// Package processbuilds reads the build each process of this node was started with (design 47, *The fingerprint handover*):
// the value MESH_FINGERPRINT the node-engine handed a daemon or a container at its start, read from the running process
// itself, never from what the declaration would hand it now. A process started before the node-engine handed the value,
// or one whose value cannot be read, says why its build is unknown instead of a fingerprint.
package processbuilds

import (
	"context"
	"encoding/json"
	"fmt"
	"os"
	"sort"
	"strconv"
	"strings"
	"time"

	nodeengine "git.novox.be/novox/mesh-contracts/go/seats/nodeengine"
	"git.novox.be/novox/mesh-sdk/go/declaration"
)

// Var is the environment value the build is handed in.
const Var = "MESH_FINGERPRINT"

// Runner runs a command and answers its output.
type Runner func(ctx context.Context, name string, args ...string) (string, error)

// Reader reads the running processes of a node.
type Reader struct {
	Node string
	Run  Runner
	// Environ reads a process's environment by its pid; nil reads /proc/<pid>/environ.
	Environ func(pid int) ([]byte, error)
}

func (r Reader) environ(pid int) ([]byte, error) {
	if r.Environ != nil {
		return r.Environ(pid)
	}
	return os.ReadFile(fmt.Sprintf("/proc/%d/environ", pid))
}

// Read answers the build of every daemon and container the declaration holds, narrowed to module when it is not empty,
// ordered by module and process.
func (r Reader) Read(ctx context.Context, d *declaration.Declaration, module string) nodeengine.ProcessBuilds {
	out := nodeengine.ProcessBuilds{Node: r.Node, Processes: []nodeengine.ProcessBuild{}}
	if d == nil {
		out.Error = "this node keeps no declaration of what it runs"
		return out
	}
	for _, res := range d.Resources {
		owner := res.DeclaredModule()
		if owner == "" {
			owner, _ = declaration.ModuleGuess(res.Identity())
		}
		if module != "" && owner != module {
			continue
		}
		switch p := res.(type) {
		case *declaration.Process:
			out.Processes = append(out.Processes, r.daemon(ctx, owner, p.Name))
		case *declaration.Container:
			out.Processes = append(out.Processes, r.container(ctx, owner, p.Name))
		}
	}
	sort.SliceStable(out.Processes, func(i, j int) bool {
		a, b := out.Processes[i], out.Processes[j]
		if a.Module != b.Module {
			return a.Module < b.Module
		}
		return a.Process < b.Process
	})
	return out
}

func (r Reader) daemon(ctx context.Context, module, name string) nodeengine.ProcessBuild {
	unit := name + ".service"
	b := nodeengine.ProcessBuild{Module: module, Process: unit, Kind: nodeengine.ProcessBuildKindService}
	said, err := r.Run(ctx, "systemctl", "show", "--property=MainPID,ActiveEnterTimestamp", "--", unit)
	if err != nil {
		b.Unknown = "the unit cannot be read: " + err.Error()
		return b
	}
	props := map[string]string{}
	for _, line := range strings.Split(said, "\n") {
		if k, v, ok := strings.Cut(strings.TrimSpace(line), "="); ok {
			props[k] = v
		}
	}
	pid, _ := strconv.Atoi(props["MainPID"])
	if pid <= 0 {
		b.Unknown = "the unit runs no process now"
		return b
	}
	if t, err := time.Parse("Mon 2006-01-02 15:04:05 MST", props["ActiveEnterTimestamp"]); err == nil {
		b.Started = &t
	}
	raw, err := r.environ(pid)
	if err != nil {
		b.Unknown = "its environment cannot be read: " + err.Error()
		return b
	}
	b.Fingerprint, b.Unknown = fingerprintIn(strings.Split(string(raw), "\x00"))
	return b
}

func (r Reader) container(ctx context.Context, module, name string) nodeengine.ProcessBuild {
	b := nodeengine.ProcessBuild{Module: module, Process: name, Kind: nodeengine.ProcessBuildKindContainer}
	var said string
	var err error
	for _, cri := range []string{"docker", "podman"} {
		if said, err = r.Run(ctx, cri, "container", "inspect", "--format", "{{json .Config.Env}}\t{{.State.StartedAt}}", name); err == nil {
			break
		}
	}
	if err != nil {
		b.Unknown = "the container cannot be read: " + err.Error()
		return b
	}
	env, started, _ := strings.Cut(strings.TrimSpace(said), "\t")
	var vars []string
	if err := json.Unmarshal([]byte(env), &vars); err != nil {
		b.Unknown = "its environment cannot be read: " + err.Error()
		return b
	}
	if t, err := time.Parse(time.RFC3339Nano, started); err == nil {
		b.Started = &t
	}
	b.Fingerprint, b.Unknown = fingerprintIn(vars)
	return b
}

// fingerprintIn is the value of the handed variable in a process's environment, or why there is none.
func fingerprintIn(vars []string) (fingerprint, unknown string) {
	for _, v := range vars {
		if value, ok := strings.CutPrefix(v, Var+"="); ok && value != "" {
			return value, ""
		}
	}
	return "", "started before the node-engine handed " + Var + ", or without it"
}