Extremely basic first crack at implementing djb's redo.

And a test program.

redo.py

@@ -0,0 +1,52 @@
+#!/usr/bin/python
+import sys, os, subprocess
+import options
+from helpers import *
+
+optspec = """
+redo [targets...]
+--
+ifchange   something something
+"""
+o = options.Options('redo', optspec)
+(opt, flags, extra) = o.parse(sys.argv[1:])
+
+targets = extra or ['it']
+
+
+def find_do_file(t):
+    p = '%s.do' % t
+    if os.path.exists(p):
+        return p
+    else:
+        return None
+    
+
+def build(t):
+    dofile = find_do_file(t)
+    if not dofile:
+        if os.path.exists(t):  # an existing source file
+            return  # success
+        else:
+            raise Exception('no rule to make %r' % t)
+    unlink(t)
+    os.putenv('REDO_TARGET', t)
+    tmpname = '%s.redo.tmp' % t
+    unlink(tmpname)
+    f = open(tmpname, 'w+')
+    log('running: %r\n' % dofile)
+    rv = subprocess.call(['sh', '-e', dofile, t , 'FIXME', tmpname],
+                         stdout=f.fileno())
+    st = os.stat(tmpname)
+    log('rv: %d (%d bytes) (%r)\n' % (rv, st.st_size, dofile))
+    if rv==0 and st.st_size:
+        os.rename(tmpname, t)
+        log('made %r\n' % t)
+    else:
+        unlink(tmpname)
+    f.close()
+    if rv != 0:
+        raise Exception('non-zero return code building %r' % t)
+
+for t in targets:
+    build(t)