feat: Add support for compiling to wasm when pthreads aren't available

Author: scottmarchant

.github/workflows/pull_request.yml

@@ -15,7 +15,6 @@ jobs:
       linux_exclude_swift_versions: "[{\"swift_version\": \"5.9\"}, {\"swift_version\": \"5.10\"}]]"
       windows_exclude_swift_versions: "[{\"swift_version\": \"5.9\"}]"
       enable_wasm_sdk_build: true
-      wasm_sdk_build_command: swift build -Xcc -D_WASI_EMULATED_PTHREAD
 
   soundness:
     name: Soundness

Sources/AsyncAlgorithms/Locking.swift

@@ -21,6 +21,9 @@ import WinSDK
 import Bionic
 #elseif canImport(wasi_pthread)
 import wasi_pthread
+#elseif canImport(WASILibc)
+// No locking on WASILibc provided by the current Swift for WebAssembly SDK as of Dec 2025.
+// That SDK is also single-threaded, so we can safely avoid locking altogether.
 #else
 #error("Unsupported platform")
 #endif
@@ -30,6 +33,8 @@ internal struct Lock {
   typealias Primitive = os_unfair_lock
   #elseif canImport(Glibc) || canImport(Musl) || canImport(Bionic) || canImport(wasi_pthread)
   typealias Primitive = pthread_mutex_t
+  #elseif canImport(WASILibc)
+  // This WASILibc variation is single threaded, provides no locks
   #elseif canImport(WinSDK)
   typealias Primitive = SRWLOCK
   #else
@@ -49,6 +54,8 @@ internal struct Lock {
     #elseif canImport(Glibc) || canImport(Musl) || canImport(Bionic) || canImport(wasi_pthread)
     let result = pthread_mutex_init(platformLock, nil)
     precondition(result == 0, "pthread_mutex_init failed")
+    #elseif canImport(WASILibc)
+    // This WASILibc variation is single threaded, provides no locks
     #elseif canImport(WinSDK)
     InitializeSRWLock(platformLock)
     #else
@@ -69,6 +76,9 @@ internal struct Lock {
     os_unfair_lock_lock(platformLock)
     #elseif canImport(Glibc) || canImport(Musl) || canImport(Bionic) || canImport(wasi_pthread)
     pthread_mutex_lock(platformLock)
+    #elseif canImport(WASILibc)
+    // This WASILibc variation is single threaded, provides no locks
+    return
     #elseif canImport(WinSDK)
     AcquireSRWLockExclusive(platformLock)
     #else
@@ -82,6 +92,9 @@ internal struct Lock {
     #elseif canImport(Glibc) || canImport(Musl) || canImport(Bionic) || canImport(wasi_pthread)
     let result = pthread_mutex_unlock(platformLock)
     precondition(result == 0, "pthread_mutex_unlock failed")
+    #elseif canImport(WASILibc)
+    // This WASILibc variation is single threaded, provides no locks
+    return
     #elseif canImport(WinSDK)
     ReleaseSRWLockExclusive(platformLock)
     #else