Use dedicated thread when invoking buildSystem.build() as it is a blocking operation (#9493)

Use Task.detachNewThread here because buildSystem.build() is a blocking
operation. Running this on the Swift Concurrency thread pool can block a
worker thread potentially causing thread pool starvation and deadlocks.
By running it on a dedicated thread, we keep the Swift Concurrency pool
available for other async work.

### Motivation:

#9441

Author: rconnell9

Sources/Basics/Concurrency/ConcurrencyHelpers.swift

@@ -14,6 +14,7 @@ import _Concurrency
 import Dispatch
 import class Foundation.NSLock
 import class Foundation.ProcessInfo
+import class Foundation.Thread
 import struct Foundation.URL
 import struct Foundation.UUID
 import func TSCBasic.tsc_await
@@ -39,6 +40,19 @@ public func unsafe_await<T: Sendable>(_ body: @Sendable @escaping () async -> T)
     return box.get()!
 }
 
+extension Task where Failure == Never {
+    /// Runs `block` in a new thread and suspends until it finishes execution.
+    ///
+    /// - note: This function should be used sparingly, such as for long-running operations that may block and therefore should not be run on the Swift Concurrency thread pool. Do not use this for operations for which there may be many concurrent invocations as it could lead to thread explosion. It is meant to be a bridge to pre-existing blocking code which can't easily be converted to use Swift concurrency features.
+    public static func detachNewThread(name: String? = nil, _ block: @Sendable @escaping () -> Success) async -> Success {
+        return await withCheckedContinuation { continuation in
+            Thread.detachNewThread {
+                Thread.current.name = name
+                return continuation.resume(returning: block())
+            }
+        }
+    }
+}
 
 extension DispatchQueue {
     // a shared concurrent queue for running concurrent asynchronous operations

Sources/Build/BuildOperation.swift

@@ -291,7 +291,7 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
             if self.cacheBuildManifest {
                 do {
                     // if buildPackageStructure returns a valid description we use that, otherwise we perform full planning
-                    if try self.buildPackageStructure() {
+                    if try await self.buildPackageStructure() {
                         // confirm the step above created the build description as expected
                         // we trust it to update the build description when needed
                         let buildDescriptionPath = self.config.buildDescriptionPath(for: .target)
@@ -829,15 +829,21 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
     }
 
     /// Build the package structure target.
-    private func buildPackageStructure() throws -> Bool {
+    private func buildPackageStructure() async throws -> Bool {
         let (buildSystem, tracker) = try self.createBuildSystem(
             buildDescription: .none,
             config: self.config
         )
         self.current = (buildSystem, tracker)
 
-        // Build the package structure target which will re-generate the llbuild manifest, if necessary.
-        let buildSuccess = buildSystem.build(target: "PackageStructure")
+        // We use Task.detachNewThread here because buildSystem.build() is a blocking
+        // operation. Running this on the Swift Concurrency thread pool can block a worker thread
+        // potentially causing thread pool starvation and deadlocks. By running it on a dedicated
+        // thread, we keep the Swift Concurrency pool available for other async work.
+        let buildSuccess = await _Concurrency.Task.detachNewThread(name: "buildPackageStructure") {
+            // Build the package structure target which will re-generate the llbuild manifest, if necessary.
+            buildSystem.build(target: "PackageStructure")
+        }
 
         // If progress has been printed this will add a newline to separate it from what could be
         // the output of the command. For instance `swift test --skip-build` may print progress for