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| 1 | +// BoundDeviceStore.swift |
| 2 | +// Which devices have been bound for sharing, kept apart from user configuration. |
| 3 | + |
| 4 | +import Foundation |
| 5 | +import Common |
| 6 | + |
| 7 | +/// The list of devices `bind` has made available, persisted so it survives a restart. |
| 8 | +/// |
| 9 | +/// This used to live in the configuration file alongside the port, the log level and |
| 10 | +/// the transfer tuning, which was wrong in three ways. |
| 11 | +/// |
| 12 | +/// The two have different owners. A port is chosen by a person and edited by hand; a |
| 13 | +/// bind list is written by a command and read by a daemon. Holding them together meant |
| 14 | +/// `bind` rewrote the whole configuration file to append one string, so a hand-edit |
| 15 | +/// made between the CLI reading the file and writing it back was lost. |
| 16 | +/// |
| 17 | +/// Worse, the CLI falls back to a default configuration when the file will not parse. |
| 18 | +/// A single malformed character therefore turned the next `bind` into a silent reset of |
| 19 | +/// the port and log level, with nothing but a warning on stderr to show for it. A |
| 20 | +/// command that shares a USB device has no business rewriting unrelated settings, and |
| 21 | +/// now it cannot: this file is the only thing bind and unbind touch. |
| 22 | +/// |
| 23 | +/// The read-modify-write is also serialised. Appending to a list means reading it, |
| 24 | +/// adding an entry and writing it back, and two `bind` commands doing that at once |
| 25 | +/// would leave only one of the devices bound. Writes are atomic, so a reader never sees |
| 26 | +/// a half-written file, but atomicity alone does not prevent a lost update. |
| 27 | +public final class BoundDeviceStore { |
| 28 | + |
| 29 | + /// Contents of the state file. An object rather than a bare array so that fields |
| 30 | + /// can be added later — a bind timestamp, say — without changing the format. |
| 31 | + private struct State: Codable { |
| 32 | + var boundDevices: [String] |
| 33 | + } |
| 34 | + |
| 35 | + private let path: String |
| 36 | + |
| 37 | + /// Locking uses a file of its own, never the state file. |
| 38 | + /// |
| 39 | + /// Saving replaces the state file by renaming a new one over it, which leaves any |
| 40 | + /// lock held on the old file attached to an inode nobody else will open. Two writers |
| 41 | + /// could then hold "the lock" on two different files and both proceed. A lock file |
| 42 | + /// that is only ever created, never replaced, is the thing they can agree on. |
| 43 | + private let lockPath: String |
| 44 | + |
| 45 | + public init(path: String = BoundDeviceStore.defaultPath()) { |
| 46 | + self.path = path |
| 47 | + self.lockPath = path + ".lock" |
| 48 | + } |
| 49 | + |
| 50 | + /// Where the list is kept. Worth reporting in diagnostics, since the answer to |
| 51 | + /// "why is this device not shared" is usually in this file. |
| 52 | + public var filePath: String { path } |
| 53 | + |
| 54 | + /// `~/.usbipd/bound-devices.json`, beside the configuration but separate from it. |
| 55 | + public static func defaultPath() -> String { |
| 56 | + let directory = FileManager.default.homeDirectoryForCurrentUser |
| 57 | + .appendingPathComponent(ServerConfig.defaultConfigDirName) |
| 58 | + return directory.appendingPathComponent("bound-devices.json").path |
| 59 | + } |
| 60 | + |
| 61 | + // MARK: - Reading |
| 62 | + |
| 63 | + /// The devices currently bound. |
| 64 | + /// |
| 65 | + /// No lock is taken. Writers replace this file by rename, so a reader sees either |
| 66 | + /// the whole previous version or the whole next one, and locking every read would |
| 67 | + /// put a system call in the path of every request for no benefit. |
| 68 | + public func boundDevices() -> [String] { |
| 69 | + guard let data = FileManager.default.contents(atPath: path) else { |
| 70 | + return migrateFromLegacyConfigIfNeeded() |
| 71 | + } |
| 72 | + guard let state = try? JSONDecoder().decode(State.self, from: data) else { |
| 73 | + return [] |
| 74 | + } |
| 75 | + return state.boundDevices |
| 76 | + } |
| 77 | + |
| 78 | + public func isBound(_ busid: String) -> Bool { |
| 79 | + return boundDevices().contains(busid) |
| 80 | + } |
| 81 | + |
| 82 | + /// When the file last changed, for callers that cache the list. |
| 83 | + public func modificationDate() -> Date? { |
| 84 | + return try? FileManager.default.attributesOfItem(atPath: path)[.modificationDate] as? Date |
| 85 | + } |
| 86 | + |
| 87 | + // MARK: - Writing |
| 88 | + |
| 89 | + /// Record a device as bound. Returns false if it already was. |
| 90 | + @discardableResult |
| 91 | + public func bind(_ busid: String) throws -> Bool { |
| 92 | + return try mutate { devices in |
| 93 | + guard !devices.contains(busid) else { return false } |
| 94 | + devices.append(busid) |
| 95 | + return true |
| 96 | + } |
| 97 | + } |
| 98 | + |
| 99 | + /// Forget a device. Returns false if it was not bound. |
| 100 | + @discardableResult |
| 101 | + public func unbind(_ busid: String) throws -> Bool { |
| 102 | + return try mutate { devices in |
| 103 | + guard let index = devices.firstIndex(of: busid) else { return false } |
| 104 | + devices.remove(at: index) |
| 105 | + return true |
| 106 | + } |
| 107 | + } |
| 108 | + |
| 109 | + /// Read, change and write back while holding an exclusive lock, so that two |
| 110 | + /// commands running at once cannot each write a list that omits the other's device. |
| 111 | + private func mutate(_ change: (inout [String]) -> Bool) throws -> Bool { |
| 112 | + try createDirectoryIfNeeded() |
| 113 | + |
| 114 | + let lock = try FileLock(path: lockPath) |
| 115 | + defer { lock.unlock() } |
| 116 | + lock.lock() |
| 117 | + |
| 118 | + var devices = boundDevices() |
| 119 | + guard change(&devices) else { return false } |
| 120 | + try write(devices) |
| 121 | + return true |
| 122 | + } |
| 123 | + |
| 124 | + private func write(_ devices: [String]) throws { |
| 125 | + let encoder = JSONEncoder() |
| 126 | + encoder.outputFormatting = [.prettyPrinted, .sortedKeys] |
| 127 | + let data = try encoder.encode(State(boundDevices: devices)) |
| 128 | + // Atomic, so a daemon reading concurrently never sees a partial list. |
| 129 | + try data.write(to: URL(fileURLWithPath: path), options: .atomic) |
| 130 | + } |
| 131 | + |
| 132 | + private func createDirectoryIfNeeded() throws { |
| 133 | + let directory = URL(fileURLWithPath: path).deletingLastPathComponent() |
| 134 | + try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true) |
| 135 | + } |
| 136 | + |
| 137 | + // MARK: - Migration |
| 138 | + |
| 139 | + /// Adopt the list from a configuration file written before the two were separated. |
| 140 | + /// |
| 141 | + /// The old key is read but not removed. Rewriting the configuration file is the |
| 142 | + /// behaviour this change exists to stop, and a stale `allowedDevices` sitting in it |
| 143 | + /// is inert — nothing reads it any more. |
| 144 | + private func migrateFromLegacyConfigIfNeeded() -> [String] { |
| 145 | + let configPath = ServerConfig.defaultConfigPath() |
| 146 | + guard let data = FileManager.default.contents(atPath: configPath), |
| 147 | + let object = try? JSONSerialization.jsonObject(with: data) as? [String: Any], |
| 148 | + let legacy = object["allowedDevices"] as? [String], |
| 149 | + !legacy.isEmpty else { |
| 150 | + return [] |
| 151 | + } |
| 152 | + |
| 153 | + // Best effort. Failing to write the new file simply means trying again next |
| 154 | + // time; the devices are still reported as bound either way. |
| 155 | + try? createDirectoryIfNeeded() |
| 156 | + try? write(legacy) |
| 157 | + return legacy |
| 158 | + } |
| 159 | +} |
| 160 | + |
| 161 | +/// An advisory lock on a file, held for as long as the object is. |
| 162 | +private final class FileLock { |
| 163 | + private let descriptor: Int32 |
| 164 | + |
| 165 | + init(path: String) throws { |
| 166 | + descriptor = open(path, O_CREAT | O_RDWR, 0o644) |
| 167 | + guard descriptor >= 0 else { |
| 168 | + throw ServerError.configurationError( |
| 169 | + "Could not open the lock file at \(path): \(String(cString: strerror(errno)))") |
| 170 | + } |
| 171 | + } |
| 172 | + |
| 173 | + func lock() { |
| 174 | + // Blocks until the other holder is done. The critical section is a small read |
| 175 | + // and write, so waiting is measured in microseconds. |
| 176 | + while flock(descriptor, LOCK_EX) != 0 && errno == EINTR { continue } |
| 177 | + } |
| 178 | + |
| 179 | + func unlock() { |
| 180 | + flock(descriptor, LOCK_UN) |
| 181 | + close(descriptor) |
| 182 | + } |
| 183 | +} |
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