[issue32345] EIO from write() is only fatal if print() contains a newline

[STINNER Victor]

Change by STINNER Victor :


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nosy: +vstinner

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[issue32345] EIO from write() is only fatal if print() contains a newline

[Creideiki]

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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[issue32345] EIO from write() is only fatal if print() contains a newline

[Antoine Pitrou]

Antoine Pitrou  added the comment:

I think the difference here is that Python calls ferror() to tell whether an 
error occurred on the underlying FILE*.  Can you adapt your C program to check 
ferror()?

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[issue32345] EIO from write() is only fatal if print() contains a newline

[Creideiki]

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

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Python 2 doesn't call write(), it calls fwrite() and friends (i.e. it uses the 
libc's buffered I/O API).  Also we don't do anything special if the printed 
string has a newline in it.  So my guess is that it's a bug in the libc.

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[issue32345] EIO from write() is only fatal if print() contains a newline

[Creideiki]

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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