feat: Incremental builds

Sources/SagaCLI/BuildCommand.swift

@@ -7,7 +7,7 @@ struct Build: ParsableCommand {
   )
 
   func run() throws {
-    print("Building site...")
+    log("Building site...")
 
     let process = Process()
     process.executableURL = URL(fileURLWithPath: "/usr/bin/env")
@@ -17,7 +17,7 @@ struct Build: ParsableCommand {
     process.waitUntilExit()
 
     if process.terminationStatus == 0 {
-      print("Build complete.")
+      log("Build complete.")
     } else {
       throw ExitCode(process.terminationStatus)
     }

Sources/SagaCLI/DevCommand.swift

@@ -27,10 +27,37 @@ struct Dev: ParsableCommand {
     }
     try cachePath.mkpath()
 
-    print("Building site...")
-    let buildResult = runBuild(cachePath: cachePath)
-    if !buildResult {
-      print("Initial build failed, starting server anyway...")
+    // Find the executable product name from Package.swift
+    guard let productName = findExecutableProduct() else {
+      print("Could not find an executable product in Package.swift")
+      throw ExitCode.failure
+    }
+
+    // Initial build
+    log("Building site...")
+    guard swiftBuild() else {
+      log("Initial build failed.")
+      throw ExitCode.failure
+    }
+
+    // Set up SIGUSR2 handler — the site process signals us when a build completes
+    let buildComplete = DispatchSemaphore(value: 0)
+    signal(SIGUSR2, SIG_IGN)
+    let sigusr2Source = DispatchSource.makeSignalSource(signal: SIGUSR2, queue: DispatchQueue(label: "Saga.Signal"))
+    sigusr2Source.setEventHandler { buildComplete.signal() }
+    sigusr2Source.resume()
+
+    // Launch the site process (stays alive, waiting for SIGUSR1 between rebuilds)
+    var siteProcess = launchSiteProcess(productName: productName, cachePath: cachePath)
+    if let siteProcess {
+      // Wait for the initial build to finish
+      buildComplete.wait()
+      guard siteProcess.isRunning else {
+        log("Initial build failed.")
+        throw ExitCode.failure
+      }
+    } else {
+      log("Failed to launch site process, starting server anyway...")
     }
 
     // Start the dev server
@@ -48,7 +75,7 @@ struct Dev: ParsableCommand {
 
     // Give the server a moment to start
     Thread.sleep(forTimeInterval: 0.5)
-    print("Development server running at http://localhost:\(port)/")
+    log("Development server running at http://localhost:\(port)/")
 
     // Open the browser
     openBrowser(url: "http://localhost:\(port)/")
@@ -66,13 +93,13 @@ struct Dev: ParsableCommand {
 
     // Start monitoring
     if !ignore.isEmpty {
-      print("Ignoring patterns: \(ignore.joined(separator: ", "))")
+      log("Ignoring patterns: \(ignore.joined(separator: ", "))")
     }
 
     var isRebuilding = false
     let rebuildLock = NSLock()
 
-    let folderMonitor = FolderMonitor(paths: paths, ignoredPatterns: defaultIgnorePatterns + ignore) {
+    let folderMonitor = FolderMonitor(paths: paths, ignoredPatterns: defaultIgnorePatterns + ignore) { changedPaths in
       rebuildLock.lock()
       guard !isRebuilding else {
         rebuildLock.unlock()
@@ -81,13 +108,39 @@ struct Dev: ParsableCommand {
       isRebuilding = true
       rebuildLock.unlock()
 
-      print("Change detected, rebuilding...")
-      let success = runBuild(cachePath: cachePath)
-      if success {
-        print("Rebuild complete.")
+      if changedPaths.contains(where: { $0.hasSuffix(".swift") }) {
+        // Swift code changed: kill the process, recompile, relaunch
+        log("Source code changed, recompiling...")
+        siteProcess?.terminate()
+        siteProcess?.waitUntilExit()
+
+        guard swiftBuild() else {
+          log("Build failed")
+          rebuildLock.lock()
+          isRebuilding = false
+          rebuildLock.unlock()
+          return
+        }
+
+        siteProcess = launchSiteProcess(productName: productName, cachePath: cachePath)
+      } else {
+        // Content changed: signal the running process to rebuild
+        log("Change detected, rebuilding...")
+        if let process = siteProcess, process.isRunning {
+          kill(process.processIdentifier, SIGUSR1)
+        } else {
+          // Process died, relaunch
+          siteProcess = launchSiteProcess(productName: productName, cachePath: cachePath)
+        }
+      }
+
+      // Wait for the site process to signal build completion
+      buildComplete.wait()
+
+      if let process = siteProcess, process.isRunning {
         server.sendReload()
       } else {
-        print("Rebuild failed.")
+        log("Rebuild failed")
       }
 
       rebuildLock.lock()
@@ -100,50 +153,19 @@ struct Dev: ParsableCommand {
     let sigintSrc = DispatchSource.makeSignalSource(signal: SIGINT, queue: signalsQueue)
     sigintSrc.setEventHandler {
       print("\nShutting down...")
+      siteProcess?.terminate()
       server.stop()
       Foundation.exit(0)
     }
     sigintSrc.resume()
     signal(SIGINT, SIG_IGN)
 
-    print("Watching for changes in: \(watch.joined(separator: ", "))")
+    log("Watching for changes in: \(watch.joined(separator: ", "))")
 
     // Prevent folderMonitor from being deallocated
     withExtendedLifetime(folderMonitor) {
       // Keep running
       dispatchMain()
     }
   }
-
-  private func runBuild(cachePath: Path) -> Bool {
-    let process = Process()
-    process.executableURL = URL(fileURLWithPath: "/usr/bin/env")
-    process.arguments = ["swift", "run"]
-    process.currentDirectoryURL = URL(fileURLWithPath: FileManager.default.currentDirectoryPath)
-
-    var env = ProcessInfo.processInfo.environment
-    env["SAGA_DEV"] = "1"
-    env["SAGA_CACHE_DIR"] = cachePath.string
-    process.environment = env
-
-    do {
-      try process.run()
-      process.waitUntilExit()
-      return process.terminationStatus == 0
-    } catch {
-      print("Build error: \(error)")
-      return false
-    }
-  }
-
-  private func openBrowser(url: String) {
-    #if os(macOS)
-      Process.launchedProcess(launchPath: "/usr/bin/open", arguments: [url])
-    #elseif os(Linux)
-      let process = Process()
-      process.executableURL = URL(fileURLWithPath: "/usr/bin/env")
-      process.arguments = ["xdg-open", url]
-      try? process.run()
-    #endif
-  }
 }

Sources/SagaCLI/FolderMonitor.swift

@@ -1,14 +1,14 @@
 import Foundation
 
 class FolderMonitor {
-  private let callback: () -> Void
+  private let callback: (Set<String>) -> Void
   private let ignoredPatterns: [String]
   private let basePath: String
   private let paths: [String]
   private var knownFiles: [String: Date] = [:]
   private var timer: DispatchSourceTimer?
 
-  init(paths: [String], ignoredPatterns: [String] = [], folderDidChange: @escaping () -> Void) {
+  init(paths: [String], ignoredPatterns: [String] = [], folderDidChange: @escaping (Set<String>) -> Void) {
     self.paths = paths
     callback = folderDidChange
     self.ignoredPatterns = ignoredPatterns
@@ -30,35 +30,30 @@ class FolderMonitor {
   private func checkForChanges() {
     let currentFiles = scanFiles()
 
-    var changed = false
+    var changedPaths: Set<String> = []
 
     // Check for new or modified files
     for (path, modDate) in currentFiles {
       if let previousDate = knownFiles[path] {
         if modDate > previousDate {
-          changed = true
-          break
+          changedPaths.insert(path)
         }
       } else {
         // New file
-        changed = true
-        break
+        changedPaths.insert(path)
       }
     }
 
     // Check for deleted files
-    if !changed {
-      for path in knownFiles.keys {
-        if currentFiles[path] == nil {
-          changed = true
-          break
-        }
+    for path in knownFiles.keys {
+      if currentFiles[path] == nil {
+        changedPaths.insert(path)
       }
     }
 
-    if changed {
+    if !changedPaths.isEmpty {
       knownFiles = currentFiles
-      callback()
+      callback(changedPaths)
     }
   }
 

Sources/SagaCLI/Utils.swift

@@ -0,0 +1,111 @@
+import Foundation
+import SagaPathKit
+
+private let logDateFormatter: DateFormatter = {
+  let f = DateFormatter()
+  f.dateFormat = "yyyy-MM-dd HH:mm:ss"
+  return f
+}()
+
+func log(_ message: String) {
+  print("\(logDateFormatter.string(from: Date())) | \(message)")
+}
+
+/// Find the first executable product name using `swift package dump-package`.
+func findExecutableProduct() -> String? {
+  let process = Process()
+  process.executableURL = URL(fileURLWithPath: "/usr/bin/env")
+  process.arguments = ["swift", "package", "dump-package"]
+  process.currentDirectoryURL = URL(fileURLWithPath: FileManager.default.currentDirectoryPath)
+
+  let pipe = Pipe()
+  process.standardOutput = pipe
+  process.standardError = FileHandle.nullDevice
+
+  do {
+    try process.run()
+    process.waitUntilExit()
+    guard process.terminationStatus == 0 else { return nil }
+
+    let data = pipe.fileHandleForReading.readDataToEndOfFile()
+    guard let json = try JSONSerialization.jsonObject(with: data) as? [String: Any],
+          let products = json["products"] as? [[String: Any]]
+    else { return nil }
+
+    // Find the first executable product
+    for product in products {
+      if let type = product["type"] as? [String: Any],
+         type["executable"] != nil,
+         let name = product["name"] as? String
+      {
+        return name
+      }
+    }
+
+    // Fall back to the first executable target (packages without explicit products)
+    if let targets = json["targets"] as? [[String: Any]] {
+      for target in targets {
+        if let type = target["type"] as? String,
+           type == "executable",
+           let name = target["name"] as? String
+        {
+          return name
+        }
+      }
+    }
+
+    return nil
+  } catch {
+    return nil
+  }
+}
+
+func swiftBuild() -> Bool {
+  let process = Process()
+  process.executableURL = URL(fileURLWithPath: "/usr/bin/env")
+  process.arguments = ["swift", "build"]
+  process.currentDirectoryURL = URL(fileURLWithPath: FileManager.default.currentDirectoryPath)
+  process.environment = ProcessInfo.processInfo.environment
+
+  do {
+    try process.run()
+    process.waitUntilExit()
+    return process.terminationStatus == 0
+  } catch {
+    print("Build error: \(error)")
+    return false
+  }
+}
+
+func launchSiteProcess(productName: String, cachePath: Path) -> Process? {
+  let binPath = FileManager.default.currentDirectoryPath + "/.build/debug/\(productName)"
+
+  let process = Process()
+  process.executableURL = URL(fileURLWithPath: binPath)
+  process.currentDirectoryURL = URL(fileURLWithPath: FileManager.default.currentDirectoryPath)
+
+  var env = ProcessInfo.processInfo.environment
+  env["SAGA_DEV"] = "1"
+  env["SAGA_CACHE_DIR"] = cachePath.string
+  env["SAGA_DEV_PID"] = "\(ProcessInfo.processInfo.processIdentifier)"
+  process.environment = env
+
+  do {
+    try process.run()
+    return process
+  } catch {
+    print("Launch error: \(error)")
+    return nil
+  }
+}
+
+func openBrowser(url: String) {
+  #if os(macOS)
+    Process.launchedProcess(launchPath: "/usr/bin/open", arguments: [url])
+  #elseif os(Linux)
+    let process = Process()
+    process.executableURL = URL(fileURLWithPath: "/usr/bin/env")
+    process.arguments = ["xdg-open", url]
+    try? process.run()
+  #endif
+}