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philips merged 1 commit into from
Sep 27, 2016
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layer: add hardlink handling information #336

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40 changes: 39 additions & 1 deletion layer.md
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Expand Up @@ -44,10 +44,46 @@ Where supported, MUST include file attributes for Additions and Modifications in
* Group Name (`gname`) *secondary to `gid`*
* Mode (`mode`)
* Extended Attributes (`xattrs`)
* Symlink reference (`linkname`)
* Symlink reference (`linkname` + symbolic link type)
* [Hardlink](#hardlinks) reference (`linkname`)

[Sparse files](https://en.wikipedia.org/wiki/Sparse_file) SHOULD NOT be used because they lack consistent support across tar implementations.

#### Hardlinks

Hardlinks are a [POSIX concept](http://pubs.opengroup.org/onlinepubs/9699919799/functions/link.html) for having one or more directory entries for the same file on the same device.
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@vbatts vbatts Sep 22, 2016

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@stevvooe i do say part of that here. So, make it more explicit?

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@stevvooe stevvooe Sep 22, 2016

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@vbatts I still think that condition needs to be added to the clause above. The issue is that two files with the same inode but different devices ids are two separate hardlinks.

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@wking wking Sep 22, 2016

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On Thu, Sep 22, 2016 at 02:14:30PM -0700, Stephen Day wrote:

@vbatts I still think that condition needs to be added to the clause
above. The issue is that two files with the same inode but different
devices ids are two separate hardlinks.

He covers that in Markdown in the next paragraph. Are you just asking for “file” → “inode” here?

Not all filesystems support hardlinks (e.g. [FAT](https://en.wikipedia.org/wiki/File_Allocation_Table)).

Hardlinks are possible with all [file types](#file-types) except `directories`.
Non-directory files are considered "hardlinked" when their link count is greater than 1.
Hardlinked files are on a same device (i.e. comparing Major:Minor pair) and have the same inode.
The corresponding files that share the link with the > 1 linkcount may be outside the directory that the changeset is being produced from, in which case the `linkname` is not recorded in the changeset.

Hardlinks are stored in a tar archive with type of a `1` char, per the [GNU Basic Tar Format][gnu-tar-standard] and [libarchive tar(5)][libarchive-tar].

While approaches to deriving new or changed hardlinks may vary, a possible approach is:

```
SET LinkMap to map[< Major:Minor String >]map[< inode integer >]< path string >
SET LinkNames to map[< src path string >]< dest path string >
FOR each path in root path
IF path type is directory
CONTINUE
ENDIF
SET filestat to stat(path)
IF filestat num of links == 1
CONTINUE
ENDIF
IF LinkMap[filestat device][filestat inode] is not empty
SET LinkNames[path] to LinkMap[filestat device][filestat inode]
ELSE
SET LinkMap[filestat device][filestat inode] to path
ENDIF
END FOR
```

With this approach, the link map and links names of a directory could be compared against that of another directory to derive additions and changes to hardlinks.

## Creating

### Initial Root Filesystem
Expand Down Expand Up @@ -279,4 +315,6 @@ Layers that have these restrictions SHOULD be tagged with an alternative mediaty
[Descriptors](descriptor.md) referencing these layers MAY include `urls` for downloading these layers.
It is implementation-defined whether or not implementations upload layers tagged with this media type.

[libarchive-tar]: https://github.com/libarchive/libarchive/wiki/ManPageTar5#POSIX_ustar_Archives
[gnu-tar-standard]: http://www.gnu.org/software/tar/manual/html_node/Standard.html
[tar-archive]: https://en.wikipedia.org/wiki/Tar_(computing)