[Bug stdio/17063] New: fclose() may fail to flush data

["ppluzhnikov at google dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17063

            Bug ID: 17063
           Summary: fclose() may fail to flush data
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: ppluzhnikov at google dot com

This is a followup to PR16532.

A program that does

  fopen(..., "w+");
  fwrite(...);
  fread(...);
  fclose();

may leave empty file on disk (fail to flush when it should have), when the
fread requests more than a page's worth of data.

At least glibc-2.19 through current trunk
(754c5a08aacb44895d1ab97c553ce424eb43f761) are affected.

./t
testing     3
testing     3 OK
testing     6
testing     6 OK
testing     9
testing     9 OK
testing  4095
testing  4095 OK
testing  4096
t: t.c:28: do_test: Assertion `pos == 6' failed.
Aborted

/// --- cut ---
#include <assert.h>
#include <stdio.h>

void do_test (int n)
{
  FILE *fp;
  const char *const fname = "/tmp/output";
  int nwritten, nread, pos;
  char line[8192];

  printf ("testing %5d\n", n);
  fp = fopen (fname, "w+");
  nwritten = fwrite ("abcabc", 1, 6, fp);
  assert (nwritten == 6);

  pos = ftello (fp);
  assert (pos == 6);

  nread = fread (line, 1, n, fp);
  assert (nread == 0);

  fclose (fp);

  fp = fopen (fname, "r");
  fseeko (fp, 0, SEEK_END);
  pos = ftello (fp);

  assert (pos == 6);
  printf ("testing %5d OK\n", n);
}

int main(int argc, char *argv[])
{
  int j, nreads[] = { 3, 6, 9, 4095, 4096, 4097, 8191, 8192 };

  for (j = 0; j < sizeof (nreads) / sizeof (nreads[0]); ++j)
    do_test (nreads[j]);

  return 0;
}

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