Remove the @convention(c) restriction from preExecProcessAction

This allows it to capture local context, and avoid using a fragile hardcoded file descriptor in the corresponding test.

Closes #148

Author: jakepetroules

Sources/Subprocess/Platforms/Subprocess+Darwin.swift

@@ -103,7 +103,7 @@ public struct PlatformOptions: Sendable {
     /// done so based on the configured `PlatformOptions`.
     ///
     /// - warning: You may ONLY call [async-signal-safe functions](https://pubs.opengroup.org/onlinepubs/9799919799/functions/V2_chap02.html) within this closure (note _"The following table defines a set of functions and function-like macros that shall be async-signal-safe."_).
-    public var preExecProcessAction: (@convention(c) @Sendable () -> Void)? = nil
+    public var preExecProcessAction: (@Sendable () -> Void)? = nil
 
     public init() {}
 }
@@ -412,6 +412,17 @@ extension Configuration {
                     try spawnConfig(&spawnAttributes, &fileActions)
                 }
 
+                final class Context {
+                    let body: @Sendable () -> ()
+                    init(body: @Sendable @escaping () -> Void) {
+                        self.body = body
+                    }
+                }
+
+                func preExecProcessAction(_ context: UnsafeMutableRawPointer) -> Void {
+                    (Unmanaged<AnyObject>.fromOpaque(context).takeRetainedValue() as! Context).body()
+                }
+
                 // Spawn
                 let spawnError: CInt = possibleExecutablePath.withCString { exePath in
                     return supplementaryGroups.withOptionalUnsafeBufferPointer { sgroups in
@@ -427,7 +438,8 @@ extension Configuration {
                             Int32(supplementaryGroups?.count ?? 0),
                             sgroups?.baseAddress,
                             self.platformOptions.createSession ? 1 : 0,
-                            self.platformOptions.preExecProcessAction,
+                            preExecProcessAction,
+                            self.platformOptions.preExecProcessAction.map { Unmanaged.passRetained(Context(body: $0)).toOpaque() }
                         )
                     }
                 }

Sources/Subprocess/Platforms/Subprocess+Linux.swift

@@ -131,6 +131,17 @@ extension Configuration {
                     errorReadFileDescriptor?.platformDescriptor() ?? -1,
                 ]
 
+                final class Context {
+                    let body: @Sendable () -> ()
+                    init(body: @Sendable @escaping () -> Void) {
+                        self.body = body
+                    }
+                }
+
+                func preExecProcessAction(_ context: UnsafeMutableRawPointer) -> Void {
+                    (Unmanaged<AnyObject>.fromOpaque(context).takeRetainedValue() as! Context).body()
+                }
+
                 // Spawn
                 var pid: pid_t = 0
                 var processDescriptor: PlatformFileDescriptor = -1
@@ -152,7 +163,8 @@ extension Configuration {
                                     CInt(supplementaryGroups?.count ?? 0),
                                     sgroups?.baseAddress,
                                     self.platformOptions.createSession ? 1 : 0,
-                                    self.platformOptions.preExecProcessAction
+                                    preExecProcessAction,
+                                    self.platformOptions.preExecProcessAction.map { Unmanaged.passRetained(Context(body: $0)).toOpaque() }
                                 )
                             }
                         }
@@ -297,7 +309,7 @@ public struct PlatformOptions: Sendable {
     /// call any necessary process setup functions.
     ///
     /// - warning: You may ONLY call [async-signal-safe functions](https://pubs.opengroup.org/onlinepubs/9799919799/functions/V2_chap02.html) within this closure (note _"The following table defines a set of functions and function-like macros that shall be async-signal-safe."_).
-    public var preExecProcessAction: (@convention(c) @Sendable () -> Void)? = nil
+    public var preExecProcessAction: (@Sendable () -> Void)? = nil
 
     public init() {}
 }

Sources/_SubprocessCShims/include/process_shims.h

@@ -44,7 +44,8 @@ int _subprocess_spawn(
     gid_t * _Nullable gid,
     int number_of_sgroups, const gid_t * _Nullable sgroups,
     int create_session,
-    void (* _Nullable configurator)(void)
+    void (* _Nonnull configurator)(void * _Nonnull context),
+    void * _Nullable configuratorContext
 );
 #endif // TARGET_OS_MAC
 
@@ -61,7 +62,8 @@ int _subprocess_fork_exec(
     gid_t * _Nullable process_group_id,
     int number_of_sgroups, const gid_t * _Nullable sgroups,
     int create_session,
-    void (* _Nullable configurator)(void)
+    void (* _Nonnull configurator)(void * _Nonnull context),
+    void * _Nullable configuratorContext
 );
 
 int _was_process_exited(int status);

Sources/_SubprocessCShims/process_shims.c

@@ -148,7 +148,8 @@ static int _subprocess_spawn_prefork(
     gid_t * _Nullable gid,
     int number_of_sgroups, const gid_t * _Nullable sgroups,
     int create_session,
-    void (* _Nullable configurator)(void)
+    void (* _Nonnull configurator)(void * _Nonnull context),
+    void * _Nullable configuratorContext
 ) {
 #define write_error_and_exit int error = errno; \
     write(pipefd[1], &error, sizeof(error));\
@@ -242,8 +243,8 @@ static int _subprocess_spawn_prefork(
         }
 
         // Run custom configuratior
-        if (configurator != NULL) {
-            configurator();
+        if (configuratorContext != NULL) {
+            configurator(configuratorContext);
         }
 
         // Use posix_spawnas exec
@@ -295,21 +296,22 @@ int _subprocess_spawn(
     gid_t * _Nullable gid,
     int number_of_sgroups, const gid_t * _Nullable sgroups,
     int create_session,
-    void (* _Nullable configurator)(void)
+    void (* _Nonnull configurator)(void * _Nonnull context),
+    void * _Nullable configuratorContext
 ) {
     int require_pre_fork = uid != NULL ||
         gid != NULL ||
         number_of_sgroups > 0 ||
         create_session > 0 ||
-        configurator != NULL;
+        configuratorContext != NULL;
 
     if (require_pre_fork != 0) {
         int rc = _subprocess_spawn_prefork(
             pid,
             exec_path,
             file_actions, spawn_attrs,
             args, env,
-            uid, gid, number_of_sgroups, sgroups, create_session, configurator
+            uid, gid, number_of_sgroups, sgroups, create_session, configurator, configuratorContext
         );
         return rc;
     }
@@ -387,7 +389,8 @@ int _subprocess_fork_exec(
     gid_t * _Nullable process_group_id,
     int number_of_sgroups, const gid_t * _Nullable sgroups,
     int create_session,
-    void (* _Nullable configurator)(void)
+    void (* _Nonnull configurator)(void * _Nonnull context),
+    void * _Nullable configuratorContext
 ) {
 #define write_error_and_exit int error = errno; \
     write(pipefd[1], &error, sizeof(error));\
@@ -573,8 +576,8 @@ int _subprocess_fork_exec(
         }
 
         // Run custom configuratior
-        if (configurator != NULL) {
-            configurator();
+        if (configuratorContext != NULL) {
+            configurator(configuratorContext);
         }
         // Finally, exec
         execve(exec_path, args, env);

Tests/SubprocessTests/SubprocessTests+Darwin.swift

@@ -43,8 +43,7 @@ struct SubprocessDarwinTests {
         let (readFD, writeFD) = try FileDescriptor.pipe()
         try await readFD.closeAfter {
             let childPID = try await writeFD.closeAfter {
-                // Allocate some constant high-numbered FD that's unlikely to be used.
-                let specialFD = try writeFD.duplicate(as: FileDescriptor(rawValue: 9000))
+                let specialFD = try writeFD.duplicate()
                 return try await specialFD.closeAfter {
                     // Make the fd non-blocking just to avoid the test hanging if it fails
                     let opts = fcntl(specialFD.rawValue, F_GETFD)
@@ -54,7 +53,7 @@ struct SubprocessDarwinTests {
                     var platformOptions = PlatformOptions()
                     platformOptions.preExecProcessAction = {
                         var pid: Int32 = getpid()
-                        if write(9000, &pid, 4) != 4 {
+                        if write(specialFD.rawValue, &pid, 4) != 4 {
                             exit(EXIT_FAILURE)
                         }
                     }