From addcde4f358061053b013374bcf7400ef28acd4f Mon Sep 17 00:00:00 2001
From: Gregory Becker <becker33@llnl.gov>
Date: Fri, 6 May 2016 12:05:27 -0700
Subject: [PATCH] Made spec hashes immutable once concrete

---
 lib/spack/spack/spec.py | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 8ff8281d5e5..b71c1d680c0 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -486,6 +486,7 @@ def __init__(self, spec_like, *dep_like, **kwargs):
         self.variants = other.variants
         self.variants.spec = self
         self.namespace = other.namespace
+        self.hash = other.hash
 
         # Specs are by default not assumed to be normal, but in some
         # cases we've read them from a file want to assume normal.
@@ -754,12 +755,16 @@ def dag_hash(self, length=None):
         """
         Return a hash of the entire spec DAG, including connectivity.
         """
-        yaml_text = yaml.dump(
-            self.to_node_dict(), default_flow_style=True, width=sys.maxint)
-#        print yaml_text
-        sha = hashlib.sha1(yaml_text)
-        return base64.b32encode(sha.digest()).lower()[:length]
-
+        if self.hash:
+            return self.hash
+        else:
+            yaml_text = yaml.dump(
+                self.to_node_dict(), default_flow_style=True, width=sys.maxint)
+            sha = hashlib.sha1(yaml_text)
+            b32_hash = base64.b32encode(sha.digest()).lower()[:length]
+            if self._concrete:
+                self.hash = b32_hash
+            return b32_hash
 
     def to_node_dict(self):
         params = dict( (name, v.value) for name, v in self.variants.items() )
@@ -2128,6 +2133,7 @@ def spec(self, name, check_valid_token = False):
         spec.dependents   = DependencyMap()
         spec.dependencies = DependencyMap()
         spec.namespace = spec_namespace
+        spec.hash = None
 
         spec._normal = False
         spec._concrete = False