health: begin work to use structured health warnings instead of strings, pipe changes into ipn.Notify (#12406)

Updates tailscale/tailscale#4136

This PR is the first round of work to move from encoding health warnings as strings and use structured data instead. The current health package revolves around the idea of Subsystems. Each subsystem can have (or not have) a Go error associated with it. The overall health of the backend is given by the concatenation of all these errors.

This PR polishes the concept of Warnable introduced by @bradfitz a few weeks ago. Each Warnable is a component of the backend (for instance, things like 'dns' or 'magicsock' are Warnables). Each Warnable has a unique identifying code. A Warnable is an entity we can warn the user about, by setting (or unsetting) a WarningState for it. Warnables have:

- an identifying Code, so that the GUI can track them as their WarningStates come and go
- a Title, which the GUIs can use to tell the user what component of the backend is broken
- a Text, which is a function that is called with a set of Args to generate a more detailed error message to explain the unhappy state

Additionally, this PR also begins to send Warnables and their WarningStates through LocalAPI to the clients, using ipn.Notify messages. An ipn.Notify is only issued when a warning is added or removed from the Tracker.

In a next PR, we'll get rid of subsystems entirely, and we'll start using structured warnings for all errors affecting the backend functionality.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

ipn/backend.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"tailscale.com/drive"
+	"tailscale.com/health"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/empty"
@@ -70,6 +71,8 @@ const (
 	NotifyNoPrivateKeys        // if set, private keys that would normally be sent in updates are zeroed out
 	NotifyInitialDriveShares   // if set, the first Notify message (sent immediately) will contain the current Taildrive Shares
 	NotifyInitialOutgoingFiles // if set, the first Notify message (sent immediately) will contain the current Taildrop OutgoingFiles
+
+	NotifyInitialHealthState // if set, the first Notify message (sent immediately) will contain the current health.State of the client
 )
 
 // Notify is a communication from a backend (e.g. tailscaled) to a frontend
@@ -138,6 +141,11 @@ type Notify struct {
 	// empty value means that there are no shares.
 	DriveShares views.SliceView[*drive.Share, drive.ShareView]
 
+	// Health is the last-known health state of the backend. When this field is
+	// non-nil, a change in health verified, and the API client should surface
+	// any changes to the user in the UI.
+	Health *health.State `json:",omitempty"`
+
 	// type is mirrored in xcode/Shared/IPN.swift
 }
 
@@ -177,6 +185,9 @@ func (n Notify) String() string {
 	if n.LocalTCPPort != nil {
 		fmt.Fprintf(&sb, "tcpport=%v ", n.LocalTCPPort)
 	}
+	if n.Health != nil {
+		sb.WriteString("Health{...} ")
+	}
 	s := sb.String()
 	return s[0:len(s)-1] + "}"
 }