Support non-blocking File#read_at on Windows

spec/std/file_spec.cr

@@ -1295,17 +1295,19 @@ describe "File" do
 
   it "reads at offset" do
     filename = datapath("test_file.txt")
-    File.open(filename) do |file|
-      file.read_at(6, 100) do |io|
-        io.gets_to_end.should eq("World\nHello World\nHello World\nHello World\nHello World\nHello World\nHello World\nHello World\nHello Worl")
-      end
+    {true, false}.each do |blocking|
+      File.open(filename, blocking: blocking) do |file|
+        file.read_at(6, 100) do |io|
+          io.gets_to_end.should eq("World\nHello World\nHello World\nHello World\nHello World\nHello World\nHello World\nHello World\nHello Worl")
+        end
 
-      file.read_at(0, 240) do |io|
-        io.gets_to_end.should eq(File.read(filename))
-      end
+        file.read_at(0, 240) do |io|
+          io.gets_to_end.should eq(File.read(filename))
+        end
 
-      file.read_at(6_i64, 5_i64) do |io|
-        io.gets_to_end.should eq("World")
+        file.read_at(6_i64, 5_i64) do |io|
+          io.gets_to_end.should eq("World")
+        end
       end
     end
   end

src/crystal/system/unix/file_descriptor.cr

@@ -219,11 +219,11 @@ module Crystal::System::FileDescriptor
     {r, w}
   end
 
-  def self.pread(fd, buffer, offset)
-    bytes_read = LibC.pread(fd, buffer, buffer.size, offset).to_i64
+  def self.pread(file, buffer, offset)
+    bytes_read = LibC.pread(file.fd, buffer, buffer.size, offset).to_i64
 
     if bytes_read == -1
-      raise IO::Error.from_errno "Error reading file"
+      raise IO::Error.from_errno("Error reading file", target: file)
     end
 
     bytes_read

src/crystal/system/win32/file_descriptor.cr

@@ -120,10 +120,6 @@ module Crystal::System::FileDescriptor
   end
 
   protected def windows_handle
-    FileDescriptor.windows_handle(fd)
-  end
-
-  def self.windows_handle(fd)
     LibC::HANDLE.new(fd)
   end
 
@@ -278,19 +274,26 @@ module Crystal::System::FileDescriptor
     {r, w}
   end
 
-  def self.pread(fd, buffer, offset)
-    handle = windows_handle(fd)
+  def self.pread(file, buffer, offset)
+    handle = file.windows_handle
 
-    overlapped = LibC::OVERLAPPED.new
-    overlapped.union.offset.offset = LibC::DWORD.new!(offset)
-    overlapped.union.offset.offsetHigh = LibC::DWORD.new!(offset >> 32)
-    if LibC.ReadFile(handle, buffer, buffer.size, out bytes_read, pointerof(overlapped)) == 0
-      error = WinError.value
-      return 0_i64 if error == WinError::ERROR_HANDLE_EOF
-      raise IO::Error.from_os_error "Error reading file", error, target: self
-    end
+    if file.system_blocking?
+      overlapped = LibC::OVERLAPPED.new
+      overlapped.union.offset.offset = LibC::DWORD.new!(offset)
+      overlapped.union.offset.offsetHigh = LibC::DWORD.new!(offset >> 32)
+      if LibC.ReadFile(handle, buffer, buffer.size, out bytes_read, pointerof(overlapped)) == 0
+        error = WinError.value
+        return 0_i64 if error == WinError::ERROR_HANDLE_EOF
+        raise IO::Error.from_os_error "Error reading file", error, target: file
+      end
 
-    bytes_read.to_i64
+      bytes_read.to_i64
+    else
+      IOCP.overlapped_operation(file, "ReadFile", file.read_timeout, offset: offset) do |overlapped|
+        ret = LibC.ReadFile(handle, buffer, buffer.size, out byte_count, overlapped)
+        {ret, byte_count}
+      end.to_i64
+    end
   end
 
   def self.from_stdio(fd)

src/crystal/system/win32/iocp.cr

@@ -168,15 +168,16 @@ module Crystal::IOCP
     end
   end
 
-  def self.overlapped_operation(file_descriptor, method, timeout, *, writing = false, &)
+  def self.overlapped_operation(file_descriptor, method, timeout, *, offset = nil, writing = false, &)
     handle = file_descriptor.windows_handle
     seekable = LibC.SetFilePointerEx(handle, 0, out original_offset, IO::Seek::Current) != 0
 
     OverlappedOperation.run(handle) do |operation|
       overlapped = operation.to_unsafe
       if seekable
-        overlapped.value.union.offset.offset = LibC::DWORD.new!(original_offset)
-        overlapped.value.union.offset.offsetHigh = LibC::DWORD.new!(original_offset >> 32)
+        start_offset = offset || original_offset
+        overlapped.value.union.offset.offset = LibC::DWORD.new!(start_offset)
+        overlapped.value.union.offset.offsetHigh = LibC::DWORD.new!(start_offset >> 32)
       end
       result, value = yield operation
 
@@ -215,8 +216,11 @@ module Crystal::IOCP
 
       # operation completed asynchronously; seek to the original file position
       # plus the number of bytes read or written (other operations might have
-      # moved the file pointer so we don't use `IO::Seek::Current` here)
-      LibC.SetFilePointerEx(handle, original_offset + byte_count, nil, IO::Seek::Set) if seekable
+      # moved the file pointer so we don't use `IO::Seek::Current` here), unless
+      # we are calling `Crystal::System::FileDescriptor.pread`
+      if seekable && !offset
+        LibC.SetFilePointerEx(handle, original_offset + byte_count, nil, IO::Seek::Set)
+      end
       byte_count
     end
   end

src/file/preader.cr

@@ -20,7 +20,7 @@ class File::PReader < IO
     count = slice.size
     count = Math.min(count, @bytesize - @pos)
 
-    bytes_read = Crystal::System::FileDescriptor.pread(@file.fd, slice[0, count], @offset + @pos)
+    bytes_read = Crystal::System::FileDescriptor.pread(@file, slice[0, count], @offset + @pos)
 
     @pos += bytes_read