Merge ~larsks/cloud-init:bug/1647118 into cloud-init:master

[Lars Kellogg-Stedman]

    Recent fedora releases use "dnf" instead of "yum" for package
    management. While there is a compatible "yum" cli available, there's
    no guarantee that it will be available.

    With this patch, cloud-init will check for /usr/bin/dnf and use that
    if it exists instead of yum.

[Scott Moser (smoser)]

How far back does dnf support go?
1 inline question.

review: Needs Information

[Lars Kellogg-Stedman (larsks)]

dnf was introduced in Fedora 18 (and has been the default since 22).

[Scott Moser (smoser)]

Lars,
did you see the question in line ?

Any reason to not use util.which('dnf') ?

[Lars Kellogg-Stedman (larsks)]

I missed the inline question, sorry!

If dnf isn't /usr/bin/dnf, I'm not sure what that says about the environment. I would leave it as is unless there is a use case that is "use dnf even when it isn't actually the system package manager".

[Scott Moser (smoser)]

But why hard code a path to /usr/bin/dnf ?
Why not allow that to be /usr/sbin/dnf or /usr/local/bin/dnf
Perhaps the system is built with dnf from upstream source and installed into /usr/local/bin, or the package maintainer decides it should go in /bin.

The chance for failure in the strict "must be named /usr/bin/dnf" is if the program is not installed there. The chance for failure in my suggested solution (trust the PATH), is name collision (a program in the PATH is named 'dnf' but is a different thing).

I've never understood why anyone would want to insist on a path of a program (/bin/ip or /bin/ls rather than just 'ls' and 'ip').

If you dont have an example likely name collision then i'd prefer util.which and let the administrator get their path set correctly.

[Scott Moser (smoser)]

I merged this, but used util.which('dnf') rather than os.path.isfile(/usr/bin/dnf).

diff --git a/cloudinit/distros/rhel.py b/cloudinit/distros/rhel.py
index e574e1b..f0e41f7 100644
--- a/cloudinit/distros/rhel.py
+++ b/cloudinit/distros/rhel.py
@@ -202,13 +202,18 @@ class Distro(distros.Distro):
         if pkgs is None:
             pkgs = []
 
-        cmd = ['yum']
-        # If enabled, then yum will be tolerant of errors on the command line
-        # with regard to packages.
-        # For example: if you request to install foo, bar and baz and baz is
-        # installed; yum won't error out complaining that baz is already
-        # installed.
-        cmd.append("-t")
+        if os.path.isfile('/usr/bin/dnf'):
+            LOG.debug('Using DNF for package management')
+            cmd = ['dnf']
+        else:
+            LOG.debug('Using YUM for package management')
+            # the '-t' argument makes yum tolerant of errors on the command
+            # line with regard to packages.
+            #
+            # For example: if you request to install foo, bar and baz and baz
+            # is installed; yum won't error out complaining that baz is already
+            # installed.
+            cmd = ['yum', '-t']
         # Determines whether or not yum prompts for confirmation
         # of critical actions. We don't want to prompt...
         cmd.append("-y")