Retry priors file write with exponential backoff

On Windows, atomic file writes (temp file + rename) can fail
intermittently with ERROR_ACCESS_DENIED if updates to the file happens
really close to the write of the file underload.

This can especially happen to `.priors` file in a tight rebuild loop.
Now writes `.priors` file in a retry loop with exponential back off.

Assisted-By: Claude

Author: cachemeifyoucan

Sources/SwiftDriver/CMakeLists.txt

@@ -113,6 +113,7 @@ add_library(SwiftDriver
   Utilities/DOTModuleDependencyGraphSerializer.swift
   Utilities/DateAdditions.swift
   Utilities/Diagnostics.swift
+  Utilities/ExponentialBackoff.swift
   Utilities/FileList.swift
   Utilities/FileMetadata.swift
   Utilities/FileType.swift

Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift

@@ -1117,9 +1117,9 @@ extension ModuleDependencyGraph {
       mockSerializedGraphVersion ?? Self.serializedGraphVersion)
 
     do {
-      try fileSystem.writeFileContents(path,
-                                       bytes: data,
-                                       atomically: true)
+      try withExponentialBackoff {
+        try fileSystem.writeFileContents(path, bytes: data, atomically: true)
+      }
     } catch {
       throw IncrementalCompilationState.WriteDependencyGraphError.couldNotWrite(
         path: path, error: error)

Sources/SwiftDriver/Utilities/ExponentialBackoff.swift

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2014 - 2026 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import class Foundation.Thread
+import struct Foundation.TimeInterval
+
+/// Retries `body` up to `maxAttempts` times with exponential backoff.
+///
+/// If all attempts fail, the error from the last attempt is thrown.
+///
+/// - Parameters:
+///   - maxAttempts: Maximum number of times to attempt `body` (default: 5).
+///   - initialDelay: The delay before the first retry, in seconds (default: 0.1).
+///   - body: The throwing operation to attempt.
+func withExponentialBackoff<T>(
+  maxAttempts: Int = 5,
+  initialDelay: TimeInterval = 0.1,
+  _ body: () throws -> T
+) throws -> T {
+  // Add a bit randomness to avoid lock stepped back off.
+  var delay = initialDelay * Double.random(in: 0.9...1.1)
+  var lastError: Error?
+
+  for _ in 0..<maxAttempts {
+    do {
+      return try body()
+    } catch {
+      lastError = error
+      Thread.sleep(forTimeInterval: delay)
+      delay *= 2
+    }
+  }
+  throw lastError!
+}