parallelize the reading again

...it is not at all clear to me why this should work and the other
attempts would not. This makes me think this doesn't actually work,
but let's keep going and see what happens.

src/tilemaker.cpp

@@ -140,7 +140,7 @@ int main(const int argc, const char* argv[]) {
 			//
 			// ...but why does this die, and our very similar use immediately
 			// below does not?
-			//boost::asio::post(pool, [&]() {
+			boost::asio::post(pool, [&]() {
 				// Determine which tiles exist in this mbtiles file.
 				//
 				// This lets us optimize the case where only a single mbtiles has
@@ -149,8 +149,16 @@ int main(const int argc, const char* argv[]) {
 				//MBTiles mbtiles;
 				//mbtiles.openForReading(input->filename);
 				//mbtiles.populateTiles(input->zooms, input->bbox);
-				input->mbtiles.populateTiles(input->zooms, input->bbox);
-			//});
+				if (tlsTiles.empty()) {
+					for (const auto& input : inputs) {
+						tlsTiles.push_back(std::make_shared<MBTiles>());
+						tlsTiles.back()->openForReading(input->filename);
+					}
+				}
+
+				//input->mbtiles.populateTiles(input->zooms, input->bbox);
+				tlsTiles[input->index]->populateTiles(input->zooms, input->bbox);
+			});
 		}
 
 		pool.join();
@@ -208,11 +216,18 @@ int main(const int argc, const char* argv[]) {
 								//std::vector<char> old = matching[0]->mbtiles.readTile(zoom, x, y);
 
 								std::string buffer(old.data(), old.size());
+								// TODO: is this valid? We have a lock, but we'll access
+								// from different threads. This might be problematic because
+								// we cache the prepared statement.
 								merged.saveTile(zoom, x, y, &buffer, false);
 								continue;
 							}
 
-							//std::cout << "z=" << std::to_string(zoom) << " x=" << std::to_string(x) << " y=" << std::to_string(y) << " has " << std::to_string(matching.size()) << " tiles" << std::endl;
+							// Multiple mbtiles want to contribute a tile at this zxy.
+							// They'll all have disjoint layers, so decompress each tile
+							// and concatenate their contents to form the new tile.
+
+							// TODO: do this
 						}
 					}
 				}