[issue32345] EIO from write() is only fatal if print() contains a newline
New submission from Creideiki :
This is Python 2.7.14 on Gentoo Linux.
I ran into an issue where a program crashes if I run it from a terminal, put it
in the background, and then close the terminal too soon. Upstream bug report:
https://github.com/micahflee/torbrowser-launcher/issues/298
It seems the cause is that a print() call without a newline ignores the EIO
returned by the write() syscall, but if there is a newline in the string, that
EIO is suddenly a fatal error.
Reproducer:
$ cat fatal.py
#!/bin/env python2.7
import time
time.sleep(5)
print('A')
print('B\nC')
print('D')
Run this program in the background under strace to see what it does, and while
it is sleeping, close its controlling terminal:
$ strace -s 256 -o fatal.log -f ./fatal.py &
[1] 17974
^d
Now look at the strace log:
$ grep write fatal.log
17978 write(1, "A\n", 2)= -1 EIO (Input/output error)
17978 write(1, "B\n", 2)= -1 EIO (Input/output error)
17978 write(2, "Traceback (most recent call last):\n", 35) = -1 EIO
(Input/output error)
17978 write(2, " File \"./fatal.py\", line 5, in \n", 41) = -1 EIO
(Input/output error)
17978 write(2, "", 4) = -1 EIO (Input/output error)
17978 write(2, "print('B\\nC')\n", 14) = -1 EIO (Input/output error)
17978 write(2, "IOError", 7)= -1 EIO (Input/output error)
17978 write(2, ": ", 2) = -1 EIO (Input/output error)
17978 write(2, "[Errno 5] Input/output error", 28) = -1 EIO (Input/output error)
17978 write(2, "\n", 1) = -1 EIO (Input/output error)
The first print('A') ran and had an EIO error, which was ignored. The second
print('B\nC') tried to write 'B', had an EIO error, and crashed. The third
print('D') was never attempted.
--
components: IO
messages: 308468
nosy: Creideiki
priority: normal
severity: normal
status: open
title: EIO from write() is only fatal if print() contains a newline
versions: Python 2.7
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[issue32345] EIO from write() is only fatal if print() contains a newline
Creideiki added the comment:
Hmm. Yes and no; you seem to be correct in that the problem is on the libc
level, but Python definitely does something special.
By the way, this is glibc 2.26.
I tried this C program:
#include
#include
int main()
{
usleep(5 * 1000 * 1000);
fwrite("A", 1, 1, stdout);
perror("A");
fwrite("\n", 1, 1, stdout);
perror("newline 1");
fwrite("B\nC", 3, 1, stdout);
perror("BC");
fwrite("\n", 1, 1, stdout);
perror("newline 2");
fwrite("D", 1, 1, stdout);
perror("D");
fwrite("\n", 1, 1, stdout);
perror("newline 3");
return 0;
}
Which behaves similarly when there's a newline in the middle of the string, in
that fwrite() returns with an error after writing the newline:
write(3, "A: Success\n", 11)= -1 EIO (Input/output error)
write(1, "A\n", 2) = -1 EIO (Input/output error)
write(3, "newline 1: Input/output error\n", 30) = -1 EIO (Input/output error)
write(1, "B\n", 2) = -1 EIO (Input/output error)
write(3, "BC: Input/output error\n", 23) = -1 EIO (Input/output error)
write(1, "\n", 1) = -1 EIO (Input/output error)
write(3, "newline 2: Input/output error\n", 30) = -1 EIO (Input/output error)
write(3, "D: Input/output error\n", 22) = -1 EIO (Input/output error)
write(1, "D\n", 2) = -1 EIO (Input/output error)
write(3, "newline 3: Input/output error\n", 30) = -1 EIO (Input/output error)
However, as can be seen from the result of the perror() calls, every fwrite()
after the first buffer flush (which happens on newlines, since stdout to a
terminal is line buffered) fails with EIO. This should mean that, at the
latest, the second Python print() call should fail. But it doesn't:
#!/bin/env python2.7
import time
time.sleep(5)
print('A')
print('E')
print('B\nC')
print('D')
write(1, "A\n", 2) = -1 EIO (Input/output error)
write(1, "E\n", 2) = -1 EIO (Input/output error)
write(1, "B\n", 2) = -1 EIO (Input/output error)
write(2, "Traceback (most recent call last):\n", 35) = -1 EIO (Input/output
error)
write(2, " File \"./fatal.py\", line 6, in \n", 41) = -1 EIO
(Input/output error)
So Python does make a difference between the implicit newline added to the end
of each print() and a newline in the middle of the user-supplied string.
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[issue32345] EIO from write() is only fatal if print() contains a newline
Creideiki added the comment:
I ran this program:
#include
#include
#include
#include
void print(const char *s, size_t l)
{
errno = 0;
fwrite(s, l, 1, stdout);
int saved_errno = errno;
fprintf(stderr,
"After \"%s\": ferror(): %i, strerror(): %s\n",
s, ferror(stdout), strerror(saved_errno));
clearerr(stdout);
}
int main()
{
usleep(5 * 1000 * 1000);
print("A", 1);
print("\n", 1);
print("B\nC", 3);
print("\n", 1);
print("D", 1);
print("\n", 1);
return 0;
}
Got this result:
write(2, "After \"A\": ferror(): 0, strerror(): Success\n", 44) = -1 EIO
(Input/output error)
write(1, "A\n", 2) = -1 EIO (Input/output error)
write(2, "After \"\n\": ferror(): 1, strerror(): Input/output error\n", 55) =
-1 EIO (Input/output error)
write(1, "B\n", 2) = -1 EIO (Input/output error)
write(2, "After \"B\nC\": ferror(): 1, strerror(): Input/output error\n", 57) =
-1 EIO (Input/output error)
write(1, "\n", 1) = -1 EIO (Input/output error)
write(2, "After \"\n\": ferror(): 1, strerror(): Input/output error\n", 55) =
-1 EIO (Input/output error)
write(2, "After \"D\": ferror(): 0, strerror(): Success\n", 44) = -1 EIO
(Input/output error)
write(1, "D\n", 2) = -1 EIO (Input/output error)
write(2, "After \"\n\": ferror(): 1, strerror(): Input/output error\n", 55) =
-1 EIO (Input/output error)
So, fwrite() with a string which does not contain a newline is buffered and
ferror() returns 0. fwrite() with a string which does contain a newline, in the
middle or at the end, flushes the buffer and makes ferror() return 1.
I think this means that after print('A') gets turned into write(1, "A\n", 2),
ferror() should still be 1.
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