Problem: bad error if extends-cache with ${section:variable}

Solution: add explicit error message

news/585.feature

+Proper error message if extends-cache tries to expand ${section:variable}

src/zc/buildout/buildout.py

@@ -1786,6 +1786,7 @@ def _default_globals():
 
     return globals_defs
 
+variable_template_split = re.compile('([$]{[^}]*})').split
 
 def _open(
         base, filename, seen, download_options,
@@ -1799,8 +1800,14 @@ def _open(
     raw_download_options = _unannotate_section(download_options)
     newest = bool_option(raw_download_options, 'newest', 'false')
     fallback = newest and not (filename in downloaded)
+    extends_cache = raw_download_options.get('extends-cache')
+    if extends_cache and variable_template_split(extends_cache)[1::2]:
+        raise ValueError(
+            "extends-cache '%s' may not contain ${section:variable} to expand."
+            % extends_cache
+        )
     download = zc.buildout.download.Download(
-        raw_download_options, cache=raw_download_options.get('extends-cache'),
+        raw_download_options, cache=extends_cache,
         fallback=fallback, hash_name=True)
     is_temp = False
     downloaded_filename = None

src/zc/buildout/tests/extends-cache.txt

@@ -550,6 +550,28 @@ file: http://localhost/faulty.cfg (downloaded as ...), line: 1
 'This is definitively not\n'
 
 
+Failing if extends-cache contains ${section:variable}
+-----------------------------------------------------
+
+Because extends-cache is used to download extends, it may not contain ${section:variable}
+as the state of all variables cannot be computed before all extends have been loaded.
+
+>>> write(server_data, 'proper.cfg', """\
+... [buildout]
+... dummy = fjhfj
+... """)
+
+>>> write('buildout.cfg', """\
+... [buildout]
+... extends = %sproper.cfg
+... extends-cache = ${buildout:dummy}
+... """ % server_url)
+>>> print_(system(buildout))
+While:
+  Initializing.
+... ValueError: extends-cache '${buildout:dummy}' may not contain ${section:variable} to expand.
+
+
 Clean up
 --------