// SPDX-License-Identifier: LGPL-3.0-only
// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®

// Crash safety of the commit protocol. A child process appends without
// flushing and dies: the parent must see exactly the committed rows. A
// damaged newer commit slot falls back to the older one. Uncommitted growth
// is truncated on the next writable open. Tombstones written before an
// unclean exit are recounted.

#include <sys/wait.h>
#include <unistd.h>

import Crafter.DB;
import std;

using namespace Crafter;
namespace fs = std::filesystem;

namespace {
    int failures = 0;

    void Check(bool condition, std::string_view what) {
        if (!condition) {
            std::println("FAIL: {}", what);
            ++failures;
        }
    }

    template<typename T>
    void CheckEqual(const T& actual, const T& expected, std::string_view what) {
        if (!(actual == expected)) {
            std::println("FAIL: {} — expected '{}', got '{}'", what, expected, actual);
            ++failures;
        }
    }

    struct Entry {
        std::string text;
        std::uint64_t n;
        static constexpr std::uint32_t version = 1;
    };

    // Runs `work` in a child. `work` must end with Die() while its table is
    // still in scope, so no destructor (and no Flush) ever runs.
    [[noreturn]] void Die() { _exit(0); }

    void InChildWithoutCleanup(const std::function<void()>& work) {
        const pid_t pid = fork();
        if (pid == 0) {
            try {
                work();
            } catch (const std::exception& error) {
                std::println("child failed: {}", error.what());
                _exit(2);
            }
            _exit(5);   // work() returned instead of dying: the test is wrong
        }
        int status = 0;
        waitpid(pid, &status, 0);
        Check(WIFEXITED(status) && WEXITSTATUS(status) == 0, "child ran to its exit point");
    }

    void Patch(const fs::path& path, std::uint64_t offset, std::byte value) {
        std::fstream file(path, std::ios::in | std::ios::out | std::ios::binary);
        file.seekp(static_cast<std::streamoff>(offset));
        file.write(reinterpret_cast<const char*>(&value), 1);
    }
}

int main() {
    const fs::path dir = fs::temp_directory_path() / "crafter-db-recover";
    fs::remove_all(dir);
    const fs::path path = dir / "log.cdb";

    // Committed rows survive, unflushed rows vanish, the table stays valid.
    InChildWithoutCleanup([&] {
        DB::Table<Entry> table(path);
        for (std::uint64_t i = 0; i < 100; i++) table.Append({ std::format("committed {}", i), i });
        table.Flush();
        for (std::uint64_t i = 100; i < 150; i++) table.Append({ std::format("lost {}", i), i });
        Die();   // no Flush, no destructor
    });
    {
        DB::Table<Entry> table(path);
        CheckEqual<std::uint64_t>(table.Size(), 100, "only flushed rows are visible after a crash");
        Check(table.Back()[&Entry::text] == "committed 99", "last committed row intact");
        table.Append({ "after crash", 100 });
        Check(table[100][&Entry::text] == "after crash", "slot after the committed rows is reusable");
        table.Flush();
    }

    // EveryAppend durability: nothing is lost.
    InChildWithoutCleanup([&] {
        DB::Table<Entry> table(path, { .durability = DB::Durability::EveryAppend });
        table.Append({ "durable", 101 });
        Die();
    });
    {
        DB::Table<Entry> table(path);
        CheckEqual<std::uint64_t>(table.Size(), 102, "EveryAppend row survived a crash");
    }

    // Uncommitted growth is truncated on open.
    std::uint64_t committedLength = 0;
    InChildWithoutCleanup([&] {
        DB::Table<Entry> table(path);
        table.Flush();
        committedLength = fs::file_size(path);
        for (std::uint64_t i = 0; i < 2000; i++) table.Append({ std::string(100, 'g'), i });   // forces new extents
        Die();
    });
    {
        const DB::Inspection info = DB::TableFile::Inspect(path);
        committedLength = info.commit.fileLength;
        Check(fs::file_size(path) > committedLength, "uncommitted extents extend the file before recovery");
        DB::Table<Entry> table(path);
        CheckEqual<std::uint64_t>(fs::file_size(path), committedLength, "writable open truncates uncommitted growth");
        CheckEqual<std::uint64_t>(table.Size(), 102, "rows unchanged");
    }

    // Tombstones written before an unclean exit are recounted.
    InChildWithoutCleanup([&] {
        DB::Table<Entry> table(path);
        table.Erase(0);
        table.Erase(1);
        Die();
    });
    {
        DB::Table<Entry> table(path);
        CheckEqual<std::uint64_t>(table.Size(), 100, "live count recounted after unclean exit");
        std::uint64_t seen = 0;
        for ([[maybe_unused]] DB::Row<Entry> row : table) seen++;
        CheckEqual<std::uint64_t>(seen, 100, "iteration honours recovered tombstones");
        table.Compact();
        table.Flush();
    }

    // An Update whose heap bytes were written but whose process died before
    // any Flush: the row must read as either the old or the new value, never
    // point at uncommitted heap bytes.
    InChildWithoutCleanup([&] {
        DB::Table<Entry> table(path);
        table.Flush();
        table.Update(5, { std::string(300, 'u'), 5 });   // long value -> heap
        table.Set<&Entry::text>(6, std::string(200, 's'));
        Die();
    });
    {
        DB::Table<Entry> table(path);
        const std::string five(table[5][&Entry::text]);
        Check(five == std::string(300, 'u') || five == "committed 5", std::format("updated row is consistent after a crash (got {} bytes)", five.size()));
        const std::string six(table[6][&Entry::text]);
        Check(six == std::string(200, 's') || six == "committed 6", "Set row is consistent after a crash");
        Check(table.Verify().Ok(), "no dangling heap references after a crashed Update");
        table.Append({ "after update crash", 7 });
        Check(table[5][&Entry::text] == five, "later appends do not change the updated row's text");
        table.Flush();
    }

    // A forked child that destroys its inherited Table must not write a
    // commit record: its counters are the parent's at fork time.
    {
        DB::Table<Entry> table(path);
        table.Append({ "parent row", 8 });
        table.Flush();
        const DB::TableStats before = table.Stats();
        table.Erase(2);                                   // tombstone after the flush
        const pid_t pid = fork();
        if (pid == 0) {
            // normal destruction path in the child
            bool threw = false;
            try {
                table.Append({ "child", 9 });
            } catch (const DB::Error&) {
                threw = true;
            }
            if (!threw) _exit(6);
            const std::string_view text = table[0][&Entry::text];   // reads work on the shared mapping
            if (text.size() == 0) _exit(7);
            table.~Table();
            _exit(0);
        }
        int status = 0;
        waitpid(pid, &status, 0);
        Check(WIFEXITED(status) && WEXITSTATUS(status) == 0, std::format("child refused to write and destroyed its table (status {})", WIFEXITED(status) ? WEXITSTATUS(status) : -1));
        const DB::Inspection info = DB::TableFile::Inspect(path);
        CheckEqual<std::uint64_t>(info.commit.commitSeq, before.commitSeq, "child wrote no commit record");
        CheckEqual<std::uint64_t>(table.Size(), before.liveCount - 1, "parent still sees its tombstone");
        table.Flush();
        const DB::Inspection after = DB::TableFile::Inspect(path);
        CheckEqual<std::uint64_t>(after.commit.liveCount, before.liveCount - 1, "parent's commit carries the tombstone");
    }

    // A damaged newer commit slot falls back to the older one.
    {
        {
            DB::Table<Entry> table(path);
            table.Append({ "slot A", 1 });
            table.Flush();                      // commit N: rows r+1
            table.Append({ "slot B", 2 });      // the destructor's clean close writes commit N+1: rows r+2
        }
        const DB::Inspection before = DB::TableFile::Inspect(path);
        const std::size_t newer = before.header.commits[0].commitSeq > before.header.commits[1].commitSeq ? 0 : 1;
        Patch(path, 3072 + newer * 512 + 8, std::byte{ 0xEE });   // corrupt rowCount of the newest slot -> CRC fails
        DB::Table<Entry> table(path);
        CheckEqual<std::uint64_t>(table.SlotCount(), before.commit.rowCount - 1, "fell back to the previous commit");
        Check(table.Back()[&Entry::text] == "slot A", "the row of the lost commit is gone");
    }

    fs::remove_all(dir);
    if (failures == 0) std::println("ok");
    return failures == 0 ? 0 : 1;
}