Generalise worker tasks, and centralise task priorities + stacks

Includes making the display driver use a worker task for flushes, so
that our double buffering actually does something useful /facepalm

18 files changed, 542 insertions, 390 deletions

diff --git a/src/audio/audio_task.cpp b/src/audio/audio_task.cpp
index b2a8062e..f0128bf3 100644
--- a/src/audio/audio_task.cpp
+++ b/src/audio/audio_task.cpp
@@ -35,157 +35,119 @@ namespace audio {
 namespace task {
 
 static const char* kTag = "task";
-static const std::size_t kStackSize = 24 * 1024;
-static const std::size_t kDrainStackSize = 1024;
 
-auto StartPipeline(Pipeline* pipeline, IAudioSink* sink) -> void {
-  // Newly created task will free this.
-  AudioTaskArgs* args = new AudioTaskArgs{.pipeline = pipeline, .sink = sink};
+void AudioTaskMain(std::unique_ptr<Pipeline> pipeline, IAudioSink* sink) {
+  std::optional<StreamInfo::Format> output_format;
+
+  std::vector<Pipeline*> elements = pipeline->GetIterationOrder();
+  std::size_t max_inputs =
+      (*std::max_element(elements.begin(), elements.end(),
+                         [](Pipeline const* first, Pipeline const* second) {
+                           return first->NumInputs() < second->NumInputs();
+                         }))
+          ->NumInputs();
+
+  // We need to be able to simultaneously map all of an element's inputs, plus
+  // its output. So preallocate that many ranges.
+  std::vector<MappableRegion<kPipelineBufferSize>> in_regions(max_inputs);
+  MappableRegion<kPipelineBufferSize> out_region;
+  std::for_each(in_regions.begin(), in_regions.end(),
+                [](const auto& region) { assert(region.is_valid); });
+  assert(out_region.is_valid);
+
+  // Each element has exactly one output buffer.
+  std::vector<HimemAlloc<kPipelineBufferSize>> buffers(elements.size());
+  std::vector<StreamInfo> buffer_infos(buffers.size());
+  std::for_each(buffers.begin(), buffers.end(),
+                [](const HimemAlloc<kPipelineBufferSize>& alloc) {
+                  assert(alloc.is_valid);
+                });
+
+  while (1) {
+    for (int i = 0; i < elements.size(); i++) {
+      std::vector<RawStream> raw_in_streams;
+      elements.at(i)->InStreams(&in_regions, &raw_in_streams);
+      RawStream raw_out_stream = elements.at(i)->OutStream(&out_region);
+
+      // Crop the input and output streams to the ranges that are safe to
+      // touch. For the input streams, this is the region that contains
+      // data. For the output stream, this is the region that does *not*
+      // already contain data.
+      std::vector<InputStream> in_streams;
+      std::for_each(raw_in_streams.begin(), raw_in_streams.end(),
+                    [&](RawStream& s) { in_streams.emplace_back(&s); });
+      OutputStream out_stream(&raw_out_stream);
+
+      elements.at(i)->OutputElement()->Process(in_streams, &out_stream);
+
+      std::for_each(in_regions.begin(), in_regions.end(),
+                    [](auto&& r) { r.Unmap(); });
+      out_region.Unmap();
+    }
 
-  ESP_LOGI(kTag, "starting audio pipeline task");
-  xTaskCreatePinnedToCore(&AudioTaskMain, "pipeline", kStackSize, args,
-                          kTaskPriorityAudioPipeline, NULL, 1);
-}
+    RawStream raw_sink_stream = elements.front()->OutStream(&out_region);
+    InputStream sink_stream(&raw_sink_stream);
 
-auto StartDrain(IAudioSink* sink) -> void {
-  auto command = new std::atomic<Command>(PLAY);
-  // Newly created task will free this.
-  AudioDrainArgs* drain_args = new AudioDrainArgs{
-      .sink = sink,
-      .command = command,
-  };
+    if (sink_stream.info().bytes_in_stream == 0) {
+      out_region.Unmap();
+      vTaskDelay(pdMS_TO_TICKS(100));
+      continue;
+    }
 
-  ESP_LOGI(kTag, "starting audio drain task");
-  xTaskCreate(&AudioDrainMain, "drain", kDrainStackSize, drain_args,
-              kTaskPriorityAudioDrain, NULL);
-}
+    if (!output_format || output_format != sink_stream.info().format) {
+      // The format of the stream within the sink stream has changed. We
+      // need to reconfigure the sink, but shouldn't do so until we've fully
+      // drained the current buffer.
+      if (xStreamBufferIsEmpty(sink->buffer())) {
+        ESP_LOGI(kTag, "reconfiguring dac");
+        output_format = sink_stream.info().format;
+        sink->Configure(*output_format);
+      }
+    }
 
-void AudioTaskMain(void* args) {
-  // Nest the body within an additional scope to ensure that destructors are
-  // called before the task quits.
-  {
-    AudioTaskArgs* real_args = reinterpret_cast<AudioTaskArgs*>(args);
-    std::unique_ptr<Pipeline> pipeline(real_args->pipeline);
-    IAudioSink* sink = real_args->sink;
-    delete real_args;
-
-    std::optional<StreamInfo::Format> output_format;
-
-    std::vector<Pipeline*> elements = pipeline->GetIterationOrder();
-    std::size_t max_inputs =
-        (*std::max_element(elements.begin(), elements.end(),
-                           [](Pipeline const* first, Pipeline const* second) {
-                             return first->NumInputs() < second->NumInputs();
-                           }))
-            ->NumInputs();
-
-    // We need to be able to simultaneously map all of an element's inputs, plus
-    // its output. So preallocate that many ranges.
-    std::vector<MappableRegion<kPipelineBufferSize>> in_regions(max_inputs);
-    MappableRegion<kPipelineBufferSize> out_region;
-    std::for_each(in_regions.begin(), in_regions.end(),
-                  [](const auto& region) { assert(region.is_valid); });
-    assert(out_region.is_valid);
-
-    // Each element has exactly one output buffer.
-    std::vector<HimemAlloc<kPipelineBufferSize>> buffers(elements.size());
-    std::vector<StreamInfo> buffer_infos(buffers.size());
-    std::for_each(buffers.begin(), buffers.end(),
-                  [](const HimemAlloc<kPipelineBufferSize>& alloc) {
-                    assert(alloc.is_valid);
-                  });
-
-    bool playing = true;
-    bool quit = false;
-    while (!quit) {
-      if (playing) {
-        for (int i = 0; i < elements.size(); i++) {
-          std::vector<RawStream> raw_in_streams;
-          elements.at(i)->InStreams(&in_regions, &raw_in_streams);
-          RawStream raw_out_stream = elements.at(i)->OutStream(&out_region);
-
-          // Crop the input and output streams to the ranges that are safe to
-          // touch. For the input streams, this is the region that contains
-          // data. For the output stream, this is the region that does *not*
-          // already contain data.
-          std::vector<InputStream> in_streams;
-          std::for_each(raw_in_streams.begin(), raw_in_streams.end(),
-                        [&](RawStream& s) { in_streams.emplace_back(&s); });
-          OutputStream out_stream(&raw_out_stream);
-
-          elements.at(i)->OutputElement()->Process(in_streams, &out_stream);
-
-          std::for_each(in_regions.begin(), in_regions.end(),
-                        [](auto&& r) { r.Unmap(); });
-          out_region.Unmap();
-        }
-
-        RawStream raw_sink_stream = elements.front()->OutStream(&out_region);
-        InputStream sink_stream(&raw_sink_stream);
-
-        if (sink_stream.info().bytes_in_stream == 0) {
-          out_region.Unmap();
-          vTaskDelay(pdMS_TO_TICKS(100));
-          continue;
-        }
-
-        if (!output_format || output_format != sink_stream.info().format) {
-          // The format of the stream within the sink stream has changed. We
-          // need to reconfigure the sink, but shouldn't do so until we've fully
-          // drained the current buffer.
-          if (xStreamBufferIsEmpty(sink->buffer())) {
-            ESP_LOGI(kTag, "reconfiguring dac");
-            output_format = sink_stream.info().format;
-            sink->Configure(*output_format);
-          }
-        }
-
-        // We've reconfigured the sink, or it was already configured correctly.
-        // Send through some data.
-        if (output_format == sink_stream.info().format &&
-            !std::holds_alternative<std::monostate>(*output_format)) {
-          // TODO: tune the delay on this, as it's currently the only way to
-          // throttle this task's CPU time. Maybe also hold off on the pipeline
-          // if the buffer is already close to full?
-          std::size_t sent = xStreamBufferSend(
-              sink->buffer(), sink_stream.data().data(),
-              sink_stream.data().size_bytes(), pdMS_TO_TICKS(10));
-          if (sent > 0) {
-            ESP_LOGI(kTag, "sunk %u bytes out of %u (%d %%)", sent,
-                     sink_stream.info().bytes_in_stream,
-                     (int)(((float)sent /
-                            (float)sink_stream.info().bytes_in_stream) *
-                           100));
-          }
-          sink_stream.consume(sent);
-        }
-
-        out_region.Unmap();
+    // We've reconfigured the sink, or it was already configured correctly.
+    // Send through some data.
+    if (output_format == sink_stream.info().format &&
+        !std::holds_alternative<std::monostate>(*output_format)) {
+      std::size_t sent =
+          xStreamBufferSend(sink->buffer(), sink_stream.data().data(),
+                            sink_stream.data().size_bytes(), 0);
+      if (sent > 0) {
+        ESP_LOGI(
+            kTag, "sunk %u bytes out of %u (%d %%)", sent,
+            sink_stream.info().bytes_in_stream,
+            (int)(((float)sent / (float)sink_stream.info().bytes_in_stream) *
+                  100));
       }
+      sink_stream.consume(sent);
     }
+
+    out_region.Unmap();
   }
-  vTaskDelete(NULL);
 }
 
 static std::byte sDrainBuf[8 * 1024];
 
-void AudioDrainMain(void* args) {
-  {
-    AudioDrainArgs* real_args = reinterpret_cast<AudioDrainArgs*>(args);
-    IAudioSink* sink = real_args->sink;
-    std::atomic<Command>* command = real_args->command;
-    delete real_args;
-
-    // TODO(jacqueline): implement PAUSE without busy-waiting.
-    while (*command != QUIT) {
-      std::size_t len = xStreamBufferReceive(sink->buffer(), sDrainBuf,
-                                             sizeof(sDrainBuf), portMAX_DELAY);
-      if (len > 0) {
-        sink->Send({sDrainBuf, len});
-      }
+void AudioDrainMain(IAudioSink* sink) {
+  while (1) {
+    std::size_t len = xStreamBufferReceive(sink->buffer(), sDrainBuf,
+                                           sizeof(sDrainBuf), portMAX_DELAY);
+    if (len > 0) {
+      sink->Send({sDrainBuf, len});
     }
   }
-  vTaskDelete(NULL);
+}
+
+auto StartPipeline(Pipeline* pipeline, IAudioSink* sink) -> void {
+  ESP_LOGI(kTag, "starting audio pipeline task");
+  tasks::StartPersistent<tasks::Type::kAudio>(
+      [=]() { AudioTaskMain(std::unique_ptr<Pipeline>(pipeline), sink); });
+}
+
+auto StartDrain(IAudioSink* sink) -> void {
+  ESP_LOGI(kTag, "starting audio drain task");
+  tasks::StartPersistent<tasks::Type::kAudioDrain>(
+      [=]() { AudioDrainMain(sink); });
 }
 
 }  // namespace task
diff --git a/src/audio/include/audio_task.hpp b/src/audio/include/audio_task.hpp
index a7b7a0fa..8269c8d4 100644
--- a/src/audio/include/audio_task.hpp
+++ b/src/audio/include/audio_task.hpp
@@ -15,20 +15,6 @@ namespace audio {
 
 namespace task {
 
-enum Command { PLAY, PAUSE, QUIT };
-
-struct AudioTaskArgs {
-  Pipeline* pipeline;
-  IAudioSink* sink;
-};
-struct AudioDrainArgs {
-  IAudioSink* sink;
-  std::atomic<Command>* command;
-};
-
-extern "C" void AudioTaskMain(void* args);
-extern "C" void AudioDrainMain(void* args);
-
 auto StartPipeline(Pipeline* pipeline, IAudioSink* sink) -> void;
 auto StartDrain(IAudioSink* sink) -> void;
 
diff --git a/src/database/CMakeLists.txt b/src/database/CMakeLists.txt
index 897bf029..c769fa33 100644
--- a/src/database/CMakeLists.txt
+++ b/src/database/CMakeLists.txt
@@ -1,7 +1,7 @@
 idf_component_register(
-  SRCS "env_esp.cpp" "database.cpp" "song.cpp" "db_task.cpp" "records.cpp" "file_gatherer.cpp" "tag_parser.cpp"
+  SRCS "env_esp.cpp" "database.cpp" "song.cpp" "records.cpp" "file_gatherer.cpp" "tag_parser.cpp"
   INCLUDE_DIRS "include"
-  REQUIRES "result" "span" "esp_psram" "fatfs" "libtags" "komihash" "cbor")
+  REQUIRES "result" "span" "esp_psram" "fatfs" "libtags" "komihash" "cbor" "tasks")
 
 target_compile_options(${COMPONENT_LIB} PRIVATE ${EXTRA_WARNINGS})
 
diff --git a/src/database/database.cpp b/src/database/database.cpp
index f5fe5240..65a500d9 100644
--- a/src/database/database.cpp
+++ b/src/database/database.cpp
@@ -18,13 +18,13 @@
 #include "leveldb/slice.h"
 #include "leveldb/write_batch.h"
 
-#include "db_task.hpp"
 #include "env_esp.hpp"
 #include "file_gatherer.hpp"
 #include "records.hpp"
 #include "result.hpp"
 #include "song.hpp"
 #include "tag_parser.hpp"
+#include "tasks.hpp"
 
 namespace database {
 
@@ -62,33 +62,33 @@ auto Database::Open(IFileGatherer* gatherer, ITagParser* parser)
     return cpp::fail(DatabaseError::ALREADY_OPEN);
   }
 
-  if (!StartDbTask()) {
-    return cpp::fail(DatabaseError::ALREADY_OPEN);
-  }
-
-  return RunOnDbTask<cpp::result<Database*, DatabaseError>>(
-             [=]() -> cpp::result<Database*, DatabaseError> {
-               leveldb::DB* db;
-               leveldb::Cache* cache = leveldb::NewLRUCache(24 * 1024);
-               leveldb::Options options;
-               options.env = sEnv.env();
-               options.create_if_missing = true;
-               options.write_buffer_size = 48 * 1024;
-               options.max_file_size = 32;
-               options.block_cache = cache;
-               options.block_size = 512;
-
-               auto status = leveldb::DB::Open(options, "/.db", &db);
-               if (!status.ok()) {
-                 delete cache;
-                 ESP_LOGE(kTag, "failed to open db, status %s",
-                          status.ToString().c_str());
-                 return cpp::fail(FAILED_TO_OPEN);
-               }
-
-               ESP_LOGI(kTag, "Database opened successfully");
-               return new Database(db, cache, gatherer, parser);
-             })
+  std::shared_ptr<tasks::Worker> worker(
+      tasks::Worker::Start<tasks::Type::kDatabase>());
+  leveldb::sBackgroundThread = std::weak_ptr<tasks::Worker>(worker);
+  return worker
+      ->Dispatch<cpp::result<Database*, DatabaseError>>(
+          [&]() -> cpp::result<Database*, DatabaseError> {
+            leveldb::DB* db;
+            leveldb::Cache* cache = leveldb::NewLRUCache(24 * 1024);
+            leveldb::Options options;
+            options.env = sEnv.env();
+            options.create_if_missing = true;
+            options.write_buffer_size = 48 * 1024;
+            options.max_file_size = 32;
+            options.block_cache = cache;
+            options.block_size = 512;
+
+            auto status = leveldb::DB::Open(options, "/.db", &db);
+            if (!status.ok()) {
+              delete cache;
+              ESP_LOGE(kTag, "failed to open db, status %s",
+                       status.ToString().c_str());
+              return cpp::fail(FAILED_TO_OPEN);
+            }
+
+            ESP_LOGI(kTag, "Database opened successfully");
+            return new Database(db, cache, gatherer, parser, worker);
+          })
       .get();
 }
 
@@ -101,9 +101,11 @@ auto Database::Destroy() -> void {
 Database::Database(leveldb::DB* db,
                    leveldb::Cache* cache,
                    IFileGatherer* file_gatherer,
-                   ITagParser* tag_parser)
+                   ITagParser* tag_parser,
+                   std::shared_ptr<tasks::Worker> worker)
     : db_(db),
       cache_(cache),
+      worker_task_(worker),
       file_gatherer_(file_gatherer),
       tag_parser_(tag_parser) {}
 
@@ -113,12 +115,13 @@ Database::~Database() {
   delete db_;
   delete cache_;
 
-  QuitDbTask();
+  leveldb::sBackgroundThread = std::weak_ptr<tasks::Worker>();
+
   sIsDbOpen.store(false);
 }
 
 auto Database::Update() -> std::future<void> {
-  return RunOnDbTask<void>([&]() -> void {
+  return worker_task_->Dispatch<void>([&]() -> void {
     // Stage 1: verify all existing songs are still valid.
     ESP_LOGI(kTag, "verifying existing songs");
     const leveldb::Snapshot* snapshot = db_->GetSnapshot();
@@ -219,7 +222,7 @@ auto Database::Update() -> std::future<void> {
 }
 
 auto Database::GetSongs(std::size_t page_size) -> std::future<Result<Song>*> {
-  return RunOnDbTask<Result<Song>*>([=, this]() -> Result<Song>* {
+  return worker_task_->Dispatch<Result<Song>*>([=, this]() -> Result<Song>* {
     Continuation<Song> c{.iterator = nullptr,
                          .prefix = CreateDataPrefix().data,
                          .start_key = CreateDataPrefix().data,
@@ -232,21 +235,22 @@ auto Database::GetSongs(std::size_t page_size) -> std::future<Result<Song>*> {
 
 auto Database::GetDump(std::size_t page_size)
     -> std::future<Result<std::string>*> {
-  return RunOnDbTask<Result<std::string>*>([=, this]() -> Result<std::string>* {
-    Continuation<std::string> c{.iterator = nullptr,
-                                .prefix = "",
-                                .start_key = "",
-                                .forward = true,
-                                .was_prev_forward = true,
-                                .page_size = page_size};
-    return dbGetPage(c);
-  });
+  return worker_task_->Dispatch<Result<std::string>*>(
+      [=, this]() -> Result<std::string>* {
+        Continuation<std::string> c{.iterator = nullptr,
+                                    .prefix = "",
+                                    .start_key = "",
+                                    .forward = true,
+                                    .was_prev_forward = true,
+                                    .page_size = page_size};
+        return dbGetPage(c);
+      });
 }
 
 template <typename T>
 auto Database::GetPage(Continuation<T>* c) -> std::future<Result<T>*> {
   Continuation<T> copy = *c;
-  return RunOnDbTask<Result<T>*>(
+  return worker_task_->Dispatch<Result<T>*>(
       [=, this]() -> Result<T>* { return dbGetPage(copy); });
 }
 
diff --git a/src/database/db_task.cpp b/src/database/db_task.cpp
deleted file mode 100644
index 5b4b34b5..00000000
--- a/src/database/db_task.cpp
+++ /dev/null
@@ -1,91 +0,0 @@
-#include "db_task.hpp"
-
-#include <functional>
-
-#include "esp_heap_caps.h"
-#include "freertos/FreeRTOS.h"
-#include "freertos/portmacro.h"
-#include "freertos/projdefs.h"
-#include "freertos/queue.h"
-#include "freertos/task.h"
-
-namespace database {
-
-static const std::size_t kDbStackSize = 256 * 1024;
-static StaticTask_t sDbStaticTask;
-static StackType_t* sDbStack = nullptr;
-
-static std::atomic<bool> sTaskRunning(false);
-static QueueHandle_t sWorkQueue;
-
-struct WorkItem {
-  std::function<void(void)>* fn;
-  bool quit;
-};
-
-auto SendToDbTask(std::function<void(void)> fn) -> void {
-  WorkItem item{
-      .fn = new std::function<void(void)>(fn),
-      .quit = false,
-  };
-  xQueueSend(sWorkQueue, &item, portMAX_DELAY);
-}
-
-template <>
-auto RunOnDbTask(std::function<void(void)> fn) -> std::future<void> {
-  std::shared_ptr<std::promise<void>> promise =
-      std::make_shared<std::promise<void>>();
-  SendToDbTask([=]() {
-    std::invoke(fn);
-    promise->set_value();
-  });
-  return promise->get_future();
-}
-
-void DatabaseTaskMain(void* args) {
-  while (true) {
-    WorkItem item;
-    if (xQueueReceive(sWorkQueue, &item, portMAX_DELAY)) {
-      if (item.fn != nullptr) {
-        std::invoke(*item.fn);
-        delete item.fn;
-      }
-      if (item.quit) {
-        break;
-      }
-    }
-  }
-  vQueueDelete(sWorkQueue);
-  sTaskRunning.store(false);
-  vTaskDelete(NULL);
-}
-
-auto StartDbTask() -> bool {
-  if (sTaskRunning.exchange(true)) {
-    return false;
-  }
-  if (sDbStack == nullptr) {
-    sDbStack = reinterpret_cast<StackType_t*>(
-        heap_caps_malloc(kDbStackSize, MALLOC_CAP_SPIRAM));
-  }
-  sWorkQueue = xQueueCreate(8, sizeof(WorkItem));
-  xTaskCreateStatic(&DatabaseTaskMain, "DB", kDbStackSize, NULL, 1, sDbStack,
-                    &sDbStaticTask);
-  return true;
-}
-
-auto QuitDbTask() -> void {
-  if (!sTaskRunning.load()) {
-    return;
-  }
-  WorkItem item{
-      .fn = nullptr,
-      .quit = true,
-  };
-  xQueueSend(sWorkQueue, &item, portMAX_DELAY);
-  while (sTaskRunning.load()) {
-    vTaskDelay(pdMS_TO_TICKS(1));
-  }
-}
-
-}  // namespace database
diff --git a/src/database/env_esp.cpp b/src/database/env_esp.cpp
index 71d4fcea..3bc68984 100644
--- a/src/database/env_esp.cpp
+++ b/src/database/env_esp.cpp
@@ -29,10 +29,12 @@
 #include "leveldb/slice.h"
 #include "leveldb/status.h"
 
-#include "db_task.hpp"
+#include "tasks.hpp"
 
 namespace leveldb {
 
+std::weak_ptr<tasks::Worker> sBackgroundThread;
+
 std::string ErrToStr(FRESULT err) {
   switch (err) {
     case FR_OK:
@@ -455,8 +457,11 @@ EspEnv::EspEnv() {}
 void EspEnv::Schedule(
     void (*background_work_function)(void* background_work_arg),
     void* background_work_arg) {
-  database::SendToDbTask(
-      [=]() { std::invoke(background_work_function, background_work_arg); });
+  auto worker = sBackgroundThread.lock();
+  if (worker) {
+    worker->Dispatch<void>(
+        [=]() { std::invoke(background_work_function, background_work_arg); });
+  }
 }
 
 }  // namespace leveldb
diff --git a/src/database/include/database.hpp b/src/database/include/database.hpp
index da0ed083..ce2ca9fe 100644
--- a/src/database/include/database.hpp
+++ b/src/database/include/database.hpp
@@ -19,6 +19,7 @@
 #include "result.hpp"
 #include "song.hpp"
 #include "tag_parser.hpp"
+#include "tasks.hpp"
 
 namespace database {
 
@@ -90,6 +91,8 @@ class Database {
   leveldb::DB* db_;
   leveldb::Cache* cache_;
 
+  std::shared_ptr<tasks::Worker> worker_task_;
+
   // Not owned.
   IFileGatherer* file_gatherer_;
   ITagParser* tag_parser_;
@@ -97,7 +100,8 @@ class Database {
   Database(leveldb::DB* db,
            leveldb::Cache* cache,
            IFileGatherer* file_gatherer,
-           ITagParser* tag_parser);
+           ITagParser* tag_parser,
+           std::shared_ptr<tasks::Worker> worker);
 
   auto dbMintNewSongId() -> SongId;
   auto dbEntomb(SongId song, uint64_t hash) -> void;
diff --git a/src/database/include/db_task.hpp b/src/database/include/db_task.hpp
deleted file mode 100644
index 39f932b0..00000000
--- a/src/database/include/db_task.hpp
+++ /dev/null
@@ -1,25 +0,0 @@
-#pragma once
-
-#include <functional>
-#include <future>
-#include <memory>
-
-namespace database {
-
-auto StartDbTask() -> bool;
-auto QuitDbTask() -> void;
-
-auto SendToDbTask(std::function<void(void)> fn) -> void;
-
-template <typename T>
-auto RunOnDbTask(std::function<T(void)> fn) -> std::future<T> {
-  std::shared_ptr<std::promise<T>> promise =
-      std::make_shared<std::promise<T>>();
-  SendToDbTask([=]() { promise->set_value(std::invoke(fn)); });
-  return promise->get_future();
-}
-
-template <>
-auto RunOnDbTask(std::function<void(void)> fn) -> std::future<void>;
-
-}  // namespace database
diff --git a/src/database/include/env_esp.hpp b/src/database/include/env_esp.hpp
index cf5a20e1..6a415ce6 100644
--- a/src/database/include/env_esp.hpp
+++ b/src/database/include/env_esp.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <memory>
 #include <mutex>
 #include <set>
 #include <string>
@@ -7,8 +8,12 @@
 #include "leveldb/env.h"
 #include "leveldb/status.h"
 
+#include "tasks.hpp"
+
 namespace leveldb {
 
+extern std::weak_ptr<tasks::Worker> sBackgroundThread;
+
 // Tracks the files locked by EspEnv::LockFile().
 //
 // We maintain a separate set instead of relying on fcntl(F_SETLK) because
diff --git a/src/drivers/CMakeLists.txt b/src/drivers/CMakeLists.txt
index 413e1ea0..30b002a0 100644
--- a/src/drivers/CMakeLists.txt
+++ b/src/drivers/CMakeLists.txt
@@ -2,5 +2,5 @@ idf_component_register(
   SRCS "touchwheel.cpp" "dac.cpp" "gpio_expander.cpp" "battery.cpp" "storage.cpp" "i2c.cpp"
   "spi.cpp" "display.cpp" "display_init.cpp" "driver_cache.cpp" "samd.cpp"
   INCLUDE_DIRS "include"
-  REQUIRES "esp_adc" "fatfs" "result" "lvgl" "span")
+  REQUIRES "esp_adc" "fatfs" "result" "lvgl" "span" "tasks")
 target_compile_options(${COMPONENT_LIB} PRIVATE ${EXTRA_WARNINGS})
diff --git a/src/drivers/display.cpp b/src/drivers/display.cpp
index f8594a5a..15c0af75 100644
--- a/src/drivers/display.cpp
+++ b/src/drivers/display.cpp
@@ -21,6 +21,7 @@
 #include "display_init.hpp"
 #include "gpio_expander.hpp"
 #include "soc/soc.h"
+#include "tasks.hpp"
 
 static const char* kTag = "DISPLAY";
 
@@ -49,6 +50,19 @@ static const int kDisplayBufferSize = (kDisplayWidth * kDisplayHeight) / 10;
 DMA_ATTR static lv_color_t sBuffer1[kDisplayBufferSize];
 DMA_ATTR static lv_color_t sBuffer2[kDisplayBufferSize];
 
+struct RenderTaskArgs {
+  std::atomic<bool>* quit;
+  QueueHandle_t work_queue;
+};
+
+struct FlushArgs {
+  lv_disp_drv_t* driver;
+  const lv_area_t* area;
+  lv_color_t* color_map;
+};
+
+void RenderMain(void* raw_args);
+
 namespace drivers {
 
 /*
@@ -138,7 +152,9 @@ auto Display::create(GpioExpander* expander,
 }
 
 Display::Display(GpioExpander* gpio, spi_device_handle_t handle)
-    : gpio_(gpio), handle_(handle) {}
+    : gpio_(gpio),
+      handle_(handle),
+      worker_task_(tasks::Worker::Start<tasks::Type::kUiFlush>()) {}
 
 Display::~Display() {}
 
@@ -225,31 +241,51 @@ void Display::SendTransaction(TransactionType type,
 void Display::OnLvglFlush(lv_disp_drv_t* disp_drv,
                           const lv_area_t* area,
                           lv_color_t* color_map) {
-  // Ideally we want to complete a single flush as quickly as possible, so grab
-  // the bus for this entire transaction sequence.
-  spi_device_acquire_bus(handle_, portMAX_DELAY);
-
-  // First we need to specify the rectangle of the display we're writing into.
-  uint16_t data[2] = {0, 0};
-
-  data[0] = SPI_SWAP_DATA_TX(area->x1, 16);
-  data[1] = SPI_SWAP_DATA_TX(area->x2, 16);
-  SendCommandWithData(displays::ST77XX_CASET, reinterpret_cast<uint8_t*>(data),
-                      4);
-
-  data[0] = SPI_SWAP_DATA_TX(area->y1, 16);
-  data[1] = SPI_SWAP_DATA_TX(area->y2, 16);
-  SendCommandWithData(displays::ST77XX_RASET, reinterpret_cast<uint8_t*>(data),
-                      4);
+  // area is stack-allocated, so it isn't safe to reference from the flush
+  // thread.
+  lv_area_t area_copy = *area;
+  worker_task_->Dispatch<void>([=, this]() {
+    // Ideally we want to complete a single flush as quickly as possible, so
+    // grab the bus for this entire transaction sequence.
+    spi_device_acquire_bus(handle_, portMAX_DELAY);
+
+    // First we need to specify the rectangle of the display we're writing into.
+    uint16_t data[2] = {0, 0};
+
+    data[0] = SPI_SWAP_DATA_TX(area_copy.x1, 16);
+    data[1] = SPI_SWAP_DATA_TX(area_copy.x2, 16);
+    SendCommandWithData(displays::ST77XX_CASET,
+                        reinterpret_cast<uint8_t*>(data), 4);
+
+    data[0] = SPI_SWAP_DATA_TX(area_copy.y1, 16);
+    data[1] = SPI_SWAP_DATA_TX(area_copy.y2, 16);
+    SendCommandWithData(displays::ST77XX_RASET,
+                        reinterpret_cast<uint8_t*>(data), 4);
+
+    // Now send the pixels for this region.
+    uint32_t size = lv_area_get_width(area) * lv_area_get_height(area);
+    SendCommandWithData(displays::ST77XX_RAMWR,
+                        reinterpret_cast<uint8_t*>(color_map), size * 2);
+
+    spi_device_release_bus(handle_);
+
+    lv_disp_flush_ready(&driver_);
+  });
+}
 
-  // Now send the pixels for this region.
-  uint32_t size = lv_area_get_width(area) * lv_area_get_height(area);
-  SendCommandWithData(displays::ST77XX_RAMWR,
-                      reinterpret_cast<uint8_t*>(color_map), size * 2);
+void RenderMain(void* raw_args) {
+  RenderTaskArgs* args = reinterpret_cast<RenderTaskArgs*>(raw_args);
+  QueueHandle_t queue = args->work_queue;
+  std::atomic<bool>* quit = args->quit;
+  delete args;
 
-  spi_device_release_bus(handle_);
+  while (!quit->load()) {
+    // TODO: flush data here! Yay speed.
+  }
 
-  lv_disp_flush_ready(&driver_);
+  vQueueDelete(queue);
+  delete quit;
+  vTaskDelete(NULL);
 }
 
 }  // namespace drivers
diff --git a/src/drivers/include/display.hpp b/src/drivers/include/display.hpp
index 9e4a0224..04bdf669 100644
--- a/src/drivers/include/display.hpp
+++ b/src/drivers/include/display.hpp
@@ -6,6 +6,7 @@
 #include "driver/spi_master.h"
 #include "lvgl/lvgl.h"
 #include "result.hpp"
+#include "tasks.hpp"
 
 #include "display_init.hpp"
 #include "gpio_expander.hpp"
@@ -37,6 +38,8 @@ class Display {
   GpioExpander* gpio_;
   spi_device_handle_t handle_;
 
+  std::unique_ptr<tasks::Worker> worker_task_;
+
   lv_disp_draw_buf_t buffers_;
   lv_disp_drv_t driver_;
   lv_disp_t* display_ = nullptr;
diff --git a/src/main/main.cpp b/src/main/main.cpp
index 29ac2c7f..bd56eb87 100644
--- a/src/main/main.cpp
+++ b/src/main/main.cpp
@@ -77,9 +77,7 @@ extern "C" void app_main(void) {
   std::shared_ptr<drivers::TouchWheel> touchwheel =
       drivers->AcquireTouchWheel();
 
-  std::atomic<bool> lvgl_quit;
-  TaskHandle_t lvgl_task_handle;
-  ui::StartLvgl(drivers.get(), &lvgl_quit, &lvgl_task_handle);
+  ui::StartLvgl(drivers.get());
 
   std::unique_ptr<audio::AudioPlayback> playback;
   if (storage) {
diff --git a/src/tasks/CMakeLists.txt b/src/tasks/CMakeLists.txt
index 0503d293..f7d7244f 100644
--- a/src/tasks/CMakeLists.txt
+++ b/src/tasks/CMakeLists.txt
@@ -1,2 +1,2 @@
-idf_component_register(SRCS "tasks.cpp" INCLUDE_DIRS ".")
+idf_component_register(SRCS "tasks.cpp" INCLUDE_DIRS "." REQUIRES "span")
 target_compile_options(${COMPONENT_LIB} PRIVATE ${EXTRA_WARNINGS})
diff --git a/src/tasks/tasks.cpp b/src/tasks/tasks.cpp
index b9fce7ec..0d9d7881 100644
--- a/src/tasks/tasks.cpp
+++ b/src/tasks/tasks.cpp
@@ -1,5 +1,204 @@
 #include "tasks.hpp"
+#include <functional>
+#include "esp_heap_caps.h"
+#include "freertos/FreeRTOS.h"
+#include "freertos/portmacro.h"
 
-const UBaseType_t kTaskPriorityLvgl = 4;
-const UBaseType_t kTaskPriorityAudioPipeline = 5;
-const UBaseType_t kTaskPriorityAudioDrain = 6;
+namespace tasks {
+
+template <Type t>
+auto Name() -> std::string;
+
+template <>
+auto Name<Type::kUi>() -> std::string {
+  return "LVGL";
+}
+template <>
+auto Name<Type::kUiFlush>() -> std::string {
+  return "DISPLAY";
+}
+template <>
+auto Name<Type::kAudio>() -> std::string {
+  return "AUDIO";
+}
+template <>
+auto Name<Type::kAudioDrain>() -> std::string {
+  return "DRAIN";
+}
+template <>
+auto Name<Type::kDatabase>() -> std::string {
+  return "DB";
+}
+
+template <Type t>
+auto AllocateStack() -> cpp::span<StackType_t>;
+
+// Decoders run on the audio task, and these sometimes require a fairly large
+// amount of stack space.
+template <>
+auto AllocateStack<Type::kAudio>() -> cpp::span<StackType_t> {
+  std::size_t size = 32 * 1024;
+  return {static_cast<StackType_t*>(heap_caps_malloc(size, MALLOC_CAP_DEFAULT)),
+          size};
+}
+template <>
+auto AllocateStack<Type::kAudioDrain>() -> cpp::span<StackType_t> {
+  std::size_t size = 1024;
+  return {static_cast<StackType_t*>(heap_caps_malloc(size, MALLOC_CAP_DEFAULT)),
+          size};
+}
+// LVGL requires only a relatively small stack. However, it can be allocated in
+// PSRAM so we give it a bit of headroom for safety.
+template <>
+auto AllocateStack<Type::kUi>() -> cpp::span<StackType_t> {
+  std::size_t size = 16 * 1024;
+  return {static_cast<StackType_t*>(heap_caps_malloc(size, MALLOC_CAP_DEFAULT)),
+          size};
+}
+// UI flushes *must* be done from internal RAM. Thankfully, there is very little
+// stack required to perform them, and the amount of stack needed is fixed.
+template <>
+auto AllocateStack<Type::kUiFlush>() -> cpp::span<StackType_t> {
+  std::size_t size = 1024;
+  return {static_cast<StackType_t*>(heap_caps_malloc(size, MALLOC_CAP_DEFAULT)),
+          size};
+}
+// Leveldb is designed for non-embedded use cases, where stack space isn't so
+// much of a concern. It therefore uses an eye-wateringly large amount of stack.
+template <>
+auto AllocateStack<Type::kDatabase>() -> cpp::span<StackType_t> {
+  std::size_t size = 256 * 1024;
+  return {static_cast<StackType_t*>(heap_caps_malloc(size, MALLOC_CAP_SPIRAM)),
+          size};
+}
+
+// 2048 bytes in internal ram
+// 302 KiB in external ram.
+
+/*
+ * Please keep the priorities below in descending order for better readability.
+ */
+
+template <Type t>
+auto Priority() -> UBaseType_t;
+
+// Realtime audio is the entire point of this device, so give this task the
+// highest priority.
+template <>
+auto Priority<Type::kAudio>() -> UBaseType_t {
+  return 10;
+}
+template <>
+auto Priority<Type::kAudioDrain>() -> UBaseType_t {
+  return 10;
+}
+// After audio issues, UI jank is the most noticeable kind of scheduling-induced
+// slowness that the user is likely to notice or care about. Therefore we place
+// this task directly below audio in terms of priority.
+template <>
+auto Priority<Type::kUi>() -> UBaseType_t {
+  return 9;
+}
+// UI flushing should use the same priority as the UI task, so as to maximise
+// the chance of the happy case: one of our cores is writing to the screen,
+// whilst the other is simultaneously preparing the next buffer to be flushed.
+template <>
+auto Priority<Type::kUiFlush>() -> UBaseType_t {
+  return 9;
+}
+// Database interactions are all inherently async already, due to their
+// potential for disk access. The user likely won't notice or care about a
+// couple of ms extra delay due to scheduling, so give this task the lowest
+// priority.
+template <>
+auto Priority<Type::kDatabase>() -> UBaseType_t {
+  return 8;
+}
+
+template <Type t>
+auto WorkerQueueSize() -> std::size_t;
+
+template <>
+auto WorkerQueueSize<Type::kDatabase>() -> std::size_t {
+  return 8;
+}
+
+template <>
+auto WorkerQueueSize<Type::kUiFlush>() -> std::size_t {
+  return 2;
+}
+
+auto PersistentMain(void* fn) -> void {
+  auto* function = reinterpret_cast<std::function<void(void)>*>(fn);
+  std::invoke(*function);
+  assert("persistent task quit!" == 0);
+  vTaskDelete(NULL);
+}
+
+auto Worker::Main(void* instance) {
+  Worker* i = reinterpret_cast<Worker*>(instance);
+  while (1) {
+    WorkItem item;
+    if (xQueueReceive(i->queue_, &item, portMAX_DELAY)) {
+      if (item.quit) {
+        break;
+      } else if (item.fn != nullptr) {
+        std::invoke(*item.fn);
+        delete item.fn;
+      }
+    }
+  }
+  i->is_task_running_.store(false);
+  i->is_task_running_.notify_all();
+  // Wait for the instance's destructor to delete this task. We do this instead
+  // of just deleting ourselves so that it's 100% certain that it's safe to
+  // delete or reuse this task's stack.
+  while (1) {
+    vTaskDelay(portMAX_DELAY);
+  }
+}
+
+Worker::Worker(const std::string& name,
+               cpp::span<StackType_t> stack,
+               std::size_t queue_size,
+               UBaseType_t priority)
+    : stack_(stack.data()),
+      queue_(xQueueCreate(queue_size, sizeof(WorkItem))),
+      is_task_running_(true),
+      task_buffer_(),
+      task_(xTaskCreateStatic(&Main,
+                              name.c_str(),
+                              stack.size(),
+                              this,
+                              priority,
+                              stack_,
+                              &task_buffer_)) {}
+
+Worker::~Worker() {
+  WorkItem item{
+      .fn = nullptr,
+      .quit = true,
+  };
+  xQueueSend(queue_, &item, portMAX_DELAY);
+  is_task_running_.wait(true);
+  vTaskDelete(task_);
+  free(stack_);
+}
+
+template <>
+auto Worker::Dispatch(const std::function<void(void)>& fn)
+    -> std::future<void> {
+  std::shared_ptr<std::promise<void>> promise =
+      std::make_shared<std::promise<void>>();
+  WorkItem item{
+      .fn = new std::function<void(void)>([=]() {
+        std::invoke(fn);
+        promise->set_value();
+      }),
+      .quit = false,
+  };
+  xQueueSend(queue_, &item, portMAX_DELAY);
+  return promise->get_future();
+}
+
+}  // namespace tasks
diff --git a/src/tasks/tasks.hpp b/src/tasks/tasks.hpp
index 47668aea..9f37131e 100644
--- a/src/tasks/tasks.hpp
+++ b/src/tasks/tasks.hpp
@@ -1,7 +1,107 @@
 #pragma once
 
+#include <atomic>
+#include <functional>
+#include <future>
+#include <memory>
+#include <string>
+
+#include "freertos/FreeRTOS.h"
 #include "freertos/portmacro.h"
+#include "freertos/projdefs.h"
+#include "freertos/queue.h"
+#include "freertos/task.h"
+#include "span.hpp"
+
+namespace tasks {
+
+/*
+ * Enumeration of every task (basically a thread) started within the firmware.
+ * These are centralised so that it is easier to reason about the relative
+ * priorities of tasks, as well as the amount and location of memory allocated
+ * to each one.
+ */
+enum class Type {
+  // The main UI task. This runs the LVGL main loop.
+  kUi,
+  // Task for flushing graphics buffers to the display.
+  kUiFlush,
+  // The main audio pipeline task.
+  kAudio,
+  // Task for flushing PCM samples to the current output.
+  kAudioDrain,
+  // Task for running database queries.
+  kDatabase,
+};
+
+template <Type t>
+auto Name() -> std::string;
+template <Type t>
+auto AllocateStack() -> cpp::span<StackType_t>;
+template <Type t>
+auto Priority() -> UBaseType_t;
+template <Type t>
+auto WorkerQueueSize() -> std::size_t;
+
+auto PersistentMain(void* fn) -> void;
+
+template <Type t>
+auto StartPersistent(const std::function<void(void)>& fn) -> void {
+  StaticTask_t* task_buffer = new StaticTask_t;
+  cpp::span<StackType_t> stack = AllocateStack<t>();
+  xTaskCreateStatic(&PersistentMain, Name<t>().c_str(), stack.size(),
+                    new std::function<void(void)>(fn), Priority<t>(),
+                    stack.data(), task_buffer);
+}
+
+class Worker {
+ private:
+  Worker(const std::string& name,
+         cpp::span<StackType_t> stack,
+         std::size_t queue_size,
+         UBaseType_t priority);
+
+  StackType_t* stack_;
+  QueueHandle_t queue_;
+  std::atomic<bool> is_task_running_;
+  StaticTask_t task_buffer_;
+  TaskHandle_t task_;
+
+  struct WorkItem {
+    std::function<void(void)>* fn;
+    bool quit;
+  };
+
+ public:
+  template <Type t>
+  static auto Start() -> Worker* {
+    return new Worker(Name<t>(), AllocateStack<t>(), WorkerQueueSize<t>(),
+                      Priority<t>());
+  }
+
+  static auto Main(void* instance);
+
+  /*
+   * Schedules the given function to be executed on the worker task, and
+   * asynchronously returns the result as a future.
+   */
+  template <typename T>
+  auto Dispatch(const std::function<T(void)>& fn) -> std::future<T> {
+    std::shared_ptr<std::promise<T>> promise =
+        std::make_shared<std::promise<T>>();
+    WorkItem item{
+        .fn = new std::function([=]() { promise->set_value(std::invoke(fn)); }),
+        .quit = false,
+    };
+    xQueueSend(queue_, &item, portMAX_DELAY);
+    return promise->get_future();
+  }
+
+  ~Worker();
+};
+
+/* Specialisation of Evaluate for functions that return nothing. */
+template <>
+auto Worker::Dispatch(const std::function<void(void)>& fn) -> std::future<void>;
 
-extern const UBaseType_t kTaskPriorityLvgl;
-extern const UBaseType_t kTaskPriorityAudioPipeline;
-extern const UBaseType_t kTaskPriorityAudioDrain;
+}  // namespace tasks
diff --git a/src/ui/include/lvgl_task.hpp b/src/ui/include/lvgl_task.hpp
index ca3fc771..b129e329 100644
--- a/src/ui/include/lvgl_task.hpp
+++ b/src/ui/include/lvgl_task.hpp
@@ -10,8 +10,6 @@
 
 namespace ui {
 
-auto StartLvgl(drivers::DriverCache* drivers,
-               std::atomic<bool>* quit,
-               TaskHandle_t* handle) -> bool;
+auto StartLvgl(drivers::DriverCache* drivers) -> void;
 
 }  // namespace ui
diff --git a/src/ui/lvgl_task.cpp b/src/ui/lvgl_task.cpp
index 12dfd34e..0e96cd41 100644
--- a/src/ui/lvgl_task.cpp
+++ b/src/ui/lvgl_task.cpp
@@ -23,6 +23,7 @@
 #include "misc/lv_color.h"
 #include "misc/lv_style.h"
 #include "misc/lv_timer.h"
+#include "tasks.hpp"
 #include "widgets/lv_label.h"
 
 #include "display.hpp"
@@ -37,24 +38,13 @@ auto tick_hook(TimerHandle_t xTimer) -> void {
   lv_tick_inc(1);
 }
 
-struct LvglArgs {
-  drivers::DriverCache* drivers;
-  std::atomic<bool>* quit;
-};
-
-void LvglMain(void* voidArgs) {
-  LvglArgs* args = reinterpret_cast<LvglArgs*>(voidArgs);
-  drivers::DriverCache* drivers = args->drivers;
-  std::atomic<bool>* quit = args->quit;
-  delete args;
-
+void LvglMain(drivers::DriverCache* drivers) {
   {
     ESP_LOGI(kTag, "init lvgl");
     lv_init();
 
     // LVGL has been initialised, so we can now start reporting ticks to it.
-    TimerHandle_t tick_timer =
-        xTimerCreate("lv_tick", pdMS_TO_TICKS(1), pdTRUE, NULL, &tick_hook);
+    xTimerCreate("lv_tick", pdMS_TO_TICKS(1), pdTRUE, NULL, &tick_hook);
 
     ESP_LOGI(kTag, "init display");
     std::shared_ptr<drivers::Display> display = drivers->AcquireDisplay();
@@ -72,37 +62,15 @@ void LvglMain(void* voidArgs) {
     lv_obj_center(label);
     lv_scr_load(label);
 
-    while (!quit->load()) {
+    while (1) {
       lv_timer_handler();
       vTaskDelay(pdMS_TO_TICKS(10));
     }
-
-    // TODO(robin? daniel?): De-init the UI stack here.
-    lv_obj_del(label);
-    lv_style_reset(&style);
-
-    xTimerDelete(tick_timer, portMAX_DELAY);
-
-    lv_deinit();
   }
-
-  vTaskDelete(NULL);
 }
 
-static const size_t kLvglStackSize = 8 * 1024;
-static StaticTask_t sLvglTaskBuffer = {};
-static StackType_t sLvglStack[kLvglStackSize] = {0};
-
-auto StartLvgl(drivers::DriverCache* drivers,
-               std::atomic<bool>* quit,
-               TaskHandle_t* handle) -> bool {
-  LvglArgs* args = new LvglArgs();
-  args->drivers = drivers;
-  args->quit = quit;
-
-  return xTaskCreateStaticPinnedToCore(&LvglMain, "LVGL", kLvglStackSize,
-                                       reinterpret_cast<void*>(args), 1,
-                                       sLvglStack, &sLvglTaskBuffer, 1);
+auto StartLvgl(drivers::DriverCache* drivers) -> void {
+  tasks::StartPersistent<tasks::Type::kUi>([=]() { LvglMain(drivers); });
 }
 
 }  // namespace ui