Use posix_spawn_file_actions_adddup2() to clear FD_CLOEXEC.

On FreeBSD, OpenBSD, Android, and Glibc ≥ 2.29, `posix_spawn_file_actions_adddup2()` automatically clears `FD_CLOEXEC` if the file descriptors passed to it are equal. Relying on this behaviour eliminates a race condition when spawning child processes.

Some older Linuxes (Amazon Linux 2 in particular) don't have this functionality, so we do a runtime check of the Glibc version.

Author: grynspan

Sources/Testing/ExitTests/ExitTest.swift

@@ -803,11 +803,26 @@ extension ExitTest {
 
 #if !SWT_TARGET_OS_APPLE
         // Set inherited those file handles that the child process needs. On
-        // Darwin, this is a no-op because we use POSIX_SPAWN_CLOEXEC_DEFAULT.
-        try stdoutWriteEnd?.setInherited(true)
-        try stderrWriteEnd?.setInherited(true)
-        try backChannelWriteEnd.setInherited(true)
-        try capturedValuesReadEnd.setInherited(true)
+        // Darwin, this isn't needed because we use POSIX_SPAWN_CLOEXEC_DEFAULT.
+        var setFileHandlesInherited = false
+#if os(Windows)
+        // Always set the file handles to be inherited on Windows. Our calls to
+        // CreateProcessW() specify which file handles are actually inherited on
+        // a per-child basis.
+        setFileHandlesInherited = true
+#elseif canImport(Glibc)
+        // On Glibc ≥ 2.29, posix_spawn_file_actions_adddup2() automatically
+        // clears FD_CLOEXEC for us in the child process.
+        var glibcVersion: (major: CInt, minor: CInt) = (0, 0)
+        swt_getGlibcVersion(&glibcVersion.major, &glibcVersion.minor)
+        setFileHandlesInherited = glibcVersion.major < 2 || (glibcVersion.major == 2 && glibcVersion.minor < 29)
+#endif
+        if setFileHandlesInherited {
+          try stdoutWriteEnd?.setInherited(true)
+          try stderrWriteEnd?.setInherited(true)
+          try backChannelWriteEnd.setInherited(true)
+          try capturedValuesReadEnd.setInherited(true)
+        }
 #endif
 
         // Spawn the child process.

Sources/Testing/ExitTests/SpawnProcess.swift

@@ -128,6 +128,8 @@ func spawnExecutable(
           } else {
 #if SWT_TARGET_OS_APPLE
             _ = posix_spawn_file_actions_addinherit_np(fileActions, fd)
+#else
+            _ = posix_spawn_file_actions_adddup2(fileActions, fd, fd)
 #endif
             highestFD = max(highestFD, fd)
           }

Sources/_TestingInternals/include/Versions.h

@@ -48,6 +48,17 @@ static const char *_Nullable swt_getWASIVersion(void) {
 }
 #endif
 
+#if defined(__GLIBC__)
+/// Get the version of glibc used when compiling the testing library.
+///
+/// This function is provided because `__GLIBC__` and `__GLIBC_MINOR__` are not
+/// available in Swift.
+static void swt_getGlibcVersion(int *outMajor, int *outMinor) {
+  *outMajor = __GLIBC__;
+  *outMinor = __GLIBC_MINOR__;
+}
+#endif
+
 SWT_ASSUME_NONNULL_END
 
 #endif