log: add default decoration filter

When a user runs 'git log', they expect a certain set of helpful
decorations. This includes:

* The HEAD ref
* Branches (refs/heads/)
* Stashes (refs/stash)
* Tags (refs/tags/)
* Remote branches (refs/remotes/)
* Replace refs (refs/replace/ or $GIT_REPLACE_REF_BASE)

Each of these namespaces was selected due to existing test cases that
verify these namespaces appear in the decorations. In particular,
stashes and replace refs can have custom colors from the
color.decorate.<slot> config option.

While one test checks for a decoration from notes, it only applies to
the tip of refs/notes/commit (or its configured ref name). Notes form
their own kind of decoration instead. Modify the expected output for the
tests in t4013 that expect this note decoration.  There are several
tests throughout the codebase that verify that --decorate-refs,
--decorate-refs-exclude, and log.excludeDecoration work as designed and
the tests continue to pass without intervention.

However, there are other refs that are less helpful to show as
decoration:

* Prefetch refs (refs/prefetch/)
* Rebase refs (refs/rebase-merge/ and refs/rebase-apply/)
* Bundle refs (refs/bundle/) [!]

[!] The bundle refs are part of a parallel series that bootstraps a repo
    from a bundle file, storing the bundle's refs into the repo's
    refs/bundle/ namespace.

In the case of prefetch refs, 96eaffebbf3d0 (maintenance: set
log.excludeDecoration durin prefetch, 2021-01-19) added logic to add
refs/prefetch/ to the log.excludeDecoration config option. Additional
feedback pointed out that having such a side-effect can be confusing and
perhaps not helpful to users. Instead, we should hide these ref
namespaces that are being used by Git for internal reasons but are not
helpful for the users to see.

The way to provide a seamless user experience without setting the config
is to modify the default decoration filters to match our expectation of
what refs the user actually wants to see.

In builtin/log.c, after parsing the --decorate-refs and
--decorate-refs-exclude options from the command-line, call
set_default_decoration_filter(). This method populates the exclusions
from log.excludeDecoration, then checks if the list of pattern
modifications are empty. If none are specified, then the default set is
restricted to the set of inclusions mentioned earlier (HEAD, branches,
etc.).  A previous change introduced the ref_namespaces array, which
includes all of these currently-used namespaces. The 'decoration' value
is non-zero when that namespace is associated with a special coloring
and fits into the list of "expected" decorations as described above,
which makes the implementation of this filter very simple.

Note that the logic in ref_filter_match() in log-tree.c follows this
matching pattern:

 1. If there are exclusion patterns and the ref matches one, then ignore
    the decoration.

 2. If there are inclusion patterns and the ref matches one, then
    definitely include the decoration.

 3. If there are config-based exclusions from log.excludeDecoration and
    the ref matches one, then ignore the decoration.

With this logic in mind, we need to ensure that we do not populate our
new defaults if any of these filters are manually set. Specifically, if
a user runs

	git -c log.excludeDecoration=HEAD log

then we expect the HEAD decoration to not appear. If we left the default
inclusions in the set, then HEAD would match that inclusion before
reaching the config-based exclusions.

A potential alternative would be to check the list of default inclusions
at the end, after the config-based exclusions. This would still create a
behavior change for some uses of --decorate-refs-exclude=<X>, and could
be overwritten somewhat with --decorate-refs=refs/ and
--decorate-refs=HEAD. However, it no longer becomes possible to include
refs outside of the defaults while also excluding some using
log.excludeDecoration.

Another alternative would be to exclude the known namespaces that are
not intended to be shown. This would reduce the visible effect of the
change for expert users who use their own custom ref namespaces. The
implementation change would be very simple to swap due to our use of
ref_namespaces:

	int i;
	struct string_list *exclude = decoration_filter->exclude_ref_pattern;

	/*
	 * No command-line or config options were given, so
	 * populate with sensible defaults.
	 */
	for (i = 0; i < NAMESPACE__COUNT; i++) {
		if (ref_namespaces[i].decoration)
			continue;

		string_list_append(exclude, ref_namespaces[i].ref);
	}

The main downside of this approach is that we expect to add new hidden
namespaces in the future, and that means that Git versions will be less
stable in how they behave as those namespaces are added.

It is critical that we provide ways for expert users to disable this
behavior change via command-line options and config keys. These changes
will be implemented in a future change.

Add a test that checks that the defaults are not added when
--decorate-refs is specified. We verify this by showing that HEAD is not
included as it normally would.  Also add a test that shows that the
default filter avoids the unwanted decorations from refs/prefetch,
refs/rebase-merge,
and refs/bundle.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/Documentation/git-log.txt b/Documentation/git-log.txt
index 20e87cecf4..b2ac89dfaf 100644
--- a/Documentation/git-log.txt
+++ b/Documentation/git-log.txt
@@ -45,13 +45,16 @@ OPTIONS
 
 --decorate-refs=<pattern>::
 --decorate-refs-exclude=<pattern>::
-	If no `--decorate-refs` is given, pretend as if all refs were
-	included.  For each candidate, do not use it for decoration if it
+	For each candidate reference, do not use it for decoration if it
 	matches any patterns given to `--decorate-refs-exclude` or if it
 	doesn't match any of the patterns given to `--decorate-refs`. The
 	`log.excludeDecoration` config option allows excluding refs from
 	the decorations, but an explicit `--decorate-refs` pattern will
 	override a match in `log.excludeDecoration`.
++
+If none of these options or config settings are given, then references are
+used as decoration if they match `HEAD`, `refs/heads/`, `refs/remotes/`,
+`refs/stash/`, or `refs/tags/`.
 
 --source::
 	Print out the ref name given on the command line by which each