Fix multiple issues with alternatives module

- Changes are no longer erroneously reported on RHEL (#12)
 - Adding new link groups on Debian works again.
   - This was broken in a previous commit by assuming the OS was RHEL
     if `update-alternatives --query <name>` had a return code of 2
 - Prefer `--display` over `--query` for determining available
   alternatives
   - --display is more distro-agnostic and simplifies the code
 - Fix missing `msg=` in `fail_json` call when `link` is missing
 - Document that `link` is required on RHEL-based distros

Tested on Ubuntu 12.04+ and CentOS 6/7

system/alternatives.py

@@ -4,6 +4,7 @@
 """
 Ansible module to manage symbolic link alternatives.
 (c) 2014, Gabe Mulley <gabe.mulley@gmail.com>
+(c) 2015, David Wittman <dwittman@gmail.com>
 
 This file is part of Ansible
 
@@ -26,7 +27,7 @@ DOCUMENTATION = '''
 module: alternatives
 short_description: Manages alternative programs for common commands
 description:
-    - Manages symbolic links using the 'update-alternatives' tool provided on debian-like systems.
+    - Manages symbolic links using the 'update-alternatives' tool
     - Useful when multiple programs are installed but provide similar functionality (e.g. different editors).
 version_added: "1.6"
 options:
@@ -41,6 +42,7 @@ options:
   link:
     description:
       - The path to the symbolic link that should point to the real executable.
+      - This option is required on RHEL-based distributions
     required: false
 requirements: [ update-alternatives ]
 '''
@@ -55,12 +57,14 @@ EXAMPLES = '''
 
 DEFAULT_LINK_PRIORITY = 50
 
+import re
+
 def main():
 
     module = AnsibleModule(
         argument_spec = dict(
             name = dict(required=True),
-            path  = dict(required=True),
+            path = dict(required=True),
             link = dict(required=False),
         ),
         supports_check_mode=True,
@@ -71,78 +75,55 @@ def main():
     path = params['path']
     link = params['link']
 
-    UPDATE_ALTERNATIVES =  module.get_bin_path('update-alternatives',True)
+    UPDATE_ALTERNATIVES = module.get_bin_path('update-alternatives',True)
 
     current_path = None
     all_alternatives = []
-    os_family = None
 
-    (rc, query_output, query_error) = module.run_command(
-        [UPDATE_ALTERNATIVES, '--query', name]
+    # Run `update-alternatives --display <name>` to find existing alternatives
+    (rc, display_output, _) = module.run_command(
+        [UPDATE_ALTERNATIVES, '--display', name]
     )
 
-    # Gather the current setting and all alternatives from the query output.
-    # Query output should look something like this on Debian systems:
-
-        # Name: java
-        # Link: /usr/bin/java
-        # Slaves:
-        #  java.1.gz /usr/share/man/man1/java.1.gz
-        # Status: manual
-        # Best: /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java
-        # Value: /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java
-
-        # Alternative: /usr/lib/jvm/java-6-openjdk-amd64/jre/bin/java
-        # Priority: 1061
-        # Slaves:
-        #  java.1.gz /usr/lib/jvm/java-6-openjdk-amd64/jre/man/man1/java.1.gz
-
-        # Alternative: /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java
-        # Priority: 1071
-        # Slaves:
-        #  java.1.gz /usr/lib/jvm/java-7-openjdk-amd64/jre/man/man1/java.1.gz
-
     if rc == 0:
-        os_family = "Debian"
-        for line in query_output.splitlines():
-            split_line = line.split(':')
-            if len(split_line) == 2:
-                key = split_line[0]
-                value = split_line[1].strip()
-                if key == 'Value':
-                    current_path = value
-                elif key == 'Alternative':
-                    all_alternatives.append(value)
-                elif key == 'Link' and not link:
-                    link = value
-    elif rc == 2:
-        os_family = "RedHat"
-        # This is the version of update-alternatives that is shipped with
-        # chkconfig on RedHat-based systems. Try again with the right options.
-        (rc, query_output, query_error) = module.run_command(
-            [UPDATE_ALTERNATIVES, '--list']
-        )
-        for line in query_output.splitlines():
-            line_name, line_mode, line_path = line.strip().split("\t")
-            if line_name != name:
-                continue
-            current_path = line_path
-            break
+        # Alternatives already exist for this link group
+        # Parse the output to determine the current path of the symlink and
+        # available alternatives
+        current_path_regex = re.compile(r'^\s*link currently points to (.*)$',
+                                        re.MULTILINE)
+        alternative_regex = re.compile(r'^(\/.*)\s-\spriority', re.MULTILINE)
+
+        current_path = current_path_regex.search(display_output).group(1)
+        all_alternatives = alternative_regex.findall(display_output)
+
+        if not link:
+            # Read the current symlink target from `update-alternatives --query`
+            # in case we need to install the new alternative before setting it.
+            #
+            # This is only compatible on Debian-based systems, as the other
+            # alternatives don't have --query available
+            rc, query_output, _ = module.run_command(
+                [UPDATE_ALTERNATIVES, '--query', name]
+            )
+            if rc == 0:
+                for line in query_output.splitlines():
+                    if line.startswith('Link:'):
+                        link = line.split()[1]
+                        break
 
     if current_path != path:
         if module.check_mode:
             module.exit_json(changed=True, current_path=current_path)
         try:
             # install the requested path if necessary
-            # (unsupported on the RedHat version)
-            if path not in all_alternatives and os_family == "Debian":
-                if link:
-                    module.run_command(
-                        [UPDATE_ALTERNATIVES, '--install', link, name, path, str(DEFAULT_LINK_PRIORITY)],
-                        check_rc=True
-                    )
-                else:
-                    module.fail_json("Needed to install the alternative, but unable to do so, as we are missking the link")
+            if path not in all_alternatives:
+                if not link:
+                    module.fail_json(msg="Needed to install the alternative, but unable to do so as we are missing the link")
+
+                module.run_command(
+                    [UPDATE_ALTERNATIVES, '--install', link, name, path, str(DEFAULT_LINK_PRIORITY)],
+                    check_rc=True
+                )
 
             # select the requested path
             module.run_command(
@@ -151,7 +132,7 @@ def main():
             )
 
             module.exit_json(changed=True)
-        except subprocess.CalledProcessError, cpe:
+        except subprocess.CalledProcessError as cpe:
             module.fail_json(msg=str(dir(cpe)))
     else:
         module.exit_json(changed=False)