Adopt C thread-locals

Motivation

In preparation for adopting FoundationEssentials where available,
we need to avoid using Thread as that isn't available. To that end,
let's migrate to using C thread-locals

Modifications

Migrate to C thread-locals

Result

No need for Thread anymore.

Author: Lukasa

NOTICE.txt

@@ -34,7 +34,7 @@ This product contains test vectors from Google's wycheproof project.
 
 ---
 
-This product contains a derivation of various scripts from SwiftNIO.
+This product contains a derivation of various files from SwiftNIO.
 
   * LICENSE (Apache License 2.0):
     * https://www.apache.org/licenses/LICENSE-2.0

Sources/CCryptoBoringSSLShims/CMakeLists.txt

@@ -14,6 +14,7 @@
 
 add_library(CCryptoBoringSSLShims STATIC
   "shims.c"
+  "threadlocal.c"
 )
 
 target_include_directories(CCryptoBoringSSLShims PUBLIC

Sources/CCryptoBoringSSLShims/include/CCryptoBoringSSLShims.h

@@ -2,7 +2,7 @@
 //
 // This source file is part of the SwiftCrypto open source project
 //
-// Copyright (c) 2019 Apple Inc. and the SwiftCrypto project authors
+// Copyright (c) 2019-2025 Apple Inc. and the SwiftCrypto project authors
 // Licensed under Apache License v2.0
 //
 // See LICENSE.txt for license information
@@ -148,6 +148,36 @@ size_t CCryptoBoringSSLShims_EC_POINT_point2oct(const EC_GROUP *group,
                                                 void *buf, size_t max_out,
                                                 BN_CTX *ctx);
 
+// MARK: Thread locals
+
+extern _Thread_local void *_CCryptoBoringSSLShims_p256_ffac_ptr;
+extern _Thread_local void *_CCryptoBoringSSLShims_p384_ffac_ptr;
+extern _Thread_local void *_CCryptoBoringSSLShims_p521_ffac_ptr;
+
+static inline void * _Nullable CCryptoBoringSSLShims_get_p256_ffac_ptr(void) {
+    return _CCryptoBoringSSLShims_p256_ffac_ptr;
+}
+
+static inline void CCryptoBoringSSLShims_set_p256_ffac_ptr(void * _Nonnull ptr) {
+    _CCryptoBoringSSLShims_p256_ffac_ptr = ptr;
+}
+
+static inline void * _Nullable CCryptoBoringSSLShims_get_p384_ffac_ptr(void) {
+    return _CCryptoBoringSSLShims_p384_ffac_ptr;
+}
+
+static inline void CCryptoBoringSSLShims_set_p384_ffac_ptr(void * _Nonnull ptr) {
+    _CCryptoBoringSSLShims_p384_ffac_ptr = ptr;
+}
+
+static inline void * _Nullable CCryptoBoringSSLShims_get_p521_ffac_ptr(void) {
+    return _CCryptoBoringSSLShims_p521_ffac_ptr;
+}
+
+static inline void CCryptoBoringSSLShims_set_p521_ffac_ptr(void * _Nonnull ptr) {
+    _CCryptoBoringSSLShims_p521_ffac_ptr = ptr;
+}
+
 #if defined(__cplusplus)
 }
 #endif // defined(__cplusplus)

Sources/CCryptoBoringSSLShims/threadlocal.c

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftCrypto open source project
+//
+// Copyright (c) 2025 Apple Inc. and the SwiftCrypto project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftCrypto project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+#include <CCryptoBoringSSLShims.h>
+
+// Once we support C23, this should become `thread_local`.
+// DO NOT TOUCH DIRECTLY, use the getters and setters from the header file.
+_Thread_local void * _Nullable _CCryptoBoringSSLShims_p256_ffac_ptr;
+_Thread_local void * _Nullable _CCryptoBoringSSLShims_p384_ffac_ptr;
+_Thread_local void * _Nullable _CCryptoBoringSSLShims_p521_ffac_ptr;

Sources/_CryptoExtras/ECToolbox/BoringSSL/ECToolbox_boring.swift

@@ -11,6 +11,7 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+@_implementationOnly import CCryptoBoringSSLShims
 import Crypto
 import CryptoBoringWrapper
 import Foundation
@@ -67,12 +68,11 @@ extension P256: OpenSSLSupportedNISTCurve {
 
     @usableFromInline
     static var __ffac: FiniteFieldArithmeticContext {
-        let key = "com.apple.swift-crypto.P256.__ffac"
-        if let value = Thread.current.threadDictionary[key] as? FiniteFieldArithmeticContext {
-            return value
+        if let valuePtr = CCryptoBoringSSLShims_get_p256_ffac_ptr() {
+            return Unmanaged<FiniteFieldArithmeticContext>.fromOpaque(valuePtr).takeUnretainedValue()
         }
         let value = try! FiniteFieldArithmeticContext(fieldSize: P256.group.order)
-        Thread.current.threadDictionary[key] = value
+        CCryptoBoringSSLShims_set_p256_ffac_ptr(Unmanaged.passRetained(value).toOpaque())
         return value
     }
 }
@@ -100,12 +100,11 @@ extension P384: OpenSSLSupportedNISTCurve {
 
     @usableFromInline
     static var __ffac: FiniteFieldArithmeticContext {
-        let key = "com.apple.swift-crypto.P384.__ffac"
-        if let value = Thread.current.threadDictionary[key] as? FiniteFieldArithmeticContext {
-            return value
+        if let valuePtr = CCryptoBoringSSLShims_get_p384_ffac_ptr() {
+            return Unmanaged<FiniteFieldArithmeticContext>.fromOpaque(valuePtr).takeUnretainedValue()
         }
         let value = try! FiniteFieldArithmeticContext(fieldSize: P384.group.order)
-        Thread.current.threadDictionary[key] = value
+        CCryptoBoringSSLShims_set_p384_ffac_ptr(Unmanaged.passRetained(value).toOpaque())
         return value
     }
 }
@@ -133,12 +132,11 @@ extension P521: OpenSSLSupportedNISTCurve {
 
     @usableFromInline
     static var __ffac: FiniteFieldArithmeticContext {
-        let key = "com.apple.swift-crypto.P521.__ffac"
-        if let value = Thread.current.threadDictionary[key] as? FiniteFieldArithmeticContext {
-            return value
+        if let valuePtr = CCryptoBoringSSLShims_get_p521_ffac_ptr() {
+            return Unmanaged<FiniteFieldArithmeticContext>.fromOpaque(valuePtr).takeUnretainedValue()
         }
         let value = try! FiniteFieldArithmeticContext(fieldSize: P521.group.order)
-        Thread.current.threadDictionary[key] = value
+        CCryptoBoringSSLShims_set_p521_ffac_ptr(Unmanaged.passRetained(value).toOpaque())
         return value
     }
 }