[idd] clipboard: pipeline remote file reads

Add two protocol-sized read-ahead buffers to each active clipboard
file stream. Fetch the next remote block on a worker while the caller
or CopyTo destination consumes the current block.

Serialize stream operations and invalidate queued work on seeks and
failures. Join the worker before releasing the remote dataset lease.

idd/LGIddHelper/CClipboardFiles.cpp

@@ -24,6 +24,8 @@
 #include <strsafe.h>
 
 #include <algorithm>
+#include <array>
+#include <condition_variable>
 #include <cstring>
 #include <cwchar>
 #include <exception>
@@ -32,6 +34,8 @@
 #include <new>
 #include <set>
 #include <stdexcept>
+#include <system_error>
+#include <thread>
 #include <unordered_map>
 #include <utility>
 
@@ -521,13 +525,41 @@ namespace
   class CClipboardFileStream final : public IStream
   {
   private:
+    static constexpr size_t PREFETCH_BLOCKS = 2U;
+
+    enum class PrefetchState
+    {
+      EMPTY,
+      QUEUED,
+      FILLING,
+      READY
+    };
+
+    struct PrefetchBlock
+    {
+      std::vector<uint8_t> data;
+      uint64_t offset     = 0;
+      ULONG length        = 0;
+      HRESULT result      = S_OK;
+      uint64_t generation = 0;
+      PrefetchState state = PrefetchState::EMPTY;
+    };
+
     std::atomic<ULONG> m_refs { 1 };
     ClipboardRemoteFileEntry m_entry;
     std::shared_ptr<CClipboardFileDatasetLease> m_lease;
+    std::mutex m_operationLock;
     std::mutex m_lock;
-    uint64_t m_offset = 0;
-    std::vector<uint8_t> m_cache;
-    uint64_t m_cacheOffset = 0;
+    std::condition_variable m_prefetchWork;
+    std::condition_variable m_prefetchReady;
+    // The consumer can drain one protocol-sized block while the worker fills
+    // the other, bounding each active stream to two MiB of read-ahead.
+    std::array<PrefetchBlock, PREFETCH_BLOCKS> m_prefetch;
+    std::thread m_prefetchThread;
+    uint64_t m_offset             = 0;
+    uint64_t m_prefetchGeneration = 1;
+    bool m_prefetchStarted        = false;
+    bool m_prefetchStop           = false;
 
     HRESULT ReadProvider(uint64_t offset, void * data, ULONG length)
     {
@@ -540,8 +572,175 @@ namespace
       return actual == length ? S_OK : STG_E_READFAULT;
     }
 
-    HRESULT ReadLocked(void * data, ULONG length, ULONG * read,
-      bool readAhead)
+    bool Contains(const PrefetchBlock& block, uint64_t offset) const
+    {
+      return block.generation == m_prefetchGeneration &&
+        block.state != PrefetchState::EMPTY &&
+        offset >= block.offset && offset - block.offset < block.length;
+    }
+
+    void InvalidatePrefetchLocked()
+    {
+      if (!++m_prefetchGeneration)
+        ++m_prefetchGeneration;
+      for (PrefetchBlock& block : m_prefetch)
+        if (block.state != PrefetchState::FILLING)
+          block.state = PrefetchState::EMPTY;
+    }
+
+    void SchedulePrefetchLocked()
+    {
+      if (!m_prefetchStarted || m_prefetchStop ||
+          m_offset >= m_entry.size)
+        return;
+
+      for (PrefetchBlock& block : m_prefetch)
+      {
+        if (block.state != PrefetchState::FILLING &&
+            block.generation != m_prefetchGeneration)
+          block.state = PrefetchState::EMPTY;
+        else if (block.state == PrefetchState::READY &&
+            block.offset + block.length <= m_offset)
+          block.state = PrefetchState::EMPTY;
+      }
+
+      uint64_t next = m_offset;
+      for (;;)
+      {
+        const PrefetchBlock * existing = nullptr;
+        for (const PrefetchBlock& block : m_prefetch)
+          if (Contains(block, next))
+          {
+            existing = &block;
+            break;
+          }
+        if (existing)
+        {
+          next = existing->offset + existing->length;
+          if (next >= m_entry.size)
+            break;
+          continue;
+        }
+
+        PrefetchBlock * empty = nullptr;
+        for (PrefetchBlock& block : m_prefetch)
+          if (block.state == PrefetchState::EMPTY)
+          {
+            empty = &block;
+            break;
+          }
+        if (!empty)
+          break;
+
+        empty->offset = next;
+        empty->length = static_cast<ULONG>((std::min<uint64_t>)(
+          FILE_READ_BYTES, m_entry.size - next));
+        empty->result = S_OK;
+        empty->generation = m_prefetchGeneration;
+        empty->state = PrefetchState::QUEUED;
+        next += empty->length;
+        m_prefetchWork.notify_one();
+        if (next >= m_entry.size)
+          break;
+      }
+    }
+
+    HRESULT StartPrefetchLocked()
+    {
+      if (m_prefetchStarted)
+        return S_OK;
+      try
+      {
+        const size_t blockBytes = static_cast<size_t>(
+          (std::min<uint64_t>)(FILE_READ_BYTES, m_entry.size));
+        for (PrefetchBlock& block : m_prefetch)
+          block.data.resize(blockBytes);
+        m_prefetchThread = std::thread(
+          &CClipboardFileStream::PrefetchThread, this);
+      }
+      catch (const std::bad_alloc&)
+      {
+        return E_OUTOFMEMORY;
+      }
+      catch (const std::length_error&)
+      {
+        return E_OUTOFMEMORY;
+      }
+      catch (const std::system_error&)
+      {
+        return E_FAIL;
+      }
+      m_prefetchStarted = true;
+      return S_OK;
+    }
+
+    void PrefetchThread()
+    {
+      std::unique_lock<std::mutex> lock(m_lock);
+      for (;;)
+      {
+        m_prefetchWork.wait(lock, [this]()
+        {
+          if (m_prefetchStop)
+            return true;
+          for (const PrefetchBlock& block : m_prefetch)
+            if (block.state == PrefetchState::QUEUED)
+              return true;
+          return false;
+        });
+        if (m_prefetchStop)
+          return;
+
+        PrefetchBlock * pending = nullptr;
+        for (PrefetchBlock& block : m_prefetch)
+          if (block.state == PrefetchState::QUEUED &&
+              (!pending || block.offset < pending->offset))
+            pending = &block;
+        if (!pending)
+          continue;
+
+        pending->state = PrefetchState::FILLING;
+        const uint64_t generation = pending->generation;
+        const uint64_t offset     = pending->offset;
+        const ULONG length        = pending->length;
+        uint8_t * const data      = pending->data.data();
+        lock.unlock();
+        const HRESULT result = ReadProvider(offset, data, length);
+        lock.lock();
+
+        if (pending->state == PrefetchState::FILLING &&
+            pending->generation == generation)
+        {
+          if (m_prefetchStop || generation != m_prefetchGeneration)
+            pending->state = PrefetchState::EMPTY;
+          else
+          {
+            pending->result = result;
+            pending->state = PrefetchState::READY;
+          }
+        }
+        SchedulePrefetchLocked();
+        m_prefetchReady.notify_all();
+      }
+    }
+
+    void StopPrefetch()
+    {
+      {
+        std::lock_guard<std::mutex> lock(m_lock);
+        if (!m_prefetchStarted)
+          return;
+        m_prefetchStop = true;
+        InvalidatePrefetchLocked();
+      }
+      m_prefetchWork.notify_all();
+      m_prefetchReady.notify_all();
+      if (m_prefetchThread.joinable())
+        m_prefetchThread.join();
+    }
+
+    HRESULT ReadLocked(std::unique_lock<std::mutex>& lock, void * data,
+      ULONG length, ULONG * read)
     {
       if (read)
         *read = 0;
@@ -552,81 +751,57 @@ namespace
 
       const ULONG requested = static_cast<ULONG>((std::min<uint64_t>)(
         length, m_entry.size - m_offset));
-      const bool direct = !readAhead || length >= FILE_READ_BYTES;
       uint8_t * output = static_cast<uint8_t *>(data);
       ULONG completed = 0;
 
+      const HRESULT started = StartPrefetchLocked();
+      if (FAILED(started))
+        return started;
+
       while (completed < requested)
       {
-        if (!direct && m_offset >= m_cacheOffset)
-        {
-          const uint64_t relative = m_offset - m_cacheOffset;
-          if (relative < m_cache.size())
+        SchedulePrefetchLocked();
+        PrefetchBlock * ready = nullptr;
+        for (PrefetchBlock& block : m_prefetch)
+          if (block.state == PrefetchState::READY &&
+              Contains(block, m_offset))
           {
-            const size_t available = m_cache.size() -
-              static_cast<size_t>(relative);
-            const ULONG amount = static_cast<ULONG>((std::min<size_t>)(
-              requested - completed, available));
-            memcpy(output + completed,
-              m_cache.data() + static_cast<size_t>(relative), amount);
-            m_offset += amount;
-            completed += amount;
-            continue;
+            ready = &block;
+            break;
           }
-        }
-
-        if (direct)
+        if (!ready)
         {
-          const ULONG amount = (std::min)(
-            FILE_READ_BYTES, requested - completed);
-          const HRESULT result = ReadProvider(
-            m_offset, output + completed, amount);
-          if (FAILED(result))
+          m_prefetchReady.wait(lock);
+          if (m_prefetchStop)
           {
             if (read)
               *read = completed;
-            return result;
+            return STG_E_READFAULT;
           }
-          m_offset += amount;
-          completed += amount;
           continue;
         }
 
-        m_cache.clear();
-        m_cacheOffset = 0;
-        const ULONG amount = static_cast<ULONG>((std::min<uint64_t>)(
-          FILE_READ_BYTES, m_entry.size - m_offset));
-        try
+        if (FAILED(ready->result))
         {
-          m_cache.resize(amount);
-        }
-        catch (const std::bad_alloc&)
-        {
-          m_cache.clear();
-          if (read)
-            *read = completed;
-          return E_OUTOFMEMORY;
-        }
-        catch (const std::length_error&)
-        {
-          m_cache.clear();
-          if (read)
-            *read = completed;
-          return E_OUTOFMEMORY;
-        }
-
-        const HRESULT result = ReadProvider(
-          m_offset, m_cache.data(), amount);
-        if (FAILED(result))
-        {
-          m_cache.clear();
+          const HRESULT result = ready->result;
+          InvalidatePrefetchLocked();
           if (read)
             *read = completed;
           return result;
         }
-        m_cacheOffset = m_offset;
+
+        const size_t relative = static_cast<size_t>(
+          m_offset - ready->offset);
+        const ULONG amount = static_cast<ULONG>((std::min<size_t>)(
+          requested - completed, ready->length - relative));
+        memcpy(output + completed, ready->data.data() + relative, amount);
+        m_offset += amount;
+        completed += amount;
+        if (relative + amount == ready->length)
+          ready->state = PrefetchState::EMPTY;
       }
 
+      SchedulePrefetchLocked();
       if (read)
         *read = completed;
       return completed == length ? S_OK : S_FALSE;
@@ -641,6 +816,11 @@ namespace
     {
     }
 
+    ~CClipboardFileStream()
+    {
+      StopPrefetch();
+    }
+
     HRESULT STDMETHODCALLTYPE QueryInterface(REFIID iid, void ** object)
       override
     {
@@ -679,8 +859,9 @@ namespace
           *read = 0;
         return STG_E_INVALIDPOINTER;
       }
-      std::lock_guard<std::mutex> lock(m_lock);
-      return ReadLocked(data, length, read, true);
+      std::lock_guard<std::mutex> operationLock(m_operationLock);
+      std::unique_lock<std::mutex> lock(m_lock);
+      return ReadLocked(lock, data, length, read);
     }
 
     HRESULT STDMETHODCALLTYPE Write(const void *, ULONG, ULONG *) override
@@ -691,6 +872,7 @@ namespace
     HRESULT STDMETHODCALLTYPE Seek(LARGE_INTEGER move, DWORD origin,
       ULARGE_INTEGER * position) override
     {
+      std::lock_guard<std::mutex> operationLock(m_operationLock);
       std::lock_guard<std::mutex> lock(m_lock);
       uint64_t base = 0;
       switch (origin)
@@ -725,6 +907,7 @@ namespace
         next = base + amount;
       }
       m_offset = next;
+      InvalidatePrefetchLocked();
       if (position)
         position->QuadPart = next;
       return S_OK;
@@ -766,8 +949,9 @@ namespace
         ULONG got = 0;
         HRESULT result;
         {
-          std::lock_guard<std::mutex> lock(m_lock);
-          result = ReadLocked(buffer.data(), wanted, &got, false);
+          std::lock_guard<std::mutex> operationLock(m_operationLock);
+          std::unique_lock<std::mutex> lock(m_lock);
+          result = ReadLocked(lock, buffer.data(), wanted, &got);
         }
         if (FAILED(result))
           return result;
@@ -819,6 +1003,7 @@ namespace
       if (!stream)
         return E_POINTER;
       *stream = nullptr;
+      std::lock_guard<std::mutex> operationLock(m_operationLock);
       std::lock_guard<std::mutex> lock(m_lock);
       try
       {