organize feature deletes destination folder, is surprising

[Thomi Richards]

I'm snapping an app that installs python packages to /usr/local/lib/python2.7/dist-packages/, and I want them to be in /usr/lib/... inside the snap.

I think I ought to be able to do this with the following in my snapcraft.yaml:

```
  wxpython:
    plugin: python
    source: whatever
    organize:
      usr/local/lib/: usr/lib/
```

However the following code from snapcraft (pluginhandler.py:326):

```
    def _organize(self):
        organize_fileset = getattr(self.code.options, 'organize', {}) or {}

        for key in organize_fileset:
            src = os.path.join(self.code.installdir, key)
            dst = os.path.join(self.code.installdir, organize_fileset[key])

            os.makedirs(os.path.dirname(dst), exist_ok=True)

            if os.path.exists(dst):
                logger.warning(
                    'Stepping over existing file for organization %r',
                    os.path.relpath(dst, self.code.installdir))
                if os.path.isdir(dst):
                    shutil.rmtree(dst)
                else:
                    os.remove(dst)
            shutil.move(src, dst)
```

...will *delete* the destination (usr/lib) before copying the source files in. This is surprising, to say the least, and I can't think of a situation where this is the intended behavior. I suggest the following logic instead:

 * If source is a directory, walk through all files recursively in src.
  * attempt to copy file to dst, reserving the part of the path after source. If it already exists, error and exit.
 * If source and dest are file paths, attempt to copy src to dest. If the destination file already exists, error and exit.

I don't think the 'organize' feature should ever delete files in the destination. The canonical example of this feature being used seems to be https://github.com/ubuntu/snappy-playpen/blob/master/ffmpeg/snapcraft.yaml which copies a file into 'bin/'. If this snap ever contains a second part with an executable in bin/ then they may get deleted, if they're built before the part with the organize stanza.

[Sergio Schvezov (sergiusens)]

https://github.com/snapcore/snapcraft/pull/780