Make the NFSv4 server work on Linux

This should absolutely not be used in production. The reason I'm adding
it, is because it allows us to validate that the NFS servers we ship are
portable.

pkg/filesystem/virtual/configuration/BUILD.bazel

@@ -9,6 +9,7 @@ go_library(
         "fuse_mount_enabled.go",
         "nfsv4_mount_darwin.go",
         "nfsv4_mount_disabled.go",
+        "nfsv4_mount_linux.go",
         "remove_stale_mounts.go",
     ],
     importpath = "github.com/buildbarn/bb-remote-execution/pkg/filesystem/virtual/configuration",

pkg/filesystem/virtual/configuration/nfsv4_mount_disabled.go

@@ -1,5 +1,5 @@
-//go:build freebsd || linux || windows
-// +build freebsd linux windows
+//go:build freebsd || windows
+// +build freebsd windows
 
 package configuration
 

pkg/filesystem/virtual/configuration/nfsv4_mount_linux.go

@@ -0,0 +1,84 @@
+//go:build linux
+// +build linux
+
+package configuration
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net"
+	"strings"
+
+	pb "github.com/buildbarn/bb-remote-execution/pkg/proto/configuration/filesystem/virtual"
+	"github.com/buildbarn/bb-storage/pkg/program"
+	"github.com/buildbarn/bb-storage/pkg/util"
+	"github.com/buildbarn/go-xdr/pkg/rpcserver"
+
+	"golang.org/x/sys/unix"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+func (m *nfsv4Mount) mount(terminationGroup program.Group, rpcServer *rpcserver.Server) error {
+	linuxConfiguration, ok := m.configuration.OperatingSystem.(*pb.NFSv4MountConfiguration_Linux)
+	if !ok {
+		return status.Error(codes.InvalidArgument, "Linux specific NFSv4 server configuration options not provided")
+	}
+	osConfiguration := linuxConfiguration.Linux
+
+	// Expose the NFSv4 server on a TCP socket.
+	sock, err := net.Listen("tcp", "localhost:")
+	if err != nil {
+		return util.StatusWrap(err, "Failed to create listening socket for NFSv4 server")
+	}
+	// TODO: Run this as part of the program.Group, so that it gets
+	// cleaned up upon shutdown.
+	go func() {
+		for {
+			c, err := sock.Accept()
+			if err != nil {
+				log.Print("Got accept error: ", err)
+			}
+			go func() {
+				err := rpcServer.HandleConnection(c, c)
+				c.Close()
+				if err != nil {
+					log.Print("Failure handling NFSv4 connection: ", err)
+				}
+			}()
+		}
+	}()
+
+	// Mount the volume exported by the NFSv4 server.
+	mountPath := m.mountPath
+	removeStaleMounts(mountPath)
+	listenAddressPort := sock.Addr().(*net.TCPAddr).AddrPort()
+	if err := unix.Mount(
+		/* source = */ ":/", // fmt.Sprintf("[%s]:/", listenAddressPort.Addr()),
+		/* target = */ mountPath,
+		/* fsType = */ "nfs4",
+		/* flags = */ 0,
+		/* data = */ strings.Join(
+			append(
+				[]string{
+					"addr=" + listenAddressPort.Addr().String(),
+					fmt.Sprintf("port=%d", listenAddressPort.Port()),
+				},
+				osConfiguration.MountOptions...,
+			),
+			","),
+	); err != nil {
+		return util.StatusWrap(err, "Mounting NFS volume failed")
+	}
+
+	// Automatically unmount upon shutdown.
+	terminationGroup.Go(func(ctx context.Context, siblingsGroup, dependenciesGroup program.Group) error {
+		<-ctx.Done()
+		if err := unix.Unmount(mountPath, 0); err != nil {
+			return util.StatusWrapf(err, "Failed to unmount %#v", mountPath)
+		}
+		return nil
+	})
+	return nil
+}