ENH: Lightweight node cache checking

Generating the hashvalue when outputs are not ready at cache check stage
when the node's directory does not exist (or no results file is in
there) leads to #3014.

This PR preempts those problems by delaying the hashval calculation.

Author: oesteban

nipype/pipeline/engine/nodes.py

@@ -293,27 +293,29 @@ def is_cached(self, rm_outdated=False):
         """
         outdir = self.output_dir()
 
-        # Update hash
-        hashed_inputs, hashvalue = self._get_hashval()
-
         # The output folder does not exist: not cached
-        if not op.exists(outdir):
-            logger.debug('[Node] Directory not found "%s".', outdir)
+        if not op.exists(outdir) or \
+                not op.exists(op.join(outdir, 'result_%s.pklz' % self.name)):
+            logger.debug('[Node] Not cached "%s".', outdir)
             return False, False
 
-        hashfile = op.join(outdir, '_0x%s.json' % hashvalue)
-        cached = op.exists(hashfile)
-
-        # Check if updated
+        # Check if there are hashfiles
         globhashes = glob(op.join(outdir, '_0x*.json'))
         unfinished = [
             path for path in globhashes
             if path.endswith('_unfinished.json')
         ]
         hashfiles = list(set(globhashes) - set(unfinished))
+
+        # Update hash
+        hashed_inputs, hashvalue = self._get_hashval()
+
+        hashfile = op.join(outdir, '_0x%s.json' % hashvalue)
         logger.debug('[Node] Hashes: %s, %s, %s, %s',
                      hashed_inputs, hashvalue, hashfile, hashfiles)
 
+        cached = op.exists(hashfile)
+
         # No previous hashfiles found, we're all set.
         if cached and len(hashfiles) == 1:
             assert(hashfile == hashfiles[0])
@@ -441,6 +443,7 @@ def run(self, updatehash=False):
             for outdatedhash in glob(op.join(self.output_dir(), '_0x*.json')):
                 os.remove(outdatedhash)
 
+        self._get_hashval()
         # Hashfile while running
         hashfile_unfinished = op.join(
             outdir, '_0x%s_unfinished.json' % self._hashvalue)