Bug 1289068

Description of problem:
Errno is incorrectly set to EINVAL even on success, for subsequent glfs_* calls.

[root@hummingbird ~]# rpm -qa | grep glusterfs
glusterfs-3.7.6-1.fc22.x86_64
glusterfs-cli-3.7.6-1.fc22.x86_64
glusterfs-devel-3.7.6-1.fc22.x86_64
glusterfs-libs-3.7.6-1.fc22.x86_64
glusterfs-client-xlators-3.7.6-1.fc22.x86_64
glusterfs-fuse-3.7.6-1.fc22.x86_64
glusterfs-server-3.7.6-1.fc22.x86_64
glusterfs-extra-xlators-3.7.6-1.fc22.x86_64
glusterfs-api-devel-3.7.6-1.fc22.x86_64
glusterfs-api-3.7.6-1.fc22.x86_64

Example:
Even though glfs_fstat() returns success (zero), errno is set to EINVAL. It doesn't matter that the second glfs_* call is glfs_stat(). Even if you have glfs_write() twice, errno is set to EINVAL for the second call.


#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>

#include "api/glfs.h"

int main () {

        glfs_t *fs = NULL;
        int ret = 0;
        glfs_fd_t *glfd = NULL;
        char *buf = NULL;
        struct stat sb;

        fs = glfs_new ("test");
        glfs_set_volfile_server (fs, "tcp", "localhost", 24007);
        glfs_set_logging (fs, NULL, 7);
        glfs_init (fs);
        glfd = glfs_creat(fs, "/file.txt", O_WRONLY, 0666);

        // Comment out this write to see the bug magically go away!
        ret = glfs_write (glfd, "qwertyuiopasdfghjklzxcvbnm", 26, 0);
        printf("errno after write(), before fstat(): %d\n", errno);
        ret = glfs_fstat(glfd, &sb);
        printf("glfs_fstat() returned: %d\n", ret);
        printf("errno after: %d\n", errno);

        ret = glfs_close(glfd);
        ret = glfs_fini (fs);
}  


Steps to Reproduce:
*******************

# gcc err.c -I /usr/include/glusterfs/ -lgfapi -O0 -ggdb -o ./err
# ./err

Actual results:
***************

# ./err
errno after write(), before fstat(): 0
glfs_fstat() returned: 0
errno after: 22

// After commenting out first call - glfs_write()

# ./err
errno after write(), before fstat(): 0
glfs_fstat() returned: 0
errno after: 0

Expected results:
*****************

# ./err
errno after write(), before fstat(): 0
glfs_fstat() returned: 0
errno after: 0


Additional info: 
****************
C programs usually check return value first and then the errno. But the control flow in golang is different.

ret, err := C.glfs_fstat(fd.fd, (*C.struct_stat)(unsafe.Pointer(stat)))
if err != nil {
    // Failure! handle error
} else {
    // Success! consume entity returned by C call. In this case "stat"
}