Factor fsimpl/gofer.host{Preadv,Pwritev} out of fsimpl/gofer.

Also fix returning EOF when 0 bytes are read.

PiperOrigin-RevId: 308089875

1 files changed, 84 insertions, 0 deletions

diff --git a/pkg/sentry/hostfd/hostfd.go b/pkg/sentry/hostfd/hostfd.go
new file mode 100644
index 000000000..70dd9cafb
--- /dev/null
+++ b/pkg/sentry/hostfd/hostfd.go
@@ -0,0 +1,84 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package hostfd provides efficient I/O with host file descriptors.
+package hostfd
+
+import (
+	"gvisor.dev/gvisor/pkg/safemem"
+	"gvisor.dev/gvisor/pkg/sync"
+)
+
+// ReadWriterAt implements safemem.Reader and safemem.Writer by reading from
+// and writing to a host file descriptor respectively. ReadWriterAts should be
+// obtained by calling GetReadWriterAt.
+//
+// Clients should usually prefer to use Preadv2 and Pwritev2 directly.
+type ReadWriterAt struct {
+	fd     int32
+	offset int64
+	flags  uint32
+}
+
+var rwpool = sync.Pool{
+	New: func() interface{} {
+		return &ReadWriterAt{}
+	},
+}
+
+// GetReadWriterAt returns a ReadWriterAt that reads from / writes to the given
+// host file descriptor, starting at the given offset and using the given
+// preadv2(2)/pwritev2(2) flags. If offset is -1, the host file descriptor's
+// offset is used instead. Users are responsible for ensuring that fd remains
+// valid for the lifetime of the returned ReadWriterAt, and must call
+// PutReadWriterAt when it is no longer needed.
+func GetReadWriterAt(fd int32, offset int64, flags uint32) *ReadWriterAt {
+	rw := rwpool.Get().(*ReadWriterAt)
+	*rw = ReadWriterAt{
+		fd:     fd,
+		offset: offset,
+		flags:  flags,
+	}
+	return rw
+}
+
+// PutReadWriterAt releases a ReadWriterAt returned by a previous call to
+// GetReadWriterAt that is no longer in use.
+func PutReadWriterAt(rw *ReadWriterAt) {
+	rwpool.Put(rw)
+}
+
+// ReadToBlocks implements safemem.Reader.ReadToBlocks.
+func (rw *ReadWriterAt) ReadToBlocks(dsts safemem.BlockSeq) (uint64, error) {
+	if dsts.IsEmpty() {
+		return 0, nil
+	}
+	n, err := Preadv2(rw.fd, dsts, rw.offset, rw.flags)
+	if rw.offset >= 0 {
+		rw.offset += int64(n)
+	}
+	return n, err
+}
+
+// WriteFromBlocks implements safemem.Writer.WriteFromBlocks.
+func (rw *ReadWriterAt) WriteFromBlocks(srcs safemem.BlockSeq) (uint64, error) {
+	if srcs.IsEmpty() {
+		return 0, nil
+	}
+	n, err := Pwritev2(rw.fd, srcs, rw.offset, rw.flags)
+	if rw.offset >= 0 {
+		rw.offset += int64(n)
+	}
+	return n, err
+}