Added an option to install only from a download cache.

src/zc/buildout/easy_install.py

@@ -104,11 +104,11 @@ _easy_install_cmd = _safe_arg(
     'from setuptools.command.easy_install import main; main()'
     )
 
-
 class Installer:
 
     _versions = {}
     _download_cache = None
+    _install_from_cache = False
 
     def __init__(self,
                  dest=None,
@@ -121,6 +121,14 @@ class Installer:
                  versions=None,
                  ):
         self._dest = dest
+
+        if self._install_from_cache:
+            if not self._download_cache:
+                raise ValueError("install_from_cache set to true with no"
+                                 " download cache")
+            links = ()
+            index = 'file://' + self._download_cache
+        
         self._links = links = list(links)
         if self._download_cache and (self._download_cache not in links):
             links.insert(0, self._download_cache)
@@ -453,7 +461,9 @@ class Installer:
 
         # XXX Need test for this
         for dist in dists:
-            if dist.has_metadata('dependency_links.txt'):
+            if (dist.has_metadata('dependency_links.txt')
+                and not self._install_from_cache
+                ):
                 for link in dist.get_metadata_lines('dependency_links.txt'):
                     link = link.strip()
                     if link not in self._links:
@@ -625,7 +635,6 @@ class Installer:
             if tmp != self._download_cache:
                 shutil.rmtree(tmp)
 
-
 def default_versions(versions=None):
     old = Installer._versions
     if versions is not None:
@@ -635,9 +644,17 @@ def default_versions(versions=None):
 def download_cache(path=-1):
     old = Installer._download_cache
     if path != -1:
+        if path:
+            path = os.path.abspath(path)
         Installer._download_cache = path
     return old
 
+def install_from_cache(setting=None):
+    old = Installer._install_from_cache
+    if setting is not None:
+        Installer._install_from_cache = bool(setting)
+    return old
+
 def install(specs, dest,
             links=(), index=None,
             executable=sys.executable, always_unzip=False,

src/zc/buildout/easy_install.txt

@@ -919,7 +919,62 @@ Now when we install the distributions:
 
 Note that we didn't download the distributions from the link server.
 
+If we remove the restriction on demo, we'll download a newer version
+from the link server:
+
+    >>> ws = zc.buildout.easy_install.install(
+    ...     ['demo'], dest,
+    ...     links=[link_server], index=link_server+'index/', 
+    ...     always_unzip=True)
+    GET 200 /demo-0.3-py2.4.egg
+
+Normally, the download cache is the prefered source of downloads, but
+not the only one.
+
+Installing solely from a download cache
+
+A download cache can be used as the basis of application sources
+releases.  In an application source release, we want to distribute an
+application that can be built without making any network accesses.  In
+this case, we distribute a download cache and tell the easy_install
+module to install from the download cache only, without making network
+accesses.  The install_from_cache function can be used to signal that
+packages should be installed only from the download cache.  The
+function always returns the previous setting.  Calling it with no
+arguments returns the current setting without changing it:
+
+    >>> zc.buildout.easy_install.install_from_cache()
+    False
+
+Calling it with a boolean value changes the setting and returns the
+previous setting:
+
+    >>> zc.buildout.easy_install.install_from_cache(True)
+    False
+
+Let's remove demo-0.3-py2.4.egg from the cache, clear the index cache,
+recreate the destination directory, and reinstall demo:
+
+    >>> remove(cache, 'demo-0.3-py2.4.egg')
+    >>> zc.buildout.easy_install.clear_index_cache()
+    >>> remove(dest)
+    >>> dest = tmpdir('sample-install')
+
+    >>> ws = zc.buildout.easy_install.install(
+    ...     ['demo'], dest,
+    ...     links=[link_server], index=link_server+'index/', 
+    ...     always_unzip=True)
+
+    >>> ls(dest)
+    d  demo-0.2-py2.4.egg
+    d  demoneeded-1.1-py2.4.egg
+
+This time, we didn't download from or even query the link server.
+
 .. Disable the download cache:
 
     >>> zc.buildout.easy_install.download_cache(None)
     '/cache'
+
+    >>> zc.buildout.easy_install.install_from_cache(False)
+    True