package installer

import (
	"errors"
	"fmt"
	"net"
	"path/filepath"
	"regexp"
	"strconv"
	"strings"
	"time"

	"golang.org/x/crypto/ssh"

	"rocketgit.ru/rsmon/worker/internal/sshinstall"
)

// SourceInstallOptions drives the remote source installation (work
// package 3 of docs/source-installation.md). It reuses the deploy
// command's SSHOptions for authentication and host-key verification and
// adds the source-build knobs. It never carries a worker token or
// control-plane credential: the built worker is staged, not configured
// or started, so no secret is ever sent to the remote host.
type SourceInstallOptions struct {
	SSHOptions

	// Repo is the public worker repository to clone or update. Empty
	// uses the sshinstall default. Only https URLs without userinfo are
	// accepted.
	Repo string
	// Branch pins the branch to build. Empty resolves the remote's
	// default branch (the public repo currently publishes "master");
	// the resolved branch and commit are recorded in the build dir.
	Branch string
	// GoVersion defaults to the pinned sshinstall toolchain (1.26.0).
	// Non-default versions have no baked checksum yet and are rejected.
	GoVersion string
	// GoArch optionally pins the Go download archive suffix (e.g.
	// "amd64"); empty derives it from the remote `uname -m`.
	GoArch string
	// BuildDir is the remote clone/build directory.
	BuildDir string
	// GoModuleProxy overrides GOPROXY for the remote build.
	GoModuleProxy string
	// ToolchainDir is where the verified Go toolchain is installed.
	// It must be an absolute path ending in /go (default
	// /usr/local/go). Replacement is atomic: the new toolchain is
	// downloaded, verified, and staged before the prior one is moved
	// aside, and the prior one is restored if the swap fails.
	ToolchainDir string
	// StageBinary is where the built worker binary is written. It must
	// be absolute and defaults to <BuildDir>/rsmon-worker. The running
	// service and its config are NOT touched until Activation runs.
	StageBinary string
	// SessionTimeout bounds each remote command. 0 uses the default
	// (30 minutes); the build step can legitimately run for minutes.
	SessionTimeout time.Duration
	// Activation drives work package 4: after the staging build, the
	// staged binary, validated environment, data dir, and the detected
	// init's service definition are installed atomically and the worker
	// is started and verified (process + /healthz). Any failure rolls
	// back to the prior working install. Empty keeps SourceInstall at the
	// staging boundary and touches no service configuration.
	Activation ActivationOptions

	// activationEnv is the once-rendered activation environment, computed
	// during option normalization so the env file is read and rendered
	// exactly once per run (no TOCTOU between preflight and activation).
	activationEnv []byte
}

// SourceInstallResult is what a source installation resolved to. The
// resolved branch and commit are recorded on the remote host in
// RecordFile, and the built binary is left at StageBinary for the next
// (service-activation) work package to install atomically.
type SourceInstallResult struct {
	Detection      sshinstall.Detection
	Plan           sshinstall.SourcePlan
	GoArch         string // resolved archive suffix, e.g. "linux-amd64"
	ToolchainDir   string
	ResolvedBranch string
	ResolvedCommit string
	RecordFile     string
	StageBinary    string
	// Activation is set when the staged binary was atomically installed
	// and the worker started/verified. It records the installed layout
	// and the supervisor used. Nil when Activation was not requested.
	Activation *ActivationResult
}

// defaultToolchainDir is the standard Go installation prefix.
const defaultToolchainDir = "/usr/local/go"

// defaultSessionTimeout bounds each remote command when the operator
// does not configure one. The staging build and first-time module
// downloads can run for minutes, so this is generous.
const defaultSessionTimeout = 30 * time.Minute

// commitRecordName is the file (inside BuildDir) that records the
// resolved branch and commit the build was produced from.
const commitRecordName = "rsmon-worker.commit"

var (
	commitPattern     = regexp.MustCompile(`^[0-9a-f]{40}$`)
	branchNamePattern = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9._/-]*$`)
)

// sourceExecutor bundles the SSH client, privilege options, and per
// command timeout used by every remote source-install step.
type sourceExecutor struct {
	client  *ssh.Client
	ssh     SSHOptions
	timeout time.Duration
}

// runPrivileged executes a command through the SSHOptions privilege path
// (root, passwordless sudo, or sudo -S) and returns bounded stdout.
func (e *sourceExecutor) runPrivileged(command string) ([]byte, error) {
	cmd, stdin := sudoWrap(e.ssh.User, e.ssh.SudoPassword, command)
	return runRemoteOutput(e.client, cmd, stdin, e.timeout)
}

// runPlain executes a command as the SSH user and returns bounded
// stdout.
func (e *sourceExecutor) runPlain(command string) ([]byte, error) {
	return runRemoteOutput(e.client, command, nil, e.timeout)
}

// fileProber builds the sshinstall.FileProber used for init-system
// detection over a live SSH session.
func (e *sourceExecutor) fileProber() sshinstall.FileProber {
	return func(paths ...string) map[string]bool {
		quoted := make([]string, len(paths))
		for i, p := range paths {
			quoted[i] = shellQuote(p)
		}
		expr := "for p in " + strings.Join(quoted, " ") + "; do [ -e \"$p\" ] && printf '%s\\n' \"$p\"; done; true"
		out, err := e.runPlain(expr)
		if err != nil {
			return nil
		}
		present := make(map[string]bool, len(paths))
		for _, line := range strings.Split(strings.TrimSpace(string(out)), "\n") {
			if line = strings.TrimSpace(line); line != "" {
				present[line] = true
			}
		}
		return present
	}
}

// SourceInstall executes the source-install flow over SSH: it reuses the
// deploy command's SSH authentication and host-key verification, detects
// the remote host, plans the pinned toolchain and package prerequisites,
// then installs packages, downloads and verifies the Go toolchain,
// clones/updates the public repository (verifying an existing checkout's
// origin matches the configured repository), checks out the resolved
// branch, builds the worker to a staging path, and only then records the
// resolved branch and commit.
//
// When Activation.Activate is set, the staged binary is then atomically
// installed together with the validated environment, data directory, and
// the detected init's service definition, and the worker is started and
// verified (process + /healthz); any activation/start/health failure
// rolls back to the prior working install. Without activation the
// running service, its configuration, and its data directory are
// deliberately untouched (the staging boundary of work package 3).
//
// Every remote step runs with the same privilege path as `deploy` (root,
// passwordless sudo, or sudo -S), every interpolated value is
// single-quoted, every step script fails closed (`set -eu` or explicit
// `&&`/retry), and every failure returns a bounded, actionable error.
// Each remote command is capped by SessionTimeout and its stdout is
// size-bounded.
func SourceInstall(opts SourceInstallOptions) (*SourceInstallResult, error) {
	opts, err := normalizeSourceOptions(opts)
	if err != nil {
		return nil, err
	}
	auth, err := sshAuth(opts.SSHOptions)
	if err != nil {
		return nil, err
	}
	hostKey, err := hostKeyCallback(opts.SSHOptions)
	if err != nil {
		return nil, err
	}
	client, err := ssh.Dial("tcp", net.JoinHostPort(opts.Host, strconv.Itoa(opts.Port)), &ssh.ClientConfig{
		User:            opts.User,
		Auth:            auth,
		HostKeyCallback: hostKey,
		Timeout:         15 * time.Second,
	})
	if err != nil {
		return nil, fmt.Errorf("connect to %s: %w", opts.Host, err)
	}
	defer client.Close() //nolint:errcheck

	executor := &sourceExecutor{client: client, ssh: opts.SSHOptions, timeout: opts.SessionTimeout}

	osRelease, err := executor.runPlain("cat /etc/os-release")
	if err != nil {
		return nil, fmt.Errorf("read remote /etc/os-release: %w", err)
	}
	uname, err := executor.runPlain("uname -m")
	if err != nil {
		return nil, fmt.Errorf("read remote machine architecture: %w", err)
	}
	detection := sshinstall.Detect(string(osRelease), executor.fileProber())
	plan, err := sshinstall.PlanSource(detection, sshinstall.SourceOptions{
		Repo:          opts.Repo,
		Branch:        opts.Branch,
		GoVersion:     opts.GoVersion,
		GoArch:        opts.GoArch,
		UnameM:        strings.TrimSpace(string(uname)),
		BuildDir:      opts.BuildDir,
		GoModuleProxy: opts.GoModuleProxy,
	})
	if err != nil {
		return nil, err
	}

	stage := opts.StageBinary
	if stage == "" {
		stage = filepath.Join(plan.BuildDir, "rsmon-worker")
	}
	result := &SourceInstallResult{
		Detection:    detection,
		Plan:         plan,
		GoArch:       plan.Toolchain.Arch,
		ToolchainDir: opts.ToolchainDir,
		StageBinary:  stage,
	}

	if len(plan.Packages) > 0 {
		script := "sh -c " + shellQuote(packageScript(detection.PackageManager, plan.Packages))
		if _, err := executor.runPrivileged(script); err != nil {
			return nil, fmt.Errorf("install prerequisites via %s: %w", detection.PackageManager, err)
		}
	}

	script := "sh -c " + shellQuote(toolchainScript(plan.Toolchain, opts.ToolchainDir))
	if _, err := executor.runPrivileged(script); err != nil {
		return nil, fmt.Errorf("install Go %s toolchain: %w", plan.Toolchain.Version, err)
	}

	// Clone or update. An existing checkout must point at the configured
	// repository, or the install fails before fetching or building.
	script = "sh -c " + shellQuote(cloneUpdateScript(plan.Repo, plan.BuildDir))
	if _, err := executor.runPrivileged(script); err != nil {
		return nil, fmt.Errorf("clone/update source repository: %w", err)
	}

	out, err := executor.runPrivileged("sh -c " + shellQuote(resolveBranchScript(plan.BuildDir)))
	if err != nil {
		return nil, fmt.Errorf("resolve remote default branch: %w", err)
	}
	branch, err := parseResolvedBranch(string(out))
	if err != nil {
		return nil, err
	}
	if plan.Branch != "" && plan.Branch != branch {
		ref := "refs/remotes/origin/" + plan.Branch
		if _, err := executor.runPrivileged("sh -c " + shellQuote(refExistsScript(plan.BuildDir, ref))); err != nil {
			return nil, fmt.Errorf("branch %q does not exist on the remote repository: %w", plan.Branch, err)
		}
		branch = plan.Branch
	}
	result.ResolvedBranch = branch

	// Checkout fails closed: a dirty tree or missing branch aborts before
	// the build, so the previous staging binary is left untouched.
	out, err = executor.runPrivileged("sh -c " + shellQuote(checkoutScript(plan.BuildDir, branch)))
	if err != nil {
		return nil, fmt.Errorf("check out branch %q: %w", branch, err)
	}
	commit, err := parseResolvedCommit(string(out))
	if err != nil {
		return nil, fmt.Errorf("resolve commit: %w", err)
	}
	result.ResolvedCommit = commit

	// Build with the repository's own flags (same -ldflags shape the
	// worker Makefile uses), resolving COMMIT from the checkout and
	// BUILD_DATE at install time. The binary is built to a temp path,
	// verified, and atomically swapped into the staging path; a failed
	// build leaves the previous staging binary in place.
	buildDate := time.Now().UTC().Format("2006-01-02T15:04:05Z")
	ldflags := fmt.Sprintf("-s -w -X main.version=dev -X main.commit=%s -X main.buildDate=%s", commit[:12], buildDate)
	script = "sh -c " + shellQuote(buildScript(opts.ToolchainDir, plan.BuildDir, plan.GoModuleProxy, stage, ldflags))
	if _, err := executor.runPrivileged(script); err != nil {
		return nil, fmt.Errorf("build worker binary: %w", err)
	}

	// The commit record is written only after a successful build, so the
	// record and the staged binary always correspond to the same commit.
	result.RecordFile = filepath.Join(plan.BuildDir, commitRecordName)
	if _, err := executor.runPrivileged("sh -c " + shellQuote(commitRecordScript(plan.BuildDir, branch, commit))); err != nil {
		return nil, fmt.Errorf("record resolved commit: %w", err)
	}

	// Work package 4: atomically install the staged build and activate
	// the worker. On any failure the remote script restores the previous
	// working install and this step returns a bounded error.
	if opts.Activation.Activate {
		if err := executor.activateWorker(opts, result); err != nil {
			return nil, err
		}
	}
	return result, nil
}

// normalizeSourceOptions validates operator input before any remote
// connection or mutation. Every value that later reaches a remote shell
// is constrained here.
func normalizeSourceOptions(o SourceInstallOptions) (SourceInstallOptions, error) {
	if o.Host == "" || o.User == "" {
		return o, errors.New("--host and --user are required for source install")
	}
	if o.Port == 0 {
		o.Port = 22
	}
	if o.Port < 1 || o.Port > 65535 {
		return o, errors.New("SSH port must be between 1 and 65535")
	}
	if o.Branch != "" && !validBranchName(o.Branch) {
		return o, fmt.Errorf("invalid branch %q: only A-Za-z0-9, dots, underscores, slashes, and hyphens are allowed", o.Branch)
	}
	if o.GoVersion != "" && !sshinstall.ValidGoVersion(o.GoVersion) {
		return o, fmt.Errorf("invalid Go version %q: only digits, letters, dots, dashes, and underscores are allowed", o.GoVersion)
	}
	if o.GoArch != "" && !sshinstall.ValidGoArch(o.GoArch) {
		return o, fmt.Errorf("invalid Go architecture %q: only letters, digits, dashes, and underscores are allowed", o.GoArch)
	}
	if err := sshinstall.ValidateRepoURL(o.Repo); err != nil {
		return o, fmt.Errorf("invalid repository: %w", err)
	}
	if o.ToolchainDir == "" {
		o.ToolchainDir = defaultToolchainDir
	}
	if !strings.HasPrefix(o.ToolchainDir, "/") || filepath.Base(o.ToolchainDir) != "go" {
		return o, fmt.Errorf("toolchain directory must be an absolute path ending in /go, got %q", o.ToolchainDir)
	}
	if o.StageBinary != "" && !strings.HasPrefix(o.StageBinary, "/") {
		return o, fmt.Errorf("staging binary path must be absolute, got %q", o.StageBinary)
	}
	if o.SessionTimeout <= 0 {
		o.SessionTimeout = defaultSessionTimeout
	}
	if o.Activation.Activate {
		// Render (and validate) the activation environment exactly once
		// here. The rendered bytes are reused at activation time, so the
		// env file is read a single time and cannot change between the
		// preflight and the remote install (env-file TOCTOU).
		env, err := o.Activation.renderEnv()
		if err != nil {
			return o, fmt.Errorf("activation: %w", err)
		}
		if err := validateRenderedEnv(env); err != nil {
			return o, fmt.Errorf("activation: %w", err)
		}
		o.activationEnv = env
	}
	return o, nil
}

// validBranchName reports whether a branch is a safe git branch name
// that can be interpolated into remote commands. The charset check is
// the command-injection boundary; the extra rules reject git-invalid or
// ambiguous refname patterns.
func validBranchName(branch string) bool {
	if !branchNamePattern.MatchString(branch) {
		return false
	}
	if strings.HasPrefix(branch, "-") || strings.HasPrefix(branch, "/") ||
		strings.Contains(branch, "..") || strings.Contains(branch, "@{") ||
		strings.Contains(branch, "//") || strings.HasSuffix(branch, ".") ||
		strings.HasSuffix(branch, "/") {
		return false
	}
	return true
}

// parseResolvedBranch turns the `git symbolic-ref` output
// ("origin/master\n") into the short branch name, validating it so
// remote-controlled output can never inject a command.
func parseResolvedBranch(raw string) (string, error) {
	branch := strings.TrimSpace(raw)
	branch = strings.TrimPrefix(branch, "origin/")
	if !validBranchName(branch) {
		return "", fmt.Errorf("remote reported an invalid default branch %q", strings.TrimSpace(raw))
	}
	return branch, nil
}

// parseResolvedCommit extracts the 40-hex commit from `git rev-parse
// HEAD` output, taking the last whitespace-separated token so unrelated
// stdout cannot satisfy the parse.
func parseResolvedCommit(raw string) (string, error) {
	fields := strings.Fields(strings.TrimSpace(raw))
	if len(fields) == 0 {
		return "", errors.New("remote reported no commit")
	}
	commit := fields[len(fields)-1]
	if !commitPattern.MatchString(commit) {
		return "", fmt.Errorf("remote reported an invalid resolved commit %q", commit)
	}
	return commit, nil
}

// quoteList renders each item as a single shell-quoted word.
func quoteList(items []string) string {
	quoted := make([]string, len(items))
	for i, item := range items {
		quoted[i] = shellQuote(item)
	}
	return strings.Join(quoted, " ")
}

// packageScript installs the minimal build prerequisites with the
// distro's package manager. It is idempotent on every supported manager
// and never installs a C compiler (the worker builds with CGO disabled).
func packageScript(pkg sshinstall.PackageManager, pkgs []string) string {
	quoted := quoteList(pkgs)
	switch pkg {
	case sshinstall.PkgApk:
		return "apk add --no-cache " + quoted
	case sshinstall.PkgApt:
		// Ubuntu/Debian need a fresh package index before installing.
		return "export DEBIAN_FRONTEND=noninteractive\napt-get update\napt-get install -y --no-install-recommends " + quoted
	case sshinstall.PkgPacman:
		return "pacman -Sy --noconfirm --needed " + quoted
	case sshinstall.PkgDnf:
		return "dnf install -y --setopt=install_weak_deps=False " + quoted
	case sshinstall.PkgYum:
		return "yum install -y " + quoted
	default:
		return ""
	}
}

// toolchainScript downloads the pinned Go toolchain, verifies its
// published SHA-256 before extraction, stages the extract on the same
// filesystem as the target, verifies the staged toolchain reports the
// target version, and only then swaps it into place. The prior toolchain
// (when present) is moved to a sibling backup first and is restored if
// the swap fails, so a failed download/verify/extract/swap always leaves
// the prior Go untouched. A present toolchain already reporting the
// target version is reused (idempotent rerun). Temp and staging
// directories are removed on success and failure.
func toolchainScript(tc sshinstall.Toolchain, toolchainDir string) string {
	parent := filepath.Dir(toolchainDir)
	want := "go" + tc.Version
	var b strings.Builder
	b.WriteString("set -eu\n")
	b.WriteString("parent=" + shellQuote(parent) + "\n")
	b.WriteString("toolchain=" + shellQuote(toolchainDir) + "\n")
	b.WriteString("want=" + shellQuote(want) + "\n")
	b.WriteString("if [ -x \"$toolchain/bin/go\" ]; then\n")
	b.WriteString("  have=\"$($toolchain/bin/go version 2>/dev/null | awk '{print $3}')\"\n")
	b.WriteString("  if [ \"$have\" = \"$want\" ]; then\n")
	b.WriteString("    echo \"go toolchain already present: $have\"\n")
	b.WriteString("    exit 0\n")
	b.WriteString("  fi\n")
	b.WriteString("fi\n")
	b.WriteString("work=\"$(mktemp -d /tmp/rsmon-toolchain-XXXXXX)\"\n")
	b.WriteString("staging=\"$(mktemp -d \"$parent/.go-staging-XXXXXX\")\"\n")
	b.WriteString("trap 'rm -rf \"$work\" \"$staging\"' EXIT HUP INT TERM\n")
	b.WriteString("archive=\"$work/go" + tc.Version + "." + tc.Arch + ".tar.gz\"\n")
	b.WriteString("curl -fsSL --retry 3 --retry-delay 2 -o \"$archive\" " + shellQuote(tc.URL) + "\n")
	b.WriteString("printf '%s  %s\\n' " + shellQuote(tc.SHA256) + " \"$archive\" | sha256sum -c -\n")
	b.WriteString("tar -C \"$staging\" -xzf \"$archive\"\n")
	b.WriteString("staged=\"$($staging/go/bin/go version | awk '{print $3}')\"\n")
	b.WriteString("if [ \"$staged\" != \"$want\" ]; then\n")
	b.WriteString("  printf 'staged toolchain failed verification: %s\\n' \"$staged\" >&2\n")
	b.WriteString("  exit 1\n")
	b.WriteString("fi\n")
	// Swap atomically on the same filesystem, preserving the prior
	// toolchain in a sibling backup with rollback on failure.
	b.WriteString("backup=\"\"\n")
	b.WriteString("if [ -e \"$toolchain\" ]; then\n")
	b.WriteString("  backup=\"$parent/.go-backup\"\n")
	b.WriteString("  rm -rf \"$backup\"\n")
	b.WriteString("  mv \"$toolchain\" \"$backup\"\n")
	b.WriteString("fi\n")
	b.WriteString("if ! mv \"$staging/go\" \"$toolchain\"; then\n")
	b.WriteString("  if [ -n \"$backup\" ]; then\n")
	b.WriteString("    mv \"$backup\" \"$toolchain\"\n")
	b.WriteString("  fi\n")
	b.WriteString("  exit 1\n")
	b.WriteString("fi\n")
	b.WriteString("if [ -n \"$backup\" ]; then\n")
	b.WriteString("  rm -rf \"$backup\"\n")
	b.WriteString("fi\n")
	b.WriteString("\"$toolchain/bin/go\" version\n")
	return b.String()
}

// cloneUpdateScript clones the repository when missing and otherwise
// fetches the latest refs, so a rerun updates in place. Before fetching
// an existing checkout, it verifies the configured repository matches
// the checkout's `remote.origin.url` exactly, so the installer can never
// fetch or build an unconfigured repository. The clone/fetch is retried
// up to three times (2s apart) because real repositories can be
// transiently unreachable (DNS, TLS, or proxy hiccups). The script fails
// closed: any exhausted retry exits non-zero.
func cloneUpdateScript(repo, buildDir string) string {
	return "set -u\n" +
		"repo=" + shellQuote(repo) + "\n" +
		"dir=" + shellQuote(buildDir) + "\n" +
		"if [ ! -d \"$dir/.git\" ]; then\n" +
		"  attempt=0\n" +
		"  while [ \"$attempt\" -lt 3 ]; do\n" +
		"    if git clone \"$repo\" \"$dir\"; then\n" +
		"      exit 0\n" +
		"    fi\n" +
		"    attempt=$((attempt + 1))\n" +
		"    sleep 2\n" +
		"  done\n" +
		"  exit 1\n" +
		"fi\n" +
		"origin=\"$(git -C \"$dir\" config --get remote.origin.url || true)\"\n" +
		"if [ \"$origin\" != \"$repo\" ]; then\n" +
		"  printf 'existing checkout origin does not match configured repository\\nconfigured: %s\\nfound: %s\\n' \"$repo\" \"$origin\" >&2\n" +
		"  exit 1\n" +
		"fi\n" +
		"attempt=0\n" +
		"while [ \"$attempt\" -lt 3 ]; do\n" +
		"  if git -C \"$dir\" fetch --prune origin; then\n" +
		"    exit 0\n" +
		"  fi\n" +
		"  attempt=$((attempt + 1))\n" +
		"  sleep 2\n" +
		"done\n" +
		"exit 1\n"
}

// resolveBranchScript prints the remote's default branch short name
// (with an "origin/" prefix) via origin/HEAD. It fails closed so a
// set-head failure aborts rather than resolving a stale default.
func resolveBranchScript(buildDir string) string {
	return "set -eu\n" +
		"git -C " + shellQuote(buildDir) + " remote set-head origin --auto >/dev/null\n" +
		"git -C " + shellQuote(buildDir) + " symbolic-ref --short refs/remotes/origin/HEAD\n"
}

// refExistsScript verifies a remote-tracking ref exists (exit 0) without
// emitting output.
func refExistsScript(buildDir, ref string) string {
	return "git -C " + shellQuote(buildDir) + " show-ref --verify --quiet " + shellQuote(ref)
}

// checkoutScript moves the local branch to the resolved remote branch
// and prints the resolved commit. It fails closed (`set -eu`): before the
// destructive `checkout -B` (which would silently discard local changes)
// it refuses when the tracked working tree is dirty, so a checkout
// failure can never be masked by a stale `rev-parse` from the previous
// checkout, and a failed checkout aborts before the build.
func checkoutScript(buildDir, branch string) string {
	return "set -eu\n" +
		"git -C " + shellQuote(buildDir) + " diff --quiet || { echo 'working tree has uncommitted changes; refusing to overwrite' >&2; exit 1; }\n" +
		"git -C " + shellQuote(buildDir) + " diff --cached --quiet || { echo 'working tree has staged changes; refusing to overwrite' >&2; exit 1; }\n" +
		"git -C " + shellQuote(buildDir) + " checkout -q -B " + shellQuote(branch) + " " + shellQuote("origin/"+branch) + "\n" +
		"git -C " + shellQuote(buildDir) + " rev-parse HEAD\n"
}

// commitRecordScript writes the resolved branch and commit to the build
// dir record file with a bounded, greppable format. It runs only after a
// successful build, so the record always matches the staged binary.
func commitRecordScript(buildDir, branch, commit string) string {
	path := shellQuote(filepath.Join(buildDir, commitRecordName))
	format := shellQuote("branch=%s\\ncommit=%s\\n")
	return "umask 022; printf " + format + " " + shellQuote(branch) + " " + shellQuote(commit) +
		" > " + path + " && chmod 0644 " + path
}

// buildScript builds the worker with the repository's own flags into the
// staging path and verifies the resulting binary runs. CGO is disabled,
// trimpath keeps the build reproducible, and both caches (GOCACHE and
// GOMODCACHE) live inside the build dir so reruns reuse them. The binary
// is built to a sibling temp path, verified, and only then atomically
// swapped over the previous staging binary, so a failed build never
// replaces it.
func buildScript(toolchainDir, buildDir, goproxy, stage, ldflags string) string {
	var b strings.Builder
	b.WriteString("set -eu\n")
	b.WriteString("cd " + shellQuote(buildDir) + "\n")
	b.WriteString("export PATH=" + shellQuote(toolchainDir+"/bin") + ":$PATH\n")
	b.WriteString("export GOCACHE=" + shellQuote(buildDir+"/.gocache") + "\n")
	b.WriteString("export GOMODCACHE=" + shellQuote(buildDir+"/.gomodcache") + "\n")
	if goproxy != "" {
		b.WriteString("export GOPROXY=" + shellQuote(goproxy) + "\n")
	}
	b.WriteString("stage=" + shellQuote(stage) + "\n")
	b.WriteString("tmp=\"$stage.new\"\n")
	b.WriteString("trap 'rm -f \"$tmp\"' EXIT HUP INT TERM\n")
	b.WriteString("CGO_ENABLED=0 ")
	b.WriteString(shellQuote(toolchainDir + "/bin/go"))
	b.WriteString(" build -trimpath -ldflags=" + shellQuote(ldflags) + " -o \"$tmp\" ./cmd/rsmon-worker\n")
	b.WriteString("\"$tmp\" --version\n")
	b.WriteString("mv -f \"$tmp\" \"$stage\"\n")
	return b.String()
}