We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
render fails if the theme has a code.css  
The `conf.py` says: 

```
# Color scheme to be used for code blocks. If your theme provides
# "assets/css/code.css" this is ignored.
```

I've [provided one](https://github.com/mgaitan/my-nikola-theme/commit/f0140d0d67771d7ee9f46df2c78548c0e757f65e) but then I can't render my site

```
(blog)tin@morochita:~/lab/blog$ nikola build
Scanning posts.....done!
ERROR: Two different tasks can't have a common target.'output/assets/css/code.css' is a target for copy_files:output/assets/css/code.css and copy_assets:output/assets/css/code.css.
(blog)tin@morochita:~/lab/blog$ 
```


--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `nikola/plugins/task/copy_assets.py`
Content:
```
1 # -*- coding: utf-8 -*-
2 
3 # Copyright © 2012-2014 Roberto Alsina and others.
4 
5 # Permission is hereby granted, free of charge, to any
6 # person obtaining a copy of this software and associated
7 # documentation files (the "Software"), to deal in the
8 # Software without restriction, including without limitation
9 # the rights to use, copy, modify, merge, publish,
10 # distribute, sublicense, and/or sell copies of the
11 # Software, and to permit persons to whom the Software is
12 # furnished to do so, subject to the following conditions:
13 #
14 # The above copyright notice and this permission notice
15 # shall be included in all copies or substantial portions of
16 # the Software.
17 #
18 # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
19 # KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
20 # WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
21 # PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS
22 # OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
23 # OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
24 # OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
25 # SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
26 
27 import codecs
28 import os
29 
30 from nikola.plugin_categories import Task
31 from nikola import utils
32 
33 
34 class CopyAssets(Task):
35     """Copy theme assets into output."""
36 
37     name = "copy_assets"
38 
39     def gen_tasks(self):
40         """Create tasks to copy the assets of the whole theme chain.
41 
42         If a file is present on two themes, use the version
43         from the "youngest" theme.
44         """
45 
46         kw = {
47             "themes": self.site.THEMES,
48             "output_folder": self.site.config['OUTPUT_FOLDER'],
49             "filters": self.site.config['FILTERS'],
50             "code_color_scheme": self.site.config['CODE_COLOR_SCHEME'],
51             "code.css_selectors": 'pre.code',
52             "code.css_close": "\ntable.codetable { width: 100%;} td.linenos {text-align: right; width: 4em;}\n",
53         }
54         has_code_css = False
55         tasks = {}
56         code_css_path = os.path.join(kw['output_folder'], 'assets', 'css', 'code.css')
57 
58         yield self.group_task()
59 
60         for theme_name in kw['themes']:
61             src = os.path.join(utils.get_theme_path(theme_name), 'assets')
62             dst = os.path.join(kw['output_folder'], 'assets')
63             for task in utils.copy_tree(src, dst):
64                 if task['name'] in tasks:
65                     continue
66                 has_code_css = task['targets'][0] == code_css_path
67                 tasks[task['name']] = task
68                 task['uptodate'] = [utils.config_changed(kw)]
69                 task['basename'] = self.name
70                 yield utils.apply_filters(task, kw['filters'])
71 
72         if not has_code_css:  # Generate it
73 
74             def create_code_css():
75                 from pygments.formatters import get_formatter_by_name
76                 formatter = get_formatter_by_name('html', style=kw["code_color_scheme"])
77                 utils.makedirs(os.path.dirname(code_css_path))
78                 with codecs.open(code_css_path, 'wb+', 'utf8') as outf:
79                     outf.write(formatter.get_style_defs(kw["code.css_selectors"]))
80                     outf.write(kw["code.css_close"])
81 
82             task = {
83                 'basename': self.name,
84                 'name': code_css_path,
85                 'targets': [code_css_path],
86                 'uptodate': [utils.config_changed(kw)],
87                 'actions': [(create_code_css, [])],
88                 'clean': True,
89             }
90             yield utils.apply_filters(task, kw['filters'])
91
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/nikola/plugins/task/copy_assets.py b/nikola/plugins/task/copy_assets.py
--- a/nikola/plugins/task/copy_assets.py
+++ b/nikola/plugins/task/copy_assets.py
@@ -45,15 +45,21 @@
 
         kw = {
             "themes": self.site.THEMES,
+            "files_folders": self.site.config['FILES_FOLDERS'],
             "output_folder": self.site.config['OUTPUT_FOLDER'],
             "filters": self.site.config['FILTERS'],
             "code_color_scheme": self.site.config['CODE_COLOR_SCHEME'],
             "code.css_selectors": 'pre.code',
+            "code.css_head": '/* code.css file generated by Nikola */\n',
             "code.css_close": "\ntable.codetable { width: 100%;} td.linenos {text-align: right; width: 4em;}\n",
         }
-        has_code_css = False
         tasks = {}
         code_css_path = os.path.join(kw['output_folder'], 'assets', 'css', 'code.css')
+        code_css_input = utils.get_asset_path('assets/css/code.css',
+                                              themes=kw['themes'],
+                                              files_folders=kw['files_folders'])
+
+        kw["code.css_input"] = code_css_input
 
         yield self.group_task()
 
@@ -63,27 +69,35 @@
             for task in utils.copy_tree(src, dst):
                 if task['name'] in tasks:
                     continue
-                has_code_css = task['targets'][0] == code_css_path
                 tasks[task['name']] = task
                 task['uptodate'] = [utils.config_changed(kw)]
                 task['basename'] = self.name
+                if code_css_input:
+                    task['file_dep'] = [code_css_input]
                 yield utils.apply_filters(task, kw['filters'])
 
-        if not has_code_css:  # Generate it
-
+        # Check whether or not there is a code.css file around.
+        if not code_css_input:
             def create_code_css():
                 from pygments.formatters import get_formatter_by_name
                 formatter = get_formatter_by_name('html', style=kw["code_color_scheme"])
                 utils.makedirs(os.path.dirname(code_css_path))
                 with codecs.open(code_css_path, 'wb+', 'utf8') as outf:
+                    outf.write(kw["code.css_head"])
                     outf.write(formatter.get_style_defs(kw["code.css_selectors"]))
                     outf.write(kw["code.css_close"])
 
+            if os.path.exists(code_css_path):
+                with codecs.open(code_css_path, 'r', 'utf-8') as fh:
+                    testcontents = fh.read(len(kw["code.css_head"])) == kw["code.css_head"]
+            else:
+                testcontents = False
+
             task = {
                 'basename': self.name,
                 'name': code_css_path,
                 'targets': [code_css_path],
-                'uptodate': [utils.config_changed(kw)],
+                'uptodate': [utils.config_changed(kw), testcontents],
                 'actions': [(create_code_css, [])],
                 'clean': True,
             }

{"golden_diff": "diff --git a/nikola/plugins/task/copy_assets.py b/nikola/plugins/task/copy_assets.py\n--- a/nikola/plugins/task/copy_assets.py\n+++ b/nikola/plugins/task/copy_assets.py\n@@ -45,15 +45,21 @@\n \n         kw = {\n             \"themes\": self.site.THEMES,\n+            \"files_folders\": self.site.config['FILES_FOLDERS'],\n             \"output_folder\": self.site.config['OUTPUT_FOLDER'],\n             \"filters\": self.site.config['FILTERS'],\n             \"code_color_scheme\": self.site.config['CODE_COLOR_SCHEME'],\n             \"code.css_selectors\": 'pre.code',\n+            \"code.css_head\": '/* code.css file generated by Nikola */\\n',\n             \"code.css_close\": \"\\ntable.codetable { width: 100%;} td.linenos {text-align: right; width: 4em;}\\n\",\n         }\n-        has_code_css = False\n         tasks = {}\n         code_css_path = os.path.join(kw['output_folder'], 'assets', 'css', 'code.css')\n+        code_css_input = utils.get_asset_path('assets/css/code.css',\n+                                              themes=kw['themes'],\n+                                              files_folders=kw['files_folders'])\n+\n+        kw[\"code.css_input\"] = code_css_input\n \n         yield self.group_task()\n \n@@ -63,27 +69,35 @@\n             for task in utils.copy_tree(src, dst):\n                 if task['name'] in tasks:\n                     continue\n-                has_code_css = task['targets'][0] == code_css_path\n                 tasks[task['name']] = task\n                 task['uptodate'] = [utils.config_changed(kw)]\n                 task['basename'] = self.name\n+                if code_css_input:\n+                    task['file_dep'] = [code_css_input]\n                 yield utils.apply_filters(task, kw['filters'])\n \n-        if not has_code_css:  # Generate it\n-\n+        # Check whether or not there is a code.css file around.\n+        if not code_css_input:\n             def create_code_css():\n                 from pygments.formatters import get_formatter_by_name\n                 formatter = get_formatter_by_name('html', style=kw[\"code_color_scheme\"])\n                 utils.makedirs(os.path.dirname(code_css_path))\n                 with codecs.open(code_css_path, 'wb+', 'utf8') as outf:\n+                    outf.write(kw[\"code.css_head\"])\n                     outf.write(formatter.get_style_defs(kw[\"code.css_selectors\"]))\n                     outf.write(kw[\"code.css_close\"])\n \n+            if os.path.exists(code_css_path):\n+                with codecs.open(code_css_path, 'r', 'utf-8') as fh:\n+                    testcontents = fh.read(len(kw[\"code.css_head\"])) == kw[\"code.css_head\"]\n+            else:\n+                testcontents = False\n+\n             task = {\n                 'basename': self.name,\n                 'name': code_css_path,\n                 'targets': [code_css_path],\n-                'uptodate': [utils.config_changed(kw)],\n+                'uptodate': [utils.config_changed(kw), testcontents],\n                 'actions': [(create_code_css, [])],\n                 'clean': True,\n             }\n", "issue": "render fails if the theme has a code.css  \nThe `conf.py` says: \n\n```\n# Color scheme to be used for code blocks. If your theme provides\n# \"assets/css/code.css\" this is ignored.\n```\n\nI've [provided one](https://github.com/mgaitan/my-nikola-theme/commit/f0140d0d67771d7ee9f46df2c78548c0e757f65e) but then I can't render my site\n\n```\n(blog)tin@morochita:~/lab/blog$ nikola build\nScanning posts.....done!\nERROR: Two different tasks can't have a common target.'output/assets/css/code.css' is a target for copy_files:output/assets/css/code.css and copy_assets:output/assets/css/code.css.\n(blog)tin@morochita:~/lab/blog$ \n```\n\n", "before_files": [{"content": "# -*- coding: utf-8 -*-\n\n# Copyright \u00a9 2012-2014 Roberto Alsina and others.\n\n# Permission is hereby granted, free of charge, to any\n# person obtaining a copy of this software and associated\n# documentation files (the \"Software\"), to deal in the\n# Software without restriction, including without limitation\n# the rights to use, copy, modify, merge, publish,\n# distribute, sublicense, and/or sell copies of the\n# Software, and to permit persons to whom the Software is\n# furnished to do so, subject to the following conditions:\n#\n# The above copyright notice and this permission notice\n# shall be included in all copies or substantial portions of\n# the Software.\n#\n# THE SOFTWARE IS PROVIDED \"AS IS\", WITHOUT WARRANTY OF ANY\n# KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE\n# WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR\n# PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS\n# OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR\n# OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR\n# OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE\n# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.\n\nimport codecs\nimport os\n\nfrom nikola.plugin_categories import Task\nfrom nikola import utils\n\n\nclass CopyAssets(Task):\n    \"\"\"Copy theme assets into output.\"\"\"\n\n    name = \"copy_assets\"\n\n    def gen_tasks(self):\n        \"\"\"Create tasks to copy the assets of the whole theme chain.\n\n        If a file is present on two themes, use the version\n        from the \"youngest\" theme.\n        \"\"\"\n\n        kw = {\n            \"themes\": self.site.THEMES,\n            \"output_folder\": self.site.config['OUTPUT_FOLDER'],\n            \"filters\": self.site.config['FILTERS'],\n            \"code_color_scheme\": self.site.config['CODE_COLOR_SCHEME'],\n            \"code.css_selectors\": 'pre.code',\n            \"code.css_close\": \"\\ntable.codetable { width: 100%;} td.linenos {text-align: right; width: 4em;}\\n\",\n        }\n        has_code_css = False\n        tasks = {}\n        code_css_path = os.path.join(kw['output_folder'], 'assets', 'css', 'code.css')\n\n        yield self.group_task()\n\n        for theme_name in kw['themes']:\n            src = os.path.join(utils.get_theme_path(theme_name), 'assets')\n            dst = os.path.join(kw['output_folder'], 'assets')\n            for task in utils.copy_tree(src, dst):\n                if task['name'] in tasks:\n                    continue\n                has_code_css = task['targets'][0] == code_css_path\n                tasks[task['name']] = task\n                task['uptodate'] = [utils.config_changed(kw)]\n                task['basename'] = self.name\n                yield utils.apply_filters(task, kw['filters'])\n\n        if not has_code_css:  # Generate it\n\n            def create_code_css():\n                from pygments.formatters import get_formatter_by_name\n                formatter = get_formatter_by_name('html', style=kw[\"code_color_scheme\"])\n                utils.makedirs(os.path.dirname(code_css_path))\n                with codecs.open(code_css_path, 'wb+', 'utf8') as outf:\n                    outf.write(formatter.get_style_defs(kw[\"code.css_selectors\"]))\n                    outf.write(kw[\"code.css_close\"])\n\n            task = {\n                'basename': self.name,\n                'name': code_css_path,\n                'targets': [code_css_path],\n                'uptodate': [utils.config_changed(kw)],\n                'actions': [(create_code_css, [])],\n                'clean': True,\n            }\n            yield utils.apply_filters(task, kw['filters'])\n", "path": "nikola/plugins/task/copy_assets.py"}], "after_files": [{"content": "# -*- coding: utf-8 -*-\n\n# Copyright \u00a9 2012-2014 Roberto Alsina and others.\n\n# Permission is hereby granted, free of charge, to any\n# person obtaining a copy of this software and associated\n# documentation files (the \"Software\"), to deal in the\n# Software without restriction, including without limitation\n# the rights to use, copy, modify, merge, publish,\n# distribute, sublicense, and/or sell copies of the\n# Software, and to permit persons to whom the Software is\n# furnished to do so, subject to the following conditions:\n#\n# The above copyright notice and this permission notice\n# shall be included in all copies or substantial portions of\n# the Software.\n#\n# THE SOFTWARE IS PROVIDED \"AS IS\", WITHOUT WARRANTY OF ANY\n# KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE\n# WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR\n# PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS\n# OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR\n# OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR\n# OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE\n# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.\n\nimport codecs\nimport os\n\nfrom nikola.plugin_categories import Task\nfrom nikola import utils\n\n\nclass CopyAssets(Task):\n    \"\"\"Copy theme assets into output.\"\"\"\n\n    name = \"copy_assets\"\n\n    def gen_tasks(self):\n        \"\"\"Create tasks to copy the assets of the whole theme chain.\n\n        If a file is present on two themes, use the version\n        from the \"youngest\" theme.\n        \"\"\"\n\n        kw = {\n            \"themes\": self.site.THEMES,\n            \"files_folders\": self.site.config['FILES_FOLDERS'],\n            \"output_folder\": self.site.config['OUTPUT_FOLDER'],\n            \"filters\": self.site.config['FILTERS'],\n            \"code_color_scheme\": self.site.config['CODE_COLOR_SCHEME'],\n            \"code.css_selectors\": 'pre.code',\n            \"code.css_head\": '/* code.css file generated by Nikola */\\n',\n            \"code.css_close\": \"\\ntable.codetable { width: 100%;} td.linenos {text-align: right; width: 4em;}\\n\",\n        }\n        tasks = {}\n        code_css_path = os.path.join(kw['output_folder'], 'assets', 'css', 'code.css')\n        code_css_input = utils.get_asset_path('assets/css/code.css',\n                                              themes=kw['themes'],\n                                              files_folders=kw['files_folders'])\n\n        kw[\"code.css_input\"] = code_css_input\n\n        yield self.group_task()\n\n        for theme_name in kw['themes']:\n            src = os.path.join(utils.get_theme_path(theme_name), 'assets')\n            dst = os.path.join(kw['output_folder'], 'assets')\n            for task in utils.copy_tree(src, dst):\n                if task['name'] in tasks:\n                    continue\n                tasks[task['name']] = task\n                task['uptodate'] = [utils.config_changed(kw)]\n                task['basename'] = self.name\n                if code_css_input:\n                    task['file_dep'] = [code_css_input]\n                yield utils.apply_filters(task, kw['filters'])\n\n        # Check whether or not there is a code.css file around.\n        if not code_css_input:\n            def create_code_css():\n                from pygments.formatters import get_formatter_by_name\n                formatter = get_formatter_by_name('html', style=kw[\"code_color_scheme\"])\n                utils.makedirs(os.path.dirname(code_css_path))\n                with codecs.open(code_css_path, 'wb+', 'utf8') as outf:\n                    outf.write(kw[\"code.css_head\"])\n                    outf.write(formatter.get_style_defs(kw[\"code.css_selectors\"]))\n                    outf.write(kw[\"code.css_close\"])\n\n            if os.path.exists(code_css_path):\n                with codecs.open(code_css_path, 'r', 'utf-8') as fh:\n                    testcontents = fh.read(len(kw[\"code.css_head\"])) == kw[\"code.css_head\"]\n            else:\n                testcontents = False\n\n            task = {\n                'basename': self.name,\n                'name': code_css_path,\n                'targets': [code_css_path],\n                'uptodate': [utils.config_changed(kw), testcontents],\n                'actions': [(create_code_css, [])],\n                'clean': True,\n            }\n            yield utils.apply_filters(task, kw['filters'])\n", "path": "nikola/plugins/task/copy_assets.py"}]}