[ruby-core:122910] [Ruby Bug#21530] Is IO#eof? supposed to always block and read?

Issue #21530 has been reported by tenderlovemaking (Aaron Patterson).

----------------------------------------
Bug #21530: Is IO#eof? supposed to always block and read?
https://bugs.ruby-lang.org/issues/21530

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
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I'm not sure whether or not this is expected behavior, but it seems like eof? blocks when called on $stdin.

For example:

```ruby
if (str = $stdin.gets)
  $stderr.puts "read #{str}"
end

if $stdin.eof? # this call waits for input
  $stderr.puts "stdin is eof"
end
```

I think this is kind of odd behavior because if you input a string but _do not_ input a newline, then hit ^D twice, `$stdin` should be at EOF, but `eof?` will block and wait for input.  If you hit ^D a third time, $stdin will be EOF, but if you input a different character it will not be EOF.

Compare this C program:

```c
#include <stdio.h>
#include <stdlib.h>

#define BUF_SIZE 4096

int main(int argc, char *argv[]) {
    char buf[BUF_SIZE];
    if (fgets(buf, BUF_SIZE, stdin)) {
        fprintf(stderr, "read %s\n", buf);
    }

    if (feof(stdin)) { // Does not block
        fprintf(stderr, "stdin is EOF\n");
    }
}
```

If you hit ^D twice with this C program, `feof` will return true for `stdin`.  I would have expected the Ruby program and the C program to behave similarly, but they don't.  Is this expected?  The documentation indeed says that `eof?` will read, but shouldn't the IO be at EOF after the second ^D?

Thank you.



-- 
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