felhom-controller/controller/internal/agentapi/features.go

package agentapi

import (
	"context"
	"errors"
	"net/http"
	"sync"
	"time"

	"gitea.dooplex.hu/admin/felhom-controller/internal/logx"
	"gitea.dooplex.hu/admin/felhom-controller/internal/util"
)

// Agent-capability probing (the publish-train backstop). A controller release that depends on
// coupled agent behavior must not fail mid-pipeline against an older agent — it detects support up
// front and refuses honestly. Detection is a ROUTE PROBE: a route that shipped together with the
// coupled semantics either answers (2xx ⇒ supported) or 404s (older agent). Transport errors and
// 5xx are INDETERMINATE — an agent problem is never claimed as "too old".

// Feature names one coupled controller↔agent capability.
type Feature string

// FeatureNetstorageVerify is the NAS verify-before-commit add semantics (agent v0.81.0): the
// coupled add behavior shipped together with GET /netstorage/verify-status, so that route IS the
// capability signal.
const FeatureNetstorageVerify Feature = "netstorage_verify"

// FeatureGuestMemoryResize is the guest RAM resize (agent v0.90.0, R-24): the resize endpoints
// (GET/POST /guest/memory) shipped together, so GET /guest/memory IS the capability signal.
const FeatureGuestMemoryResize Feature = "guest_memory_resize"

// FeatureBackupAgeState is R-88 Part 2 (agent v0.105.0): GET /backup/due carries `age_state`,
// distinguishing "never backed up" (absent) from "could not tell" (unknown). There is no route
// probe for it — the signal is a FIELD on an existing route, so the version floor is the gate and
// an empty field means legacy.
const FeatureBackupAgeState Feature = "backup_age_state"

// SupportState is a probe verdict. The zero value is SupportUnknown (fail-open: unknown never
// refuses — the existing agent-error paths speak honestly when the agent is down).
type SupportState int

const (
	// SupportUnknown — the probe could not decide (transport error, timeout, auth, 5xx).
	SupportUnknown SupportState = iota
	// SupportYes — the agent answered 2xx on the feature's probe route.
	SupportYes
	// SupportNo — the agent answered 404: it predates the route, and with it the coupled semantics.
	SupportNo
)

// String returns the state's wire/template vocabulary: "yes" | "no" | "unknown".
func (s SupportState) String() string {
	switch s {
	case SupportYes:
		return "yes"
	case SupportNo:
		return "no"
	default:
		return "unknown"
	}
}

// SupportProber is the minimal agent surface a probe needs. *Client satisfies it, and so does the
// web layer's netAgent seam — tests inject fakes there.
type SupportProber interface {
	NetVerifyStatus(ctx context.Context) (NetVerifyStatus, error)
}

// featureProbes maps each coupled feature to its route probe (the FALLBACK path for agents that
// predate the v0.82.0 version header).
//
// CONVENTION (publish-train rules doc, felhom.eu/documentation/runbooks/publish-train-rules.md):
// every future coupled feature adds a row here AND a featureMinAgent row, plus a Supports gate
// call at its entry point, and declares MinAgent in its CHANGELOG header.
var featureProbes = map[Feature]func(ctx context.Context, p SupportProber) error{
	FeatureNetstorageVerify: func(ctx context.Context, p SupportProber) error {
		_, err := p.NetVerifyStatus(ctx)
		return err
	},
	// The memory-resize prober needs GET /guest/memory, not NetVerifyStatus. Rather than couple the
	// shared SupportProber (and every unrelated prober/fake) to the memory surface, the probe
	// type-asserts the ONE method it needs — the memory feature is only ever probed with a
	// GuestMemory-capable prober (the web memAgent seam / *Client). A prober without it → a non-404
	// error → SupportUnknown (fail-open), never a false "supported".
	FeatureGuestMemoryResize: func(ctx context.Context, p SupportProber) error {
		gm, ok := p.(interface {
			GuestMemory(ctx context.Context) (GuestMemoryInfo, error)
		})
		if !ok {
			return errNoMemoryProbe
		}
		_, err := gm.GuestMemory(ctx)
		return err
	},
}

// errNoMemoryProbe classifies to SupportUnknown (not a *StatusError 404), so a prober that cannot be
// asked never reads as "unsupported".
var errNoMemoryProbe = errors.New("agentapi: prober does not support the guest-memory probe")

// featureMinAgent maps each coupled feature to the MINIMUM agent version that carries its coupled
// semantics (the CHANGELOG `MinAgent:` header value). Used by Supports when the agent's version is
// KNOWN (the v0.82.0 X-Felhom-Agent-Version channel) — a direct comparison, no probe traffic. A
// feature missing here (or an unparseable table value) falls back to the probe.
var featureMinAgent = map[Feature]string{
	FeatureNetstorageVerify:  "0.81.0",
	FeatureGuestMemoryResize: "0.90.0",
	// R-88 Part 2: /backup/due carries age_state, distinguishing "never backed up" from "cannot tell".
	FeatureBackupAgeState: "0.105.0",
}

// AgentVersionReporter is optionally implemented by a SupportProber (*Client is one): it reports
// the last strictly-validated agent version seen on its traffic ("" = unknown → probe fallback).
type AgentVersionReporter interface {
	AgentVersion() string
}

// supportTTL bounds how long a probe verdict (either polarity) is trusted. An agent updated
// mid-window flips within this — no invalidation plumbing by design.
const supportTTL = 5 * time.Minute

type supportEntry struct {
	state SupportState
	at    time.Time
}

// SupportCache caches per-feature probe verdicts. Yes and No are cached for supportTTL; Unknown is
// NEVER cached (a down agent re-probes on the next call, so recovery is immediate). The zero value
// is ready to use.
type SupportCache struct {
	mu      sync.Mutex
	now     func() time.Time // test seam; nil → time.Now
	entries map[Feature]supportEntry
}

// Supports reports whether the agent behind p provides feature f.
//
// Order (v0.115.0): (1) the agent's VERSION is known (the v0.82.0 header channel, strictly
// validated at capture) AND the feature has a MinAgent row → pure semver comparison, NO probe
// traffic, no cache involvement (the cache stays probe-only); (2) otherwise — pre-0.82 agent, no
// traffic yet, or a table gap — the v0.114.0 probe path, byte-identical (cache inside the TTL
// window, probe on miss). The probe runs OUTSIDE the lock — concurrent misses may double-probe
// (harmless: the probe is one cheap GET).
func (sc *SupportCache) Supports(ctx context.Context, p SupportProber, f Feature) SupportState {
	state, _ := sc.SupportsWithSource(ctx, p, f)
	return state
}

// SupportsWithSource is Supports plus the DECISION SOURCE ("version" | "probe-cache" |
// "probe" | "unregistered") — the v0.116.0 observability extension so the gate line can
// say HOW the verdict was reached. Behavior is byte-identical to v0.115.0 Supports.
func (sc *SupportCache) SupportsWithSource(ctx context.Context, p SupportProber, f Feature) (SupportState, string) {
	probe, ok := featureProbes[f]
	if !ok {
		return SupportUnknown, "unregistered" // unregistered feature — never refuse on a table gap
	}
	if vr, hasVer := p.(AgentVersionReporter); hasVer {
		if state, decided := supportsByVersion(vr.AgentVersion(), f); decided {
			return state, "version"
		}
	}
	sc.mu.Lock()
	nowFn := sc.now
	if nowFn == nil {
		nowFn = time.Now
	}
	if e, hit := sc.entries[f]; hit && nowFn().Sub(e.at) < supportTTL {
		sc.mu.Unlock()
		return e.state, "probe-cache"
	}
	sc.mu.Unlock()

	state := classifySupportErr(probe(ctx, p))
	if state != SupportUnknown {
		sc.mu.Lock()
		if sc.entries == nil {
			sc.entries = map[Feature]supportEntry{}
		}
		sc.entries[f] = supportEntry{state: state, at: nowFn()}
		sc.mu.Unlock()
	}
	return state, "probe"
}

// Supports probes (cached, TTL 5m, both polarities) whether the connected agent provides the
// feature. 2xx ⇒ Yes. 404 ⇒ No. Anything else ⇒ Unknown (never "too old"). The web layer drives
// the same machinery through its netAgent seam (Server.netFeatures) so tests can fake the probe.
func (c *Client) Supports(ctx context.Context, f Feature) SupportState {
	state, source := c.features.SupportsWithSource(ctx, c, f)
	logx.Debugf(c.logger, "[agentapi] Supports(%s) = %s (source=%s agent_version=%q)",
		f, state, source, c.AgentVersion())
	return state
}

// supportsByVersion decides a feature by version comparison alone. decided=false (unknown/garbage
// version, missing MinAgent row, unparseable table value) sends the caller to the probe fallback —
// a bad version string must never be trusted in EITHER direction.
func supportsByVersion(agentVer string, f Feature) (state SupportState, decided bool) {
	if agentVer == "" {
		return SupportUnknown, false
	}
	minStr, ok := featureMinAgent[f]
	if !ok {
		return SupportUnknown, false
	}
	av, err := util.ParseVersion(agentVer)
	if err != nil {
		return SupportUnknown, false // capture validates the shape, but stay defensive
	}
	mv, err := util.ParseVersion(minStr)
	if err != nil {
		return SupportUnknown, false // a broken table row falls back to probing, never refuses
	}
	if av.Compare(mv) >= 0 {
		return SupportYes, true
	}
	return SupportNo, true
}

// classifySupportErr maps a probe outcome to a SupportState. ONLY a typed HTTP 404 means
// "unsupported" — every other error (transport, timeout, 401, 5xx, envelope problems) is Unknown,
// so a merely-down agent is never reported as outdated. Typed errors only; never string-match.
func classifySupportErr(err error) SupportState {
	if err == nil {
		return SupportYes
	}
	var se *StatusError
	if errors.As(err, &se) && se.Code == http.StatusNotFound {
		return SupportNo
	}
	return SupportUnknown
}

// AgentVersionReporter witness. Asserted at SupportsWithSource (`p.(AgentVersionReporter)`); a failed
// assertion falls back from the version gate to the live probe. That degrade is benign — both paths
// decide correctly — but *Client is the production prober and losing the version path would silently
// turn every MinAgent floor into a probe round-trip, which is a behaviour change nobody would see.
var _ AgentVersionReporter = (*Client)(nil)