Remove Stubs.cpp and reimplement what it has in Swift. (#929)

This PR removes Stubs.cpp, which currently houses some thunks for
functions that are conditionally unavailable in glibc, and replaces it
with runtime function lookups in Swift. Is there potentially a one-time
non-zero performance cost? Yes. Is that performance cost prohibitive
given that the functions are only looked up once and then cached? No.

These functions won't get called on Linux if `SWT_NO_DYNAMIC_LINKING` is
defined but we don't currently support that combination anyway. Even if
you're using a statically-linked Swift standard library, we'd expect
Linux to still support calling `dlsym()`.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ExitTests/SpawnProcess.swift

@@ -26,6 +26,18 @@ typealias ProcessID = HANDLE
 typealias ProcessID = Never
 #endif
 
+#if os(Linux) && !SWT_NO_DYNAMIC_LINKING
+/// Close file descriptors above a given value when spawing a new process.
+///
+/// This symbol is provided because the underlying function was added to glibc
+/// relatively recently and may not be available on all targets. Checking
+/// `__GLIBC_PREREQ()` is insufficient because `_DEFAULT_SOURCE` may not be
+/// defined at the point spawn.h is first included.
+private let _posix_spawn_file_actions_addclosefrom_np = symbol(named: "posix_spawn_file_actions_addclosefrom_np").map {
+  unsafeBitCast($0, to: (@convention(c) (UnsafeMutablePointer<posix_spawn_file_actions_t>, CInt) -> CInt).self)
+}
+#endif
+
 /// Spawn a process and wait for it to terminate.
 ///
 /// - Parameters:
@@ -141,17 +153,18 @@ func spawnExecutable(
       // Close all other file descriptors open in the parent.
       flags |= CShort(POSIX_SPAWN_CLOEXEC_DEFAULT)
 #elseif os(Linux)
+#if !SWT_NO_DYNAMIC_LINKING
       // This platform doesn't have POSIX_SPAWN_CLOEXEC_DEFAULT, but we can at
       // least close all file descriptors higher than the highest inherited one.
       // We are assuming here that the caller didn't set FD_CLOEXEC on any of
       // these file descriptors.
-      _ = swt_posix_spawn_file_actions_addclosefrom_np(fileActions, highestFD + 1)
+      _ = _posix_spawn_file_actions_addclosefrom_np?(fileActions, highestFD + 1)
+#endif
 #elseif os(FreeBSD)
       // Like Linux, this platform doesn't have POSIX_SPAWN_CLOEXEC_DEFAULT.
       // Unlike Linux, all non-EOL FreeBSD versions (≥13.1) support
-      // `posix_spawn_file_actions_addclosefrom_np`. Therefore, we don't need
-      // `swt_posix_spawn_file_actions_addclosefrom_np` to guard the
-      // availability of this function.
+      // `posix_spawn_file_actions_addclosefrom_np`, and FreeBSD does not use
+      // glibc nor guard symbols behind `_DEFAULT_SOURCE`.
       _ = posix_spawn_file_actions_addclosefrom_np(fileActions, highestFD + 1)
 #elseif os(OpenBSD)
       // OpenBSD does not have posix_spawn_file_actions_addclosefrom_np().

Sources/Testing/ExitTests/WaitFor.swift

@@ -94,6 +94,17 @@ private nonisolated(unsafe) let _waitThreadNoChildrenCondition = {
   return result
 }()
 
+#if os(Linux) && !SWT_NO_DYNAMIC_LINKING
+/// Set the name of the current thread.
+///
+/// This function declaration is provided because `pthread_setname_np()` is
+/// only declared if `_GNU_SOURCE` is set, but setting it causes build errors
+/// due to conflicts with Swift's Glibc module.
+private let _pthread_setname_np = symbol(named: "pthread_setname_np").map {
+  unsafeBitCast($0, to: (@convention(c) (pthread_t, UnsafePointer<CChar>) -> CInt).self)
+}
+#endif
+
 /// Create a waiter thread that is responsible for waiting for child processes
 /// to exit.
 private let _createWaitThread: Void = {
@@ -152,7 +163,9 @@ private let _createWaitThread: Void = {
 #if SWT_TARGET_OS_APPLE
       _ = pthread_setname_np("Swift Testing exit test monitor")
 #elseif os(Linux)
-      _ = swt_pthread_setname_np(pthread_self(), "SWT ExT monitor")
+#if !SWT_NO_DYNAMIC_LINKING
+      _ = _pthread_setname_np?(pthread_self(), "SWT ExT monitor")
+#endif
 #elseif os(FreeBSD)
       _ = pthread_set_name_np(pthread_self(), "SWT ex test monitor")
 #elseif os(OpenBSD)

Sources/_TestingInternals/CMakeLists.txt

@@ -12,7 +12,6 @@ include(LibraryVersion)
 include(TargetTriple)
 add_library(_TestingInternals STATIC
   Discovery.cpp
-  Stubs.cpp
   Versions.cpp
   WillThrow.cpp)
 target_include_directories(_TestingInternals PUBLIC

Sources/_TestingInternals/Stubs.cpp

@@ -117,25 +117,6 @@ static char *_Nullable *_Null_unspecified swt_environ(void) {
 }
 #endif
 
-#if defined(__linux__)
-/// Set the name of the current thread.
-///
-/// This function declaration is provided because `pthread_setname_np()` is
-/// only declared if `_GNU_SOURCE` is set, but setting it causes build errors
-/// due to conflicts with Swift's Glibc module.
-SWT_EXTERN int swt_pthread_setname_np(pthread_t thread, const char *name);
-#endif
-
-#if defined(__GLIBC__)
-/// Close file descriptors above a given value when spawing a new process.
-///
-/// This symbol is provided because the underlying function was added to glibc
-/// relatively recently and may not be available on all targets. Checking
-/// `__GLIBC_PREREQ()` is insufficient because `_DEFAULT_SOURCE` may not be
-/// defined at the point spawn.h is first included.
-SWT_EXTERN int swt_posix_spawn_file_actions_addclosefrom_np(posix_spawn_file_actions_t *fileActions, int from);
-#endif
-
 #if !defined(__ANDROID__)
 #if __has_include(<signal.h>) && defined(si_pid)
 /// Get the value of the `si_pid` field of a `siginfo_t` structure.