health: add an ETag to UnhealthyState for change detection

Updates tailscale/corp#30596

Signed-off-by: James Sanderson <jsanderson@tailscale.com>

health/state.go

@@ -4,6 +4,9 @@
 package health
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
 	"time"
 
 	"tailscale.com/tailcfg"
@@ -35,6 +38,36 @@ type UnhealthyState struct {
 	DependsOn           []WarnableCode        `json:",omitempty"`
 	ImpactsConnectivity bool                  `json:",omitempty"`
 	PrimaryAction       *UnhealthyStateAction `json:",omitempty"`
+
+	// ETag identifies a specific version of an UnhealthyState. If the contents
+	// of the other fields of two UnhealthyStates are the same, the ETags will
+	// be the same. If the contents differ, the ETags will also differ. The
+	// implementation is not defined and the value is opaque: it might be a
+	// hash, it might be a simple counter. Implementations should not rely on
+	// any specific implementation detail or format of the ETag string other
+	// than string (in)equality.
+	ETag string `json:",omitzero"`
+}
+
+// hash computes a deep hash of UnhealthyState which will be stable across
+// different runs of the same binary.
+func (u UnhealthyState) hash() []byte {
+	hasher := sha256.New()
+	enc := json.NewEncoder(hasher)
+
+	// hash.Hash.Write never returns an error, so this will only fail if u is
+	// not marshalable, in which case we have much bigger problems.
+	_ = enc.Encode(u)
+	return hasher.Sum(nil)
+}
+
+// withETag returns a copy of UnhealthyState with an ETag set. The ETag will be
+// the same for all UnhealthyState instances that are equal. If calculating the
+// ETag errors, it returns a copy of the UnhealthyState with an empty ETag.
+func (u UnhealthyState) withETag() UnhealthyState {
+	u.ETag = ""
+	u.ETag = hex.EncodeToString(u.hash())
+	return u
 }
 
 // UnhealthyStateAction represents an action (URL and link) to be presented to
@@ -107,7 +140,8 @@ func (t *Tracker) CurrentState() *State {
 			// that are unhealthy.
 			continue
 		}
-		wm[w.Code] = *w.unhealthyState(ws)
+		state := w.unhealthyState(ws)
+		wm[w.Code] = state.withETag()
 	}
 
 	for id, msg := range t.lastNotifiedControlMessages {
@@ -127,7 +161,7 @@ func (t *Tracker) CurrentState() *State {
 			}
 		}
 
-		wm[state.WarnableCode] = state
+		wm[state.WarnableCode] = state.withETag()
 	}
 
 	return &State{