Add a "fork" of musl as //fusl.

fusl/src/stdio/popen.c

+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <spawn.h>
+#include "stdio_impl.h"
+#include "syscall.h"
+
+extern char **__environ;
+
+FILE *popen(const char *cmd, const char *mode)
+{
+        int p[2], op, e;
+        pid_t pid;
+        FILE *f;
+        posix_spawn_file_actions_t fa;
+
+        if (*mode == 'r') {
+                op = 0;
+        } else if (*mode == 'w') {
+                op = 1;
+        } else {
+                errno = EINVAL;
+                return 0;
+        }
+        
+        if (pipe2(p, O_CLOEXEC)) return NULL;
+        f = fdopen(p[op], mode);
+        if (!f) {
+                __syscall(SYS_close, p[0]);
+                __syscall(SYS_close, p[1]);
+                return NULL;
+        }
+        FLOCK(f);
+
+        /* If the child's end of the pipe happens to already be on the final
+         * fd number to which it will be assigned (either 0 or 1), it must
+         * be moved to a different fd. Otherwise, there is no safe way to
+         * remove the close-on-exec flag in the child without also creating
+         * a file descriptor leak race condition in the parent. */
+        if (p[1-op] == 1-op) {
+                int tmp = fcntl(1-op, F_DUPFD_CLOEXEC, 0);
+                if (tmp < 0) {
+                        e = errno;
+                        goto fail;
+                }
+                __syscall(SYS_close, p[1-op]);
+                p[1-op] = tmp;
+        }
+
+        e = ENOMEM;
+        if (!posix_spawn_file_actions_init(&fa)) {
+                if (!posix_spawn_file_actions_adddup2(&fa, p[1-op], 1-op)) {
+                        if (!(e = posix_spawn(&pid, "/bin/sh", &fa, 0,
+                            (char *[]){ "sh", "-c", (char *)cmd, 0 }, __environ))) {
+                                posix_spawn_file_actions_destroy(&fa);
+                                f->pipe_pid = pid;
+                                if (!strchr(mode, 'e'))
+                                        fcntl(p[op], F_SETFD, 0);
+                                __syscall(SYS_close, p[1-op]);
+                                FUNLOCK(f);
+                                return f;
+                        }
+                }
+                posix_spawn_file_actions_destroy(&fa);
+        }
+fail:
+        fclose(f);
+        __syscall(SYS_close, p[1-op]);
+
+        errno = e;
+        return 0;
+}