Add tmpfs IOFS (#1212)

* Define an interface for IOFS handling. Add no-op and temporary directory implementations.

* Move IOFS stuff out into separate file, add basic tmpfs implementation for linux only

* Switch between directory and tmpfs based on platform and config

* Respect FN_IOFS_OPTS

* Make directory iofs default on all platforms

* At least try to clean up a bit on failure

* Add backout if IOFS creation fails

* Add comment about iofs.Close

api/agent/iofs_linux.go

@@ -0,0 +1,38 @@
+package agent
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/fnproject/fn/api/common"
+	"golang.org/x/sys/unix"
+)
+
+type tmpfsIOFS struct {
+	directoryIOFS
+}
+
+func (t *tmpfsIOFS) Close() error {
+	if err := unix.Unmount(t.AgentPath(), 0); err != nil {
+		// At this point we don't have a lot of choice but to leak the directory and mount
+		return err
+	}
+	return t.directoryIOFS.Close()
+}
+
+func newTmpfsIOFS(ctx context.Context, cfg *Config) (*tmpfsIOFS, error) {
+	dirIOFS, err := newDirectoryIOFS(ctx, cfg)
+	if err != nil {
+		return nil, err
+	}
+	if err = unix.Mount("tmpfs", dirIOFS.AgentPath(), "tmpfs", uintptr(unix.MS_NOEXEC|unix.MS_NOSUID|unix.MS_NODEV), cfg.IOFSOpts); err != nil {
+		// Best effort to clean up after failure. If the dirIOFS.Close() fails we're not going to see the error though...
+		if err := dirIOFS.Close(); err != nil {
+			common.Logger(ctx).WithError(err).Error("failed to cleanup iofs dir")
+		}
+		return nil, fmt.Errorf("cannot mount/create tmpfs at %s", dirIOFS.AgentPath())
+	}
+	return &tmpfsIOFS{*dirIOFS}, nil
+}
+
+var _ iofs = &tmpfsIOFS{}