Avoid blocking a Swift Task in the read() syscall

Add a DispatchIO-based helper which returns the data asynchronously.

Author: jakepetroules

Sources/SWBServiceCore/ServiceHostConnection.swift

@@ -148,10 +148,13 @@ final class ServiceHostConnection: @unchecked Sendable {
                 #endif
 
                 // Read data.
-                let result = read(self.inputFD.rawValue, tmp.baseAddress, numericCast(tmpBufferSize))
-                if result < 0 {
-                    if errno == EINTR { continue }
-                    error = ServiceHostIOError(message: "read from client failed", cause: SWBUtil.POSIXError(errno, context: "read"))
+                let result: Int
+                do {
+                    let buf = try await DispatchFD(fileDescriptor: self.inputFD).readChunk(upToLength: tmpBufferSize)
+                    result = buf.count
+                    buf.copyBytes(to: tmp)
+                } catch let readError {
+                    error = ServiceHostIOError(message: "read from client failed", cause: readError)
                     break
                 }
                 if result == 0 {

Sources/SWBUtil/Dispatch+Async.swift

@@ -13,7 +13,7 @@
 // This file contains helpers used to bridge GCD and Swift Concurrency.
 // In the long term, these ideally all go away.
 
-private import Foundation
+import Foundation
 
 /// Runs an async function and synchronously waits for the response.
 /// - warning: This function is extremely dangerous because it blocks the calling thread and may lead to deadlock, and should only be used as a temporary transitional aid.
@@ -41,6 +41,24 @@ public func runAsyncAndBlock<T: Sendable, E>(_ block: @Sendable @escaping () asy
     return try result.value!.get()
 }
 
+extension DispatchFD {
+    public func readChunk(upToLength maxLength: Int) async throws -> SWBDispatchData {
+        return try await withCheckedThrowingContinuation { continuation in
+            SWBDispatchIO.read(
+                fromFileDescriptor: self,
+                maxLength: maxLength,
+                runningHandlerOn: .global()
+            ) { data, error in
+                if error != 0 {
+                    continuation.resume(throwing: POSIXError(error))
+                    return
+                }
+                continuation.resume(returning: data)
+            }
+        }
+    }
+}
+
 extension AsyncThrowingStream where Element == UInt8, Failure == any Error {
     /// Returns an async stream which reads bytes from the specified file descriptor. Unlike `FileHandle.bytes`, it does not block the caller.
     @available(macOS, deprecated: 15.0, message: "Use the AsyncSequence-returning overload.")

Sources/SWBUtil/SWBDispatch.swift

@@ -15,7 +15,7 @@
 
 private import Dispatch
 public import SWBLibc
-import Foundation
+public import Foundation
 
 #if canImport(System)
 public import System
@@ -123,6 +123,12 @@ extension SWBDispatchData: RandomAccessCollection {
     }
 }
 
+extension SWBDispatchData: DataProtocol {
+    public var regions: DispatchData.Regions {
+        dispatchData.regions
+    }
+}
+
 /// Thin wrapper for `DispatchSemaphore` to isolate it from the rest of the codebase and help migration away from it.
 internal final class SWBDispatchSemaphore: Sendable {
     private let semaphore: DispatchSemaphore
@@ -183,6 +189,13 @@ public final class SWBDispatchIO: Sendable {
         io = DispatchIO(type: .stream, fileDescriptor: numericCast(fileDescriptor), queue: queue.queue, cleanupHandler: cleanupHandler)
     }
 
+    public static func read(fromFileDescriptor fileDescriptor: DispatchFD, maxLength: Int, runningHandlerOn queue: SWBQueue, handler: @escaping (SWBDispatchData, Int32) -> Void) {
+        // Most of the dispatch APIs take a parameter called "fileDescriptor". On Windows (except makeReadSource and makeWriteSource) it is actually a HANDLE, so convert it accordingly.
+        DispatchIO.read(fromFileDescriptor: numericCast(fileDescriptor.rawValue), maxLength: maxLength, runningHandlerOn: queue.queue) { data, error in
+            handler(SWBDispatchData(data), error)
+        }
+    }
+
     public static func stream(fileDescriptor: DispatchFD, queue: SWBQueue, cleanupHandler: @escaping (Int32) -> Void) -> SWBDispatchIO {
         // Most of the dispatch APIs take a parameter called "fileDescriptor". On Windows (except makeReadSource and makeWriteSource) it is actually a HANDLE, so convert it accordingly.
         SWBDispatchIO(fileDescriptor: numericCast(fileDescriptor.rawValue), queue: queue, cleanupHandler: cleanupHandler)