Bug 974489

Description of problem:

(Bug found by Matt Booth)

The following commit:
https://github.com/libguestfs/libguestfs/commit/e71b2c11f59b3f8ae0c4b31c4ab5b0d1bfcdf181
(also present in libguestfs 1.22) changes various calls to
guestfs_exists to guestfs_is_file.  This was done to make
fuzz-testing of inspection easier.

Unfortunately at least this change:

-  if (guestfs_exists (g, "/etc/redhat-release") > 0) {
+  if (guestfs_is_file (g, "/etc/redhat-release") > 0) { 

breaks inspection of Fedora guests since /etc/redhat-release
is a symlink:

$ ls -l /etc/redhat-release 
lrwxrwxrwx. 1 root root 14 Jan 17 08:28 /etc/redhat-release -> fedora-release

I haven't checked all the other distros but it is possible
the change could break those as well.

What we really want to do is test that these are files or
symlinks to files, since if the test succeeds we usually
open the file and read its contents.

Matt suggests changing the is_file etc APIs to add a
followsymlinks optional boolean flag.  Possible APIs that
would be changed like this are:

is-blockdev          test if block device
is-chardev           test if character device
is-dir               test if a directory
is-fifo              test if FIFO (named pipe)
is-file              test if a regular file
is-socket            test if socket

By using the once_had_no_optargs flag in the generator this
change should be backwards compatible.

Version-Release number of selected component (if applicable):

libguestfs 1.22.2
libguestfs >= 1.23.2

How reproducible:

100%

Steps to Reproduce:
1. Inspect (eg. run virt-inspector) on a Fedora guest.

Actual results:

Inspection "half works", but it doesn't detect that it's a Fedora
VM:

<?xml version="1.0"?>
<operatingsystems>
  <operatingsystem>
    <root>/dev/fedora/root</root>
    <name>linux</name>
    <arch>x86_64</arch>
    <major_version>0</major_version>
    <minor_version>0</minor_version>

Note there is no <distro>fedora</distro> and the major version number
are bogus.

Expected results:

<distro>fedora</distro> and <major_version/> set to some non-zero value.
List of applications shown.  etc.

Additional info: