fetch_strategy: add a source_id to fetches

This will be included in the full hash of packages.

lib/spack/spack/fetch_strategy.py

@@ -141,6 +141,14 @@ def cachable(self):
             bool: True if can cache, False otherwise.
         """
 
+    def source_id(self):
+        """A unique ID for the source.
+
+        The returned value is added to the content which determines the full
+        hash for a package using `str()`.
+        """
+        raise NotImplementedError
+
     def __str__(self):  # Should be human readable URL.
         return "FetchStrategy.__str___"
 
@@ -160,6 +168,11 @@ class FetchStrategyComposite(object):
     matches = FetchStrategy.matches
     set_stage = FetchStrategy.set_stage
 
+    def source_id(self):
+        component_ids = tuple(i.source_id() for i in self)
+        if all(component_ids):
+            return component_ids
+
 
 class URLFetchStrategy(FetchStrategy):
     """FetchStrategy that pulls source code from a URL for an archive,
@@ -197,6 +210,9 @@ def curl(self):
             self._curl = which('curl', required=True)
         return self._curl
 
+    def source_id(self):
+        return self.digest
+
     @_needs_stage
     def fetch(self):
         if self.archive_file:
@@ -602,6 +618,16 @@ def git(self):
     def cachable(self):
         return bool(self.commit or self.tag)
 
+    def source_id(self):
+        return self.commit or self.tag
+
+    def get_source_id(self):
+        if not self.branch:
+            return
+        output = self.git('ls-remote', self.url, self.branch, output=str)
+        if output:
+            return output.split()[0]
+
     def fetch(self):
         if self.stage.source_path:
             tty.msg("Already fetched %s" % self.stage.source_path)
@@ -744,6 +770,18 @@ def svn(self):
     def cachable(self):
         return bool(self.revision)
 
+    def source_id(self):
+        return self.revision
+
+    def get_source_id(self):
+        output = self.svn('info', self.url, output=str)
+        if not output:
+            return None
+        lines = output.split('\n')
+        for line in lines:
+            if line.startswith('Revision:'):
+                return line.split()[-1]
+
     @_needs_stage
     def fetch(self):
         if self.stage.source_path:
@@ -839,6 +877,14 @@ def hg(self):
     def cachable(self):
         return bool(self.revision)
 
+    def source_id(self):
+        return self.revision
+
+    def get_source_id(self):
+        output = self.hg('id', self.url, output=str)
+        if output:
+            return output.strip()
+
     @_needs_stage
     def fetch(self):
         if self.stage.source_path: