chore: import upstream snapshot with attribution

Sources/CShim/include/exec_command.h

@@ -0,0 +1,56 @@
+/*
+ * Copyright © 2025-2026 Apple Inc. and the Containerization project authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef exec_command_h
+#define exec_command_h
+
+#if defined(__linux__) || defined(__APPLE__)
+
+#include <sys/types.h>
+#include <unistd.h>
+
+struct exec_command_attrs {
+  int setpgid;
+  /// parent group id
+  pid_t pgid;
+  /// set the controlling terminal
+  int setctty;
+  /// controlling terminal fd
+  int ctty;
+  /// set the process as session leader
+  int setsid;
+  /// set the process user id
+  uid_t uid;
+  /// set the process group id
+  gid_t gid;
+  /// signal mask for the child process
+  int mask;
+  /// parent death signal (Linux only, 0 to disable)
+  int pdeathSignal;
+  /// make the new process group the foreground process group
+  int setfgpgrp;
+};
+
+void exec_command_attrs_init(struct exec_command_attrs *attrs);
+
+/// spawn a new child process with the provided attrs
+int exec_command(pid_t *result, const char *executable, char *const argv[],
+                 char *const envp[], const int file_handles[],
+                 const int file_handle_count, const char *working_directory,
+                 struct exec_command_attrs *attrs);
+
+#endif /* defined(__linux__) || defined(__APPLE__) */
+#endif /* exec_command_h */