//===----------------------------------------------------------------------===//
// Copyright © 2026 Apple Inc. and the Containerization project authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//===----------------------------------------------------------------------===//

#if os(Linux)

import ContainerizationError
import ContainerizationOS
import Foundation
import Logging
import Synchronization

final class IOPair: Sendable {
    private let io: Mutex<IO>
    private let logger: Logger?
    private let reason: String

    private struct IO {
        let from: IOCloser
        let to: IOCloser
        let buffer: UnsafeMutableBufferPointer<UInt8>
        var closed: Bool
        var registeredFd: Int32?
        // [Nucleic vendored patch] Backpressure state: bytes read from `from` that `to` couldn't
        // take yet (its buffer was full), plus whether `to` is currently registered for EPOLLOUT
        // to flush them. While `pending` is non-empty the relay reads nothing more — the source
        // pipe backs up and throttles the *producing process* instead of this thread. The write
        // fd used to be BLOCKING (only registered fds get O_NONBLOCK, and it never was), so one
        // slow-drained stream parked the shared ProcessSupervisor poller thread — freezing every
        // exec's stdio and every control-plane relay in the container at once.
        var pending: [UInt8]
        var pendingOffset: Int
        var writeFdRegistered: Bool

        func drain() {
            let readFrom = OSFile(fd: from.fileDescriptor)
            let writeTo = OSFile(fd: to.fileDescriptor)

            while true {
                let r = readFrom.read(buffer)
                if r.read > 0 {
                    let view = UnsafeMutableBufferPointer(
                        start: buffer.baseAddress,
                        count: r.read
                    )

                    let w = writeTo.write(view)
                    if w.wrote != r.read {
                        return
                    }
                }

                switch r.action {
                case .eof, .again, .error(_):
                    return
                default:
                    break
                }
            }
        }

        mutating func close(logger: Logger?) {
            if self.closed {
                return
            }

            // [Nucleic vendored patch] Flush what we can IN ORDER: the pending backlog first,
            // then (only if it fully flushed) a best-effort drain of the source. Draining with
            // unsent pending bytes would reorder the stream.
            let writeTo = OSFile(fd: to.fileDescriptor)
            while pendingOffset < pending.count {
                let offset = pendingOffset
                let result = pending.withUnsafeMutableBufferPointer { buf in
                    writeTo.write(
                        UnsafeMutableBufferPointer(
                            start: buf.baseAddress!.advanced(by: offset),
                            count: buf.count - offset))
                }
                if result.wrote > 0 { pendingOffset += result.wrote }
                if result.action != .success { break }
            }
            if pendingOffset >= pending.count {
                // Try and drain IO first.
                self.drain()
            }

            // Remove the fds from our global epoll instance first.
            if let fd = self.registeredFd {
                do {
                    try ProcessSupervisor.default.unregisterFd(fd)
                } catch {
                    logger?.error("failed to delete fd from epoll \(fd): \(error)")
                }
                self.registeredFd = nil
            }
            // [Nucleic vendored patch] The write fd may be registered for backpressure flushing.
            if self.writeFdRegistered {
                do {
                    try ProcessSupervisor.default.unregisterFd(to.fileDescriptor)
                } catch {
                    logger?.error("failed to delete write fd from epoll \(to.fileDescriptor): \(error)")
                }
                self.writeFdRegistered = false
            }

            do {
                try self.from.close()
            } catch {
                logger?.error("failed to close reader fd for IOPair: \(error)")
            }

            do {
                try self.to.close()
            } catch {
                logger?.error("failed to close writer fd for IOPair: \(error)")
            }
            self.buffer.deallocate()
            self.closed = true
        }
    }

    init(
        readFrom: IOCloser,
        writeTo: IOCloser,
        reason: String,
        logger: Logger? = nil
    ) {
        let buffer = UnsafeMutableBufferPointer<UInt8>.allocate(capacity: Int(getpagesize()))
        self.io = Mutex(
            IO(
                from: readFrom,
                to: writeTo,
                buffer: buffer,
                closed: false,
                registeredFd: nil,
                pending: [],
                pendingOffset: 0,
                writeFdRegistered: false
            ))
        self.reason = reason
        self.logger = logger
    }

    func relay(ignoreHup: Bool = false) throws {
        self.logger?.info("setting up relay for \(reason)")

        let (readFromFd, writeToFd) = self.io.withLock { io in
            io.registeredFd = io.from.fileDescriptor
            return (io.from.fileDescriptor, io.to.fileDescriptor)
        }

        // [Nucleic vendored patch] The write fd must be non-blocking BEFORE the first relay write.
        // `Epoll.add` only sets O_NONBLOCK on fds it registers, and the write fd is registered only
        // on demand (EPOLLOUT backpressure below) — so without this, the very first full-buffer
        // write blocked the shared poller thread.
        let flags = fcntl(writeToFd, F_GETFL)
        if flags == -1 || fcntl(writeToFd, F_SETFL, flags | O_NONBLOCK) == -1 {
            self.logger?.error(
                "failed to set relay write fd non-blocking",
                metadata: ["fd": "\(writeToFd)", "errno": "\(errno)"])
        }

        try ProcessSupervisor.default.registerFd(readFromFd, mask: .input) { mask in
            self.io.withLock { io in
                if io.closed {
                    return
                }

                if mask.isHangup && !mask.readyToRead {
                    self.logger?.debug("received EPOLLHUP with no EPOLLIN")
                    if !ignoreHup {
                        io.close(logger: self.logger)
                    }
                    return
                }

                self.pump(&io, mask: mask, ignoreHup: ignoreHup)
            }
        }
    }

    /// [Nucleic vendored patch] One relay pass, non-blocking end to end: flush any pending
    /// backlog toward `to`, then (only once it's empty) drain `from`. On a full destination the
    /// remainder is stashed in `pending` and the write fd registered for EPOLLOUT, whose edge
    /// re-enters this pump — so backpressure suspends the relay instead of blocking or spinning
    /// the shared poller thread. Must be called with the `io` lock held.
    private func pump(_ io: inout IO, mask: Epoll.Mask, ignoreHup: Bool) {
        let readFrom = OSFile(fd: io.from.fileDescriptor)
        let writeTo = OSFile(fd: io.to.fileDescriptor)

        // Flush the pending backlog first; reads stay suspended until it clears.
        while io.pendingOffset < io.pending.count {
            let offset = io.pendingOffset
            let result = io.pending.withUnsafeMutableBufferPointer { buf in
                writeTo.write(
                    UnsafeMutableBufferPointer(
                        start: buf.baseAddress!.advanced(by: offset),
                        count: buf.count - offset))
            }
            if result.wrote > 0 { io.pendingOffset += result.wrote }
            switch result.action {
            case .success:
                continue
            case .again:
                self.ensureWriteRegistered(&io)
                return
            default:
                self.logger?.error("stopping relay: write failed during backlog flush")
                io.close(logger: self.logger)
                return
            }
        }
        if !io.pending.isEmpty {
            io.pending = []
            io.pendingOffset = 0
            self.unregisterWrite(&io)
        }

        // Loop so we drain fully (edge-triggered epoll requires reading until EAGAIN).
        while true {
            let r = readFrom.read(io.buffer)
            if r.read > 0 {
                let view = UnsafeMutableBufferPointer(
                    start: io.buffer.baseAddress,
                    count: r.read
                )

                let w = writeTo.write(view)
                if w.wrote != r.read {
                    switch w.action {
                    case .again:
                        // Destination full: stash the remainder and suspend reads until its
                        // EPOLLOUT edge flushes it. (A later `read` re-reports EOF if this
                        // chunk was the stream's last, so no EOF is lost by returning here.)
                        io.pending = Array(
                            UnsafeBufferPointer(
                                start: io.buffer.baseAddress!.advanced(by: w.wrote),
                                count: r.read - w.wrote))
                        io.pendingOffset = 0
                        self.ensureWriteRegistered(&io)
                        return
                    default:
                        self.logger?.error("stopping relay: short write for stdio")
                        io.close(logger: self.logger)
                        return
                    }
                }
            }

            switch r.action {
            case .error(let errno):
                self.logger?.error("failed with errno \(errno) while reading for fd \(io.from.fileDescriptor)")
                fallthrough
            case .eof:
                self.logger?.debug("closing relay for \(io.from.fileDescriptor)")
                io.close(logger: self.logger)
                return
            case .again:
                if mask.isHangup && !ignoreHup {
                    self.logger?.error("received EPOLLHUP and EAGAIN exiting")
                    io.close(logger: self.logger)
                }
                return
            default:
                break
            }
        }
    }

    /// [Nucleic vendored patch] Register the write fd for EPOLLOUT so the pending backlog is
    /// flushed when the destination drains. Registration failure closes the pair — without the
    /// flush wakeup the relay would hang with data stranded. Must be called with the lock held.
    private func ensureWriteRegistered(_ io: inout IO) {
        guard !io.writeFdRegistered else { return }
        let writeToFd = io.to.fileDescriptor
        do {
            try ProcessSupervisor.default.registerFd(writeToFd, mask: .output) { _ in
                self.io.withLock { io in
                    guard !io.closed else { return }
                    // An empty mask: HUP/EOF handling rides the read fd's own events.
                    self.pump(&io, mask: [], ignoreHup: true)
                }
            }
            io.writeFdRegistered = true
        } catch {
            self.logger?.error("failed to register relay write fd for backpressure: \(error)")
            io.close(logger: self.logger)
        }
    }

    /// [Nucleic vendored patch] Drop the EPOLLOUT registration once the backlog has flushed.
    /// Must be called with the lock held.
    private func unregisterWrite(_ io: inout IO) {
        guard io.writeFdRegistered else { return }
        io.writeFdRegistered = false
        do {
            try ProcessSupervisor.default.unregisterFd(io.to.fileDescriptor)
        } catch {
            self.logger?.error("failed to unregister relay write fd: \(error)")
        }
    }

    func close() {
        self.io.withLock { io in
            self.logger?.info("closing relay for \(reason)")
            io.close(logger: self.logger)
        }
    }
}

#endif