ipn/ipnlocal: add basic support for netmap caching (#18530)

This commit is based on ff0978ab, and extends #18497 to connect network map
caching to the LocalBackend. As implemented, only "whole" netmap values are
stored, and we do not yet handle incremental updates. As-written, the feature must
be explicitly enabled via the TS_USE_CACHED_NETMAP envknob, and must be
considered experimental.

Updates #12639

Co-Authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I48a1e92facfbf7fb3a8e67cff7f2c9ab4ed62c83
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/diskcache.go

@@ -0,0 +1,56 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ipnlocal
+
+import (
+	"tailscale.com/feature/buildfeatures"
+	"tailscale.com/ipn/ipnlocal/netmapcache"
+	"tailscale.com/types/netmap"
+)
+
+// diskCache is the state netmap caching to disk.
+type diskCache struct {
+	// all fields guarded by LocalBackend.mu
+
+	dir   string // active profile cache directory
+	cache *netmapcache.Cache
+}
+
+func (b *LocalBackend) writeNetmapToDiskLocked(nm *netmap.NetworkMap) error {
+	if !buildfeatures.HasCacheNetMap || nm == nil || nm.Cached {
+		return nil
+	}
+	b.logf("writing netmap to disk cache")
+
+	dir, err := b.profileMkdirAllLocked(b.pm.CurrentProfile().ID(), "netmap-cache")
+	if err != nil {
+		return err
+	}
+	if c := b.diskCache; c.cache == nil || c.dir != dir {
+		b.diskCache.cache = netmapcache.NewCache(netmapcache.FileStore(dir))
+		b.diskCache.dir = dir
+	}
+	return b.diskCache.cache.Store(b.currentNode().Context(), nm)
+}
+
+func (b *LocalBackend) loadDiskCacheLocked() (om *netmap.NetworkMap, ok bool) {
+	if !buildfeatures.HasCacheNetMap {
+		return nil, false
+	}
+	dir, err := b.profileMkdirAllLocked(b.pm.CurrentProfile().ID(), "netmap-cache")
+	if err != nil {
+		b.logf("profile data directory: %v", err)
+		return nil, false
+	}
+	if c := b.diskCache; c.cache == nil || c.dir != dir {
+		b.diskCache.cache = netmapcache.NewCache(netmapcache.FileStore(dir))
+		b.diskCache.dir = dir
+	}
+	nm, err := b.diskCache.cache.Load(b.currentNode().Context())
+	if err != nil {
+		b.logf("load netmap from cache: %v", err)
+		return nil, false
+	}
+	return nm, true
+}