[Bug 37940] New: Invalid file buffering behavior causes data corruption

https://bugs.winehq.org/show_bug.cgi?id=37940

Bug ID: 37940
           Summary: Invalid file buffering behavior causes data corruption
           Product: Wine
           Version: 1.7.34
          Hardware: x86
                OS: Linux
            Status: UNCONFIRMED
          Severity: major
          Priority: P2
         Component: msvcrt
          Assignee: [e-mail]
          Reporter: [e-mail]
      Distribution: ---

Created attachment 50543
  --> https://bugs.winehq.org/attachment.cgi?id=50543
Compiled binary to reproduce

msvcrt file routines sometimes corrupt file data by invalid buffer flushing.
Reproduced by following code (compiled binary attached):

char data[68200];
    char buf[sizeof(data)];
    for (int i = 0; i < sizeof(data); i++)
        data[i] = rand();

FILE* f = _wfsopen(L"test.bin", L"wb+", 64);

rewind(f);
    fwrite(data, 1, 68100, f);

// Following calls possible corrupt buffer
    rewind(f);
    fread(buf, 1, 40000, f);
    fread(buf, 1, 40000, f);

// Following call causes wrong buffer flushing
    fwrite(&data[68100], 1, 100, f);

fclose(f);

f = fopen("test.bin", "rb");
    fread(buf, 1, sizeof(buf), f);
    fclose(f);
    printf("Memcmp: %d\n", memcmp(data, buf, sizeof(buf)));

Windows output:
Memcmp: 0

Wine output:
Memcmp: -1

Windows file size: 68200
Wine file size: 72296

Bug causes at least cl.exe in Microsoft Visual Studio 2008/2010. Reproduced
when cl.exe generates precompiled header.

test.h:
#pragma once
#include "Windows.h"
#include <iostream>

test.cpp:
#include "test.h"

int main()
{
   std::cout << "WORKS!" << std::endl;
   return 0;
}

Command to reproduce:
rm test.pch; wine cl.exe /Yctest.h test.cpp