[Xen-devel] More Generic Config Files

["Dan Kendall" <Dan.Kendall@xxxxxxxxxx>]

Dear All,

I wanted to have a bit more control over the vars I use in my domain config 
files so that I could achieve more with a single file and just change the var 
values passed on the command line.  A couple of examples:

(*)combining a vm 'type' (e.g. www, cvs) and an 'id', checking to see if a disk 
image for such exists and cloning it from a 'type' master if not.

(*)getting a known offset for certain octets in mac addresses dependant on the 
'type' so that dhcp allocates predictable results.

The first of these was possible using a temp file to hold state, created using 
inner functions within the check() function for each var.  The second appears 
to be impossible.  

The attached (and inline) patch creates two new functions that can be called 
from the config file to register 'postcheck' functions and subsequently call 
them with the full dictionary of defined vars.  Changes made to the vars' 
values in the postchecks are maintained across calls and are carried into the 
config proper.

My python skills aren't great so any suggestions more than welcome, especially 
regards to 'hiding' the vars_{ex|im}port functions.  I appreciate there are 
more pressing areas of work but this makes my life easier and may do others' 
too.  If there are sanctioned ways of getting a similar level of functionality 
please let me know.



# HG changeset patch
# User dkendall <dan.kendall@xxxxxxxxxx>
# Date 1161957184 -3600
# Node ID 6c528a82d57b5fc32c517ebaab5b7c985f2b338c
# Parent  4a320d26fc24bc49ae24f31dec2bf006a9ddc7a8
Create postcheck() that calls postcheck routines registered using the new
xm_vars.reg_postcheck().  Each of these receives dictionary of the vars
and changes made to var values are permanent.  Kinda like an all in one
check() that knows about all defined vars.  Allows for more generic config
files.
Signed-off-by: dan kendall <dan.kendall@xxxxxxxxxx>

diff -r 4a320d26fc24 -r 6c528a82d57b tools/python/xen/xm/help.py
--- a/tools/python/xen/xm/help.py       Thu Oct 26 16:56:16 2006 +0100
+++ b/tools/python/xen/xm/help.py       Fri Oct 27 14:53:04 2006 +0100
@@ -35,6 +35,7 @@ class Vars:
         self.help = help
         self.env = env
         self.vars = []
+        self.postchecks = []
 
     def var(self, name, use=None, check=None):
         """Define a configuration variable.
@@ -58,6 +59,34 @@ class Vars:
         else:
             for v in self.vars:
                 v.doCheck(self.env)
+
+    def reg_postcheck(self, pcheck):
+        """Register routine to run after check
+        """
+        self.postchecks.append(pcheck)
+
+    def vars_export_dict(self):
+        """Export the var list to a dictionary
+        """
+        d = {}
+        for v in self.vars:
+            d[v.name] = self.env[v.name]
+        return d
+
+    def vars_import_dict(self,d):
+        """Import var list from dictionary
+           Note we only import already declared vars
+        """
+        for v in self.vars:
+            self.env[v.name] = d[v.name]
+
+    def postcheck(self):
+        """Perform any register post check routines
+        """
+        for c in self.postchecks:
+            d = self.vars_export_dict()
+            c(d)
+            self.vars_import_dict(d)
 
     def doHelp(self, out=sys.stderr):
         """Print help for the variables.
@@ -86,7 +115,12 @@ class Var:
     def doCheck(self, env):
         """Execute the check and set the variable to the new value.
         """
-        if not self.check: return
+        #ensure variable gets back to config variable 'space' even 
+        #if no check registered
+        #if not self.check: return
+        if not self.check:
+            env[self.name] = ''
+            return
         try: 
             env[self.name] = self.check(self.name, env.get(self.name))
         except StandardError, ex:


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Attachment: help.patch
Description: help.patch

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