[ruby-core:85343] [Ruby trunk Bug#14434] IO#reopen fails after EPIPE

Issue #14434 has been reported by nobu (Nobuyoshi Nakada).

----------------------------------------
Bug #14434: IO#reopen fails after EPIPE
https://bugs.ruby-lang.org/issues/14434

* Author: nobu (Nobuyoshi Nakada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
Consider the following code, which emulates `yes | head -1`.

```ruby
IO.popen("head -1", "w") do |f|
  f.sync = false
  stdout = STDOUT.dup
  STDOUT.reopen(f)
  loop{puts "y"} rescue break $!
ensure
  STDOUT.reopen(stdout)
end
```

This fails with `Errno::EPIPE` in `IO#reopen`.

```
Traceback (most recent call last):
	4: from -:1:in `<main>'
	3: from -:1:in `popen'
	2: from -:7:in `block in <main>'
	1: from -:7:in `ensure in block in <main>'
-:7:in `reopen': Broken pipe (Errno::EPIPE)
```

BTW, this "broken pipe" IO can't even close.

```ruby
r, w = IO.pipe
w.sync = false # make buffered
w.print("foo")
r.close
# w.reopen(STDOUT) rescue p $! #=> #<Errno::EPIPE: Broken pipe>
w.close rescue p $! #=> #<Errno::EPIPE: Broken pipe>
```

This is caused by flushing buffered data.
I think there is no way to recover "broken pipe" FD, so nothing can be done for the remained data and should be discarded gently.

```diff
diff --git i/io.c w/io.c
index 0a4e66f5ed..5eca6762f2 100644
--- i/io.c
+++ w/io.c
@@ -7150,8 +7150,7 @@ io_reopen(VALUE io, VALUE nfile)
 	}
     }
     if (fptr->mode & FMODE_WRITABLE) {
-        if (io_fflush(fptr) < 0)
-            rb_sys_fail(0);
+        fptr_finalize_flush(fptr, TRUE, FALSE);
     }
     else {
 	io_tell(fptr);
@@ -7177,7 +7176,8 @@ io_reopen(VALUE io, VALUE nfile)
     if (fd != fd2) {
 	if (IS_PREP_STDIO(fptr) || fd <= 2 || !fptr->stdio_file) {
 	    /* need to keep FILE objects of stdin, stdout and stderr */
-	    if (rb_cloexec_dup2(fd2, fd) < 0)
+	    fd = (fd < 0) ? rb_cloexec_dup(fd2) : rb_cloexec_dup2(fd2, fd);
+	    if (fd < 0)
 		rb_sys_fail_path(orig->pathv);
             rb_update_max_fd(fd);
 	}
```



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