glep-0074: Specify compressed file formats

Signed-off-by: Michał Górny <mgorny@gentoo.org>

1 files changed, 71 insertions, 10 deletions

diff --git a/glep-0074.rst b/glep-0074.rst
index 3d7bbbd..7f53302 100644
--- a/glep-0074.rst
+++ b/glep-0074.rst
@@ -27,7 +27,8 @@ Changes
 =======
 
 v1.3
-  Formally specified the current set of hash algorithms supported.
+  Formally specified the current set of hash algorithms and compressed
+  Manifest formats supported.
 
 v1.2
   Specified the newline convention used for Manifests.
@@ -432,9 +433,8 @@ compression and this specification.
 
 The compressed Manifest files are required to be suffixed for their
 compression algorithm. This suffix should be used to recognize
-the compression and decompress Manifests transparently. The exact list
-of algorithms and their corresponding suffixes are outside the scope
-of this specification.
+the compression and decompress Manifests transparently. The supported
+formats are specified in `compressed file formats`_ section.
 
 The top-level Manifest file must not be compressed. Since the OpenPGP
 signature covers the uncompressed text and is compressed itself,
@@ -455,6 +455,46 @@ uncompressed content and the specification is free to choose either
 of the files using the same base name.
 
 
+Compressed file formats
+-----------------------
+
+.. table:: Table 2. Defined compressed file formats
+   :widths: auto
+
+   ===========  ======  ====================  ===========
+   Tool name    Suffix  Specification         Notes
+   ===========  ======  ====================  ===========
+   bzip2        .bz2    (none known)
+   gzip         .gz     RFC 1952 [#RFC1952]_  Recommended
+   lz4          .lz4    (none known)
+   lzip         .lz     RFC draft [#LZIP]_
+   lzma         .lzma   (none known)          Deprecated
+   lzop         .lzo    (none known)
+   xz           .xz     xz [#XZ]_
+   zstd         .zst    RFC 8878 [#RFC8878]_
+   ===========  ======  ====================  ===========
+
+Any new formats must be added to this specification prior to being used
+for Manifest files. Adding a new compressed file format is considered
+a backwards-compatible change to the GLEP. It is recommended that new
+formats use their reference (most common) file suffixes.
+
+An implementation can implement an arbitrary subset of the listed
+formats. For best interoperability, it should implement at least
+the recommended formats. Using deprecated formats should be avoided.
+
+If multiple Manifest variants coexist using different compressed file
+formats, the implementation may choose to use an arbitrary subset
+of them. However, all of them must be verified against the hashes stored
+in the containing Manifest. Should they be decompressed, the resulting
+contents must be identical.
+
+If the compressed file format is unsupported and a variant using
+a supported format coexists, the other variant should be used. However,
+at least one supported variant must exist for the verification
+to succeed.
+
+
 Combining multiple Manifest trees (informational)
 -------------------------------------------------
 
@@ -1033,12 +1073,19 @@ into a compressed sub-Manifest in the top directory (e.g.
 ``Manifest.sub.gz``), and including a ``MANIFEST`` entry for this file
 in a signed, uncompressed top-level Manifest.
 
-The existence of additional entries for uncompressed Manifest checksums
-was debated. However, plain entries for the uncompressed file would
-be confusing if only the compressed file existed, and conflicting
-if both uncompressed and compressed variants existed. Furthermore,
-it has been pointed out that ``DIST`` entries do not have
-an uncompressed variant either.
+The existence of additional entries for checksums of Manifest contents
+after uncompressing was debated. However, plain entries for
+the uncompressed file would be confusing if only the compressed file
+existed. Furthermore, it has been pointed out that ``DIST`` entries
+do not have an uncompressed variant either.
+
+The specification permits coexistence of multiple variants of the same
+Manifest file using different compression for historical compatibility.
+However, there does not seem to be any real benefit from including
+a compressed Manifest file if the uncompressed variant needs to exist
+anyway. Providing different compressed variants could technically
+improve interoperability, though the same result could probably
+be achieved by using a more commonly supported format (e.g. gzip).
 
 
 Performance considerations
@@ -1171,6 +1218,20 @@ References
    (archived at 2017-11-29)
    (https://web.archive.org/web/20171129084214/http://www.larc.usp.br/~pbarreto/WhirlpoolPage.html)
 
+.. [#RFC1952] RFC 1952: GZIP file format specification version 4.3
+   (https://www.rfc-editor.org/rfc/rfc1952)
+
+.. [#LZIP] RFC draft: Lzip Compressed Format and the 'application/lzip'
+   Media Type
+   (https://datatracker.ietf.org/doc/html/draft-diaz-lzip)
+
+.. [#XZ] The .xz File Format
+   (https://tukaani.org/xz/xz-file-format.txt)
+
+.. [#RFC8878] RFC 8878: Zstandard Compression and the 'application/zstd'
+   Media Type
+   (https://www.rfc-editor.org/rfc/rfc8878)
+
 .. [#C08] Cappos, J et al. (2008). "Attacks on Package Managers"
    (https://www2.cs.arizona.edu/stork/packagemanagersecurity/attacks-on-package-managers.html)