Create mq package.

Create package mq to implement POSIX message queues, and define initial
struct definitions.

Updates #136

pkg/sentry/kernel/mq/BUILD

@@ -0,0 +1,31 @@
+load("//tools:defs.bzl", "go_library")
+load("//tools/go_generics:defs.bzl", "go_template_instance")
+
+package(licenses = ["notice"])
+
+go_template_instance(
+    name = "message_list",
+    out = "message_list.go",
+    package = "mq",
+    prefix = "msg",
+    template = "//pkg/ilist:generic_list",
+    types = {
+        "Element": "*Message",
+        "Linker": "*Message",
+    },
+)
+
+go_library(
+    name = "mq",
+    srcs = [
+        "message_list.go",
+        "mq.go",
+    ],
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/sentry/fs",
+        "//pkg/sync",
+        "//pkg/waiter",
+    ],
+)

pkg/sentry/kernel/mq/mq.go

@@ -0,0 +1,96 @@
+// Copyright 2021 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 mq provides an implementation for POSIX message queues.
+package mq
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/waiter"
+)
+
+const (
+	maxPriority = linux.MQ_PRIO_MAX - 1 // Highest possible message priority.
+)
+
+// Queue represents a POSIX message queue.
+//
+// +stateify savable
+type Queue struct {
+	// owner is the registry's owner. Immutable.
+	owner fs.FileOwner
+
+	// perms is the registry's access permissions. Immutable.
+	perms fs.FilePermissions
+
+	// mu protects all the fields below.
+	mu sync.Mutex `state:"nosave"`
+
+	// senders is a queue of currently blocked senders. Senders are notified
+	// when space isi available in the queue for a new message.
+	senders waiter.Queue
+
+	// receivers is a queue of currently blocked receivers. Receivers are
+	// notified when a new message is inserted in the queue.
+	receivers waiter.Queue
+
+	// messages is a list of messages currently in the queue.
+	messages msgList
+
+	// subscriber represents a task registered to receive async notification
+	// from this queue.
+	subscriber Subscriber
+
+	// nonBlock is true if this queue is non-blocking.
+	nonBlock bool
+
+	// messageCount is the number of messages currently in the queue.
+	messageCount int64
+
+	// maxMessageCount is the maximum number of messages that the queue can
+	// hold.
+	maxMessageCount int64
+
+	// maxMessageSize is the maximum size of a message held by the queue.
+	maxMessageSize uint64
+
+	// byteCount is the number of bytes of data in all messages in the queue.
+	byteCount uint64
+}
+
+// Message holds a message exchanged through a Queue via mq_timedsend(2) and
+// mq_timedreceive(2), and additional info relating to the message.
+//
+// +stateify savable
+type Message struct {
+	msgEntry
+
+	// Text is the message's sent content.
+	Text string
+
+	// Size is the message's size in bytes.
+	Size uint64
+
+	// Priority is the message's priority.
+	Priority uint32
+}
+
+// Subscriber represents a task registered for async notification from a Queue.
+//
+// +stateify savable
+type Subscriber struct {
+	// TODO: Add fields when mq_notify(2) is implemented.
+}