hash);
-    if (!existing_data) {
-      // We found a hash that matches, but there's no data record? Weird.
-      auto new_data = std::make_shared<TrackData>();
-      new_data->id = dbMintNewTrackId();
-      new_data->filepath = path;
-      new_data->tags_hash = hash;
-      new_data->modified_at = modified;
-      dbIngestTagHashes(*tags, new_data->individual_tag_hashes);
-      dbPutTrackData(*new_data);
-      auto t = std::make_shared<Track>(new_data, tags);
-      dbCreateIndexesForTrack(*t);
-      db_->Put(leveldb::WriteOptions{}, EncodePathKey(path),
-               TrackIdToBytes(new_data->id));
-      return;
-    }
+    // Make sure the file-based metadata on the TrackData is up to date.
+    data->filepath = path;
+    data->tags_hash = hash;
+    data->modified_at = {info.fdate, info.ftime};
 
-    if (existing_data->is_tombstoned) {
-      ESP_LOGI(kTag, "exhuming track %lu", existing_data->id);
-      existing_data->is_tombstoned = false;
-      existing_data->modified_at = modified;
-      dbPutTrackData(*existing_data);
-      auto t = std::make_shared<Track>(existing_data, tags);
-      dbCreateIndexesForTrack(*t);
-      db_->Put(leveldb::WriteOptions{}, EncodePathKey(path),
-               TrackIdToBytes(existing_data->id));
-    } else if (existing_data->filepath !=
-               std::pmr::string{path.data(), path.size()}) {
-      ESP_LOGW(kTag, "hash collision: %s, %s, %s",
-               tags->title().value_or("no title").c_str(),
-               tags->artist().value_or("no artist").c_str(),
-               tags->album().value_or("no album").c_str());
-    }
+    // Apply all the actual database changes as one atomic batch. This makes
+    // the whole 'new track' operation atomic, and also reduces the amount of
+    // lock contention when adding many tracks at once.
+    leveldb::WriteBatch batch;
+    dbIngestTagHashes(*tags, data->individual_tag_hashes, batch);
+
+    dbCreateIndexesForTrack(*data, *tags, batch);
+    batch.Put(EncodeDataKey(data->id), EncodeDataValue(*data));
+    batch.Put(EncodeHashKey(data->tags_hash), EncodeHashValue(data->id));
+    batch.Put(EncodePathKey(path), TrackIdToBytes(data->id));
+
+    db_->Write(leveldb::WriteOptions(), &batch);
   });
 
   uint64_t end_time = esp_timer_get_time();
@@ -536,22 +536,6 @@ auto Database::dbMintNewTrackId() -> TrackId {
   return next_track_id_++;
 }
 
-auto Database::dbEntomb(TrackId id, uint64_t hash) -> void {
-  std::string key = EncodeHashKey(hash);
-  std::string val = EncodeHashValue(id);
-  if (!db_->Put(leveldb::WriteOptions(), key, val).ok()) {
-    ESP_LOGE(kTag, "failed to entomb #%llx (id #%lx)", hash, id);
-  }
-}
-
-auto Database::dbPutTrackData(const TrackData& s) -> void {
-  std::string key = EncodeDataKey(s.id);
-  std::string val = EncodeDataValue(s);
-  if (!db_->Put(leveldb::WriteOptions(), key, val).ok()) {
-    ESP_LOGE(kTag, "failed to write data for #%lx", s.id);
-  }
-}
-
 auto Database::dbGetTrackData(TrackId id) -> std::shared_ptr<TrackData> {
   std::string key = EncodeDataKey(id);
   std::string raw_val;
@@ -562,33 +546,19 @@ auto Database::dbGetTrackData(TrackId id) -> std::shared_ptr<TrackData> {
   return ParseDataValue(raw_val);
 }
 
-auto Database::dbPutHash(const uint64_t& hash, TrackId i) -> void {
-  std::string key = EncodeHashKey(hash);
-  std::string val = EncodeHashValue(i);
-  if (!db_->Put(leveldb::WriteOptions(), key, val).ok()) {
-    ESP_LOGE(kTag, "failed to write hash for #%lx", i);
-  }
-}
-
-auto Database::dbGetHash(const uint64_t& hash) -> std::optional<TrackId> {
-  std::string key = EncodeHashKey(hash);
-  std::string raw_val;
-  if (!db_->Get(leveldb::ReadOptions(), key, &raw_val).ok()) {
-    ESP_LOGW(kTag, "no key found for hash #%llx", hash);
-    return {};
-  }
-  return ParseHashValue(raw_val);
+auto Database::dbCreateIndexesForTrack(const Track& track,
+                                       leveldb::WriteBatch& batch) -> void {
+  dbCreateIndexesForTrack(track.data(), track.tags(), batch);
 }
 
-auto Database::dbCreateIndexesForTrack(const Track& track) -> void {
+auto Database::dbCreateIndexesForTrack(const TrackData& data,
+                                       const TrackTags& tags,
+                                       leveldb::WriteBatch& batch) -> void {
   for (const IndexInfo& index : getIndexes()) {
-    leveldb::WriteBatch writes;
-    auto entries = Index(collator_, index, track);
+    auto entries = Index(collator_, index, data, tags);
     for (const auto& it : entries) {
-      writes.Put(EncodeIndexKey(it.first),
-                 {it.second.data(), it.second.size()});
+      batch.Put(EncodeIndexKey(it.first), {it.second.data(), it.second.size()});
     }
-    db_->Write(leveldb::WriteOptions(), &writes);
   }
 }
 
@@ -597,9 +567,8 @@ auto Database::dbRemoveIndexes(std::shared_ptr<TrackData> data) -> void {
   if (!tags) {
     return;
   }
-  Track track{data, tags};
   for (const IndexInfo& index : getIndexes()) {
-    auto entries = Index(collator_, index, track);
+    auto entries = Index(collator_, index, *data, *tags);
     for (auto it = entries.rbegin(); it != entries.rend(); it++) {
       auto key = EncodeIndexKey(it->first);
       auto status = db_->Delete(leveldb::WriteOptions{}, key);
@@ -626,16 +595,14 @@ auto Database::dbRemoveIndexes(std::shared_ptr<TrackData> data) -> void {
 }
 
 auto Database::dbIngestTagHashes(const TrackTags& tags,
-                                 std::pmr::unordered_map<Tag, uint64_t>& out)
-    -> void {
-  leveldb::WriteBatch batch{};
+                                 std::pmr::unordered_map<Tag, uint64_t>& out,
+                                 leveldb::WriteBatch& batch) -> void {
   for (const auto& tag : tags.allPresent()) {
     auto val = tags.get(tag);
     auto hash = tagHash(val);
     batch.Put(EncodeTagHashKey(hash), tagToString(val));
     out[tag] = hash;
   }
-  db_->Write(leveldb::WriteOptions{}, &batch);
 }
 
 auto Database::dbRecoverTagsFromHashes(