package selfupdate

import (
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"regexp"
	"sort"
	"sync"
)

// The self-update folder keeps what it needs and no more (R-1368 part 2; space list 2026-11-10, place H12).
//
// WHAT WAS WRONG. Execute downloads each new agent to <StateDir>/selfupdate/felhom-agent-<version> and removes that
// file only when the download or its checksum fails. A good update left it there for ever: measured 2026-11-10, 49 /
// 48 / 33 binaries = 740 / 724 / 509 MB on demo-hp, demo-felhom and Tester 1 — one for each version the box ever got.
//
// WHAT THE FOLDER IS NEEDED FOR. The root wrapper (felhom-os-apply, mode agent_update) reads the staged file ONCE,
// copies it to its own root-owned folder and flips from that copy. After that hand-off nothing reads the staged file:
//   - the rollback (felhom-selfupdate-guarded `rollback`) needs two files and neither is a staged binary:
//     `pending.json` in this folder, and `/usr/local/bin/felhom-agent.prev` outside it;
//   - the commit needs `pending.json` only.
//
// So a staged binary is dead weight as soon as its update ended. Pinned by TestPrune_RollbackReadsNoStagedBinary (the
// wrapper's text) and TestPrune_RealRollbackStillWorksAfterNUpdates (the wrapper's rollback verb, run for real).
//
// THE RULE. After an update ended HEALTHY (the wrapper's `commit` returned success), remove every staged binary but
// the one of the running version. Nothing is removed:
//   - while a pending marker exists or cannot be read — an update is in flight, and that is the only time a rollback
//     can fire;
//   - after a rollback, or at a plain start — only the commit path calls this, so the binary of a version that failed
//     stays for the operator to look at until the next good update;
//   - that is not a regular file with exactly the name Execute gives a staged binary. `pending.json`, a folder, a
//     symlink, and any name this code does not know are never touched.
//
// NOT REACHED BY THIS: `/usr/local/bin/felhom-agent.bak-*` (space list H13). Those files are root-owned, in a folder
// the agent cannot write, and no code of the agent makes them (a person's copy before a hand deploy). R-1368 part 4.

// stagedNameRe is the exact name Execute gives a staged binary: "felhom-agent-" + a version that versionRe accepts.
// Kept in step with versionRe by TestPrune_NameShapeFollowsTheVersionRule.
var stagedNameRe = regexp.MustCompile(`^felhom-agent-([0-9]+\.[0-9]+\.[0-9]+(-[0-9A-Za-z.]+)?)$`)

// stagingMu makes a download and a prune take turns. Without it a prune that runs while Execute downloads the NEXT
// version would remove that half-written file (its name is a staged binary's and it is not the running version).
var stagingMu sync.Mutex

// pruneStaged removes the staged binaries the folder no longer needs and returns the names it removed, sorted. It
// refuses (removes nothing, returns an error) while an update is in flight. A file it cannot remove is logged and
// skipped: a failed clean-up must never fail the commit that came before it.
func pruneStaged(stateDir, runningVersion string, logger *slog.Logger) ([]string, error) {
	logger = orDefaultLogger(logger)
	stagingMu.Lock()
	defer stagingMu.Unlock()

	if !versionRe.MatchString(runningVersion) {
		return nil, fmt.Errorf("selfupdate prune: refusing — running version %q is not a version this code knows; it could not name the file to keep", runningVersion)
	}
	pm, err := readPending(stateDir)
	if err != nil {
		return nil, fmt.Errorf("selfupdate prune: refusing — the pending marker cannot be read: %w", err)
	}
	if pm != nil {
		return nil, fmt.Errorf("selfupdate prune: refusing — an update to %s is pending (a rollback may still fire)", pm.NewVersion)
	}
	dir := stagingDir(stateDir)
	entries, err := os.ReadDir(dir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil, nil
		}
		return nil, fmt.Errorf("selfupdate prune: read %s: %w", dir, err)
	}
	keep := "felhom-agent-" + runningVersion
	var removed []string
	for _, ent := range entries {
		name := ent.Name()
		if name == keep || !stagedNameRe.MatchString(name) {
			continue // the running version's binary, pending.json, or a name this code does not know
		}
		if !ent.Type().IsRegular() {
			logger.Warn("selfupdate prune: a staged-binary name that is not a regular file — left alone", "name", name, "type", ent.Type().String())
			continue
		}
		// name came from ReadDir and matched stagedNameRe, so it holds no path separator: this is one exact file.
		if err := os.Remove(filepath.Join(dir, name)); err != nil {
			logger.Warn("selfupdate prune: cannot remove an old staged binary — left for the next good update", "name", name, "err", err)
			continue
		}
		logger.Debug("selfupdate prune: removed an old staged binary", "name", name)
		removed = append(removed, name)
	}
	sort.Strings(removed)
	logger.Info("selfupdate prune: the self-update folder holds what it needs", "kept", keep, "removed", len(removed))
	return removed, nil
}