Documentation: mark `--object-format=sha256` as experimental

After eff45daab8 ("repository: enable SHA-256 support by default",
2020-07-29), vanilla builds of Git enable the user to run, e.g.,

  git init --object-format=sha256

and hack away. This can be a good way to gain experience with the
SHA-256 world, e.g., to find bugs that

  GIT_TEST_DEFAULT_HASH=sha256 make test

doesn't spot.

But it really is a separate world: Such SHA-256 repos will live entirely
separate from the (by now fairly large) set of SHA-1 repos. Interacting
across the border is possible in principle, e.g., through "diff + apply"
(or "format-patch + am"), but even that has its limitations: Applying a
SHA-256 diff in a SHA-1 repo works in the simple case, but if you need
to resort to `-3`, you're out of luck.

Similarly, "push + pull" should work, but you really will be operating
mostly offset from the rest of the world. That might be ok by the time
you initialize your repository, and it might be ok for several months
after that, but there might come a day when you're starting to regret
your use of `git init --object-format=sha256` and have dug yourself into
a fairly deep hole.

There are currently topics in flight to document our data formats and
protocols regarding SHA-256 and in some cases (midx and commit-graph),
we're considering adjusting how the file formats indicate which object
format to use.

Wherever `--object-format` is mentioned in our documentation, let's make
it clear that using it with "sha256" is experimental. If we later need
to explain why we can't handle data we generated back in 2020, we can
always point to this paragraph we're adding here.

By "include::"-ing a small blurb, we should be able to be consistent
throughout the documentation and can eventually gradually tone down the
severity of this text. One day, we might even use it to start phasing
out `--object-format=sha1`, but let's not get ahead of ourselves...

There's also `extensions.objectFormat`, but it's only mentioned three
times. Twice where we're adding this new disclaimer and in the third
spot we already have a "do not edit" warning. From there, interested
readers should eventually find this new one that we're adding here.

Because `GIT_DEFAULT_HASH` provides another entry point to this
functionality, document the experimental nature of it too.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/git-index-pack.txt b/Documentation/git-index-pack.txt
index ac74d058e01d13830b276616dce25d5745b64699..af0c26232c1e775cf5f4d56ee67c07b6a73a9104 100644 (file)
--- a/Documentation/git-index-pack.txt
+++ b/Documentation/git-index-pack.txt
@@ -100,6 +100,8 @@ OPTIONS
        value is set or outside a repository.
 +
 This option cannot be used with --stdin.
++
+include::object-format-disclaimer.txt[]
 
 NOTES
 -----

diff --git a/Documentation/git-init.txt b/Documentation/git-init.txt
index ddfe265da5b6524f30d972bfea990bb2a89b5611..f35f70f13d0385e24ba2c0727e3720b4a068a048 100644 (file)
--- a/Documentation/git-init.txt
+++ b/Documentation/git-init.txt
@@ -53,6 +53,8 @@ current working directory.
 
 Specify the given object format (hash algorithm) for the repository.  The valid
 values are 'sha1' and (if enabled) 'sha256'.  'sha1' is the default.
++
+include::object-format-disclaimer.txt[]
 
 --template=<template_directory>::
 

diff --git a/Documentation/git-show-index.txt b/Documentation/git-show-index.txt
index 39b1d8eaa1456f60af07de2e30f0653f9d5efd64..e49318a5a0aec7f46c6d23dbff5afcb920fae236 100644 (file)
--- a/Documentation/git-show-index.txt
+++ b/Documentation/git-show-index.txt
@@ -44,6 +44,8 @@ OPTIONS
        valid values are 'sha1' and (if enabled) 'sha256'.  The default is the
        algorithm for the current repository (set by `extensions.objectFormat`), or
        'sha1' if no value is set or outside a repository..
++
+include::object-format-disclaimer.txt[]
 
 GIT
 ---

diff --git a/Documentation/git.txt b/Documentation/git.txt
index 3e50065198891b8f889d7905d8e0573ca791f02f..e0a2690ba20b7910ac7de0159463a5c268757627 100644 (file)
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -497,7 +497,8 @@ double-quotes and respecting backslash escapes. E.g., the value
        If this variable is set, the default hash algorithm for new
        repositories will be set to this value. This value is currently
        ignored when cloning; the setting of the remote repository
-       is used instead. The default is "sha1".
+       is used instead. The default is "sha1". THIS VARIABLE IS
+       EXPERIMENTAL! See `--object-format` in linkgit:git-init[1].
 
 Git Commits
 ~~~~~~~~~~~

diff --git a/Documentation/object-format-disclaimer.txt b/Documentation/object-format-disclaimer.txt
new file mode 100644 (file)
index 0000000..4cb106f
--- /dev/null
+++ b/Documentation/object-format-disclaimer.txt
@@ -0,0 +1,6 @@
+THIS OPTION IS EXPERIMENTAL! SHA-256 support is experimental and still
+in an early stage.  A SHA-256 repository will in general not be able to
+share work with "regular" SHA-1 repositories.  It should be assumed
+that, e.g., Git internal file formats in relation to SHA-256
+repositories may change in backwards-incompatible ways.  Only use
+`--object-format=sha256` for testing purposes.