Non-Master PTY output discarded on child process exit

Hi! I use "non-master" term as a replacement for *****. I cannot find a documented safe way to read an output from non-master PTY because unlike Linux, on macOS it is discarded immediately after the child process exits. The situation is similar, regardless of whether I use forkpty(), or open /dev/ptmx directly. Here is my example:

Code Block C#undef NDEBUG#define CHECK assert#include <assert.h>#include <errno.h>#include <fcntl.h>#include <limits.h>#include <stdio.h>#include <stdlib.h>#include <sys/wait.h>#include <unistd.h>#if defined(__APPLE__)#include <util.h>#else#include <pty.h>#endifint main(void){    int master;    char name[PATH_MAX];    pid_t pid = forkpty(&master, name, NULL, NULL);    CHECK(pid != -1);    if (pid == 0) {  // child        //char *args[] = {"/bin/echo", "01234567890123456789012345678901234567890123456789", NULL};        //execv(args[0], args);        char buf[] =            "01234567890123456789012345678901234567890123456789"   // 50            "01234567890123456789012345678901234567890123456789";  // 100        CHECK(printf("%s", buf) == sizeof buf - 1);        CHECK(fflush(stdout) == 0);        exit(EXIT_SUCCESS);    }    // parent#define BUFSIZE 64    char buf[BUFSIZE];    //int non-master = open(name, O_WRONLY);  // here!    //CHECK(non-master != -1);    sleep(2);  // simulate process scheduling    int len;    do {        len = read(master, buf, BUFSIZE);        if (len == -1) {            printf("read = %d, err = %d\n", len, errno);  // EIO on Linux            break;        }        printf("read %d bytes: %.*s\n", len, len, buf);    } while (len != 0);    int status;    CHECK(waitpid(pid, &status, 0) == pid);    CHECK(WIFEXITED(status));    CHECK(WEXITSTATUS(status) == EXIT_SUCCESS);    CHECK(close(master) != -1);    return 0;}

On Linux, the output is:

Code Block sh$ ./a.outread 64 bytes: 0123456789012345678901234567890123456789012345678901234567890123read 36 bytes: 456789012345678901234567890123456789read = -1, err = 5

but on macOS:

Code Block sh$ ./a.outread 0 bytes:

There is a known workaround -- keep an open non-master fd in the parent process (see unix.stackexchange.com/a/478969), but it really looks like a "hacky" way, which btw causes a bug on Linux (read() never returns), and which isn't documented anywhere, right? So from this point of view, the code above should work as good as on Linux, and many projects use PTY this way (w/o non-master fd), which turns out to not work.

For example, Python's Pexpect will fail on macOS if there is a delay between spawn() and the first .expect():

Code Block pythonimport pexpect, timechild = pexpect.spawn('python3 -c \'print("%s", flush=True)\'' % ("0123456789" * 10))time.sleep(2)child.expect("0123456789" * 10)

results to:

Code Block sh$ python3 pexpect_test.pyTraceback (most recent call last):  File "pexpect_test.py", line 5, in <module>    child.expect("0123456789" * 10)  File "/usr/local/lib/python3.8/site-packages/pexpect/spawnbase.py", line 343, in expect    return self.expect_list(compiled_pattern_list,  File "/usr/local/lib/python3.8/site-packages/pexpect/spawnbase.py", line 372, in expect_list    return exp.expect_loop(timeout)  File "/usr/local/lib/python3.8/site-packages/pexpect/expect.py", line 179, in expect_loop    return self.eof(e)  File "/usr/local/lib/python3.8/site-packages/pexpect/expect.py", line 122, in eof    raise excpexpect.exceptions.EOF: End Of File (EOF). Empty string style platform.

Is this the expected behavior that should be specified in openpty(3) manual page, or is it a bug in macOS Catalina 10.15.7?