fixup! zc.recipe.egg: Improve on the fly pathces.

Only build egg at this point if we have some patches to apply otherwise
buildout would install dependent egg without logging this operation, which
would cause many tests to fail.

Also cover the whole scenario of egg patching in a new doctest.

src/zc/buildout/easy_install.py

@@ -851,6 +851,7 @@ class Installer:
                 else:
                     logger.debug('Adding required %r', str(req))
                 self._log_requirement(ws, req)
+                if patch_dict and req.project_name in patch_dict:
                     self._env.scan(
                         self.build(str(req), {}, patch_dict=patch_dict))
                 for dist in self._get_dist(req, ws,

zc.recipe.egg_/src/zc/recipe/egg/patches.rst

+
+Patching eggs before installation
+---------------------------------
+
+The SlapOS extensions of ``zc.recipe.egg`` supports applying patches before installing eggs.
+
+The syntax is to use a version with the magic string ``SlapOSPatched`` plus the number of
+patches to apply.
+
+
+Let's use a patch for demoneeded egg:
+
+    >>> write(sample_buildout, 'demoneeded.patch',
+    ... r"""diff -ru before/demoneeded-1.1/eggrecipedemoneeded.py after/demoneeded-1.1/eggrecipedemoneeded.py
+    ... --- before/demoneeded-1.1/eggrecipedemoneeded.py 2020-09-08 09:27:36.000000000 +0200
+    ... +++ after/demoneeded-1.1/eggrecipedemoneeded.py 2020-09-08 09:46:16.482243822 +0200
+    ... @@ -1,3 +1,3 @@
+    ... -y=1
+    ... +y="patched demoneeded"
+    ...  def f():
+    ...    pass
+    ... \ No newline at end of file
+    ... """)
+
+First, we install demoneeded directly:
+
+    >>> write(sample_buildout, 'buildout.cfg',
+    ... """
+    ... [buildout]
+    ... parts = demoneeded
+    ...
+    ... [demoneeded]
+    ... recipe = zc.recipe.egg:eggs
+    ... eggs = demoneeded
+    ... find-links = %(server)s
+    ... index = %(server)s/index
+    ... demoneeded-patches =
+    ...   ./demoneeded.patch#4b8ad56711dd0d898a2b7957e9604079
+    ... demoneeded-patch-options = -p2
+    ...
+    ... [versions]
+    ... demoneeded = 1.1+SlapOSPatched001
+    ... """ % dict(server=link_server))
+
+When running buildout, we have a warning that a different version is installed, but that's not fatal.
+
+    >>> print_(system(buildout), end='')
+    Installing demoneeded.
+    patching file eggrecipedemoneeded.py
+    Installing demoneeded 1.1
+    Caused installation of a distribution:
+    demoneeded 1.1+slapospatched001
+    with a different version.
+
+The installed egg has the slapospatched001 marker
+
+    >>> ls(sample_buildout, 'eggs')
+    d  demoneeded-1.1+slapospatched001-pyN.N.egg
+    -  setuptools-0.7-py2.3.egg
+    d  zc.buildout-1.0-py2.3.egg
+
+The code of the egg has been patched:
+
+    >>> import glob
+    >>> import os.path
+    >>> cat(glob.glob(os.path.join(sample_buildout, 'eggs', 'demoneeded-1.1+slapospatched001*', '*'))[0])
+    y="patched demoneeded"
+    def f():
+      pass
+
+Reset the state and also remove the installed egg
+
+    >>> remove('.installed.cfg')
+    >>> rmdir(glob.glob(os.path.join(sample_buildout, 'eggs', 'demoneeded-1.1+slapospatched001*'))[0])
+
+
+In the previous example we applied patches to an egg installed directly, but
+the same technique can be used to apply patches on eggs installed as dependencies.
+
+In this example we install demo and apply a patch to demoneeded, which is a dependency to demo. 
+
+    >>> write(sample_buildout, 'buildout.cfg',
+    ... """
+    ... [buildout]
+    ... parts = demo
+    ...
+    ... [demo]
+    ... recipe = zc.recipe.egg
+    ... eggs = demo
+    ... find-links = %(server)s
+    ... index = %(server)s/index
+    ... demoneeded-patches =
+    ...   ./demoneeded.patch#4b8ad56711dd0d898a2b7957e9604079
+    ... demoneeded-patch-options = -p2
+    ...
+    ... [versions]
+    ... demoneeded = 1.1+SlapOSPatched001
+    ... """ % dict(server=link_server))
+
+When running buildout, we also have that warning that a different version is installed.
+
+    >>> print_(system(buildout), end='')
+    Installing demo.
+    Getting distribution for 'demo'.
+    Got demo 0.3.
+    patching file eggrecipedemoneeded.py
+    Installing demoneeded 1.1
+    Caused installation of a distribution:
+    demoneeded 1.1+slapospatched001
+    with a different version.
+    Generated script '/sample-buildout/bin/demo'.
+
+The installed egg has the slapospatched001 marker
+
+    >>> ls(sample_buildout, 'eggs')
+    d  demo-0.3-pyN.N.egg
+    d  demoneeded-1.1+slapospatched001-pyN.N.egg
+    -  setuptools-0.7-py2.3.egg
+    d  zc.buildout-1.0-py2.3.egg
+
+If we run the demo script we see the patch was applied:
+
+    >>> print_(system(join(sample_buildout, 'bin', 'demo')), end='')
+    3 patched demoneeded

zc.recipe.egg_/src/zc/recipe/egg/tests.py

@@ -110,6 +110,29 @@ def test_suite():
                 ]),
             ),
         ))
+
+    suite = unittest.TestSuite((
+        doctest.DocFileSuite(
+            'patches.rst',
+            setUp=setUp, tearDown=zc.buildout.testing.buildoutTearDown,
+            optionflags=doctest.NORMALIZE_WHITESPACE | doctest.ELLIPSIS,
+            checker=renormalizing.RENormalizing([
+               zc.buildout.testing.normalize_path,
+               zc.buildout.testing.normalize_endings,
+               zc.buildout.testing.normalize_script,
+               zc.buildout.testing.normalize_egg_py,
+               zc.buildout.tests.normalize_bang,
+               zc.buildout.tests.normalize_S,
+               zc.buildout.testing.not_found,
+               zc.buildout.testing.easy_install_deprecated,
+               (re.compile(r'[d-]  zc.buildout(-\S+)?[.]egg(-link)?'),
+                'zc.buildout.egg'),
+               (re.compile(r'[d-]  setuptools-[^-]+-'), 'setuptools-X-'),
+               (re.compile(r'eggs\\\\demo'), 'eggs/demo'),
+               (re.compile(r'[a-zA-Z]:\\\\foo\\\\bar'), '/foo/bar'),
+               ])
+            ),
+    ))
     return suite
 
 if __name__ == '__main__':