fetch --prune: prune only based on explicit refspecs

The old behavior of "fetch --prune" was to prune whatever was being
fetched.  In particular, "fetch --prune --tags" caused tags not only
to be fetched, but also to be pruned.  This is inappropriate because
there is only one tags namespace that is shared among the local
repository and all remotes.  Therefore, if the user defines a local
tag and then runs "git fetch --prune --tags", then the local tag is
deleted.  Moreover, "--prune" and "--tags" can also be configured via
fetch.prune / remote.<name>.prune and remote.<name>.tagopt, making it
even less obvious that an invocation of "git fetch" could result in
tag lossage.

Since the command "git remote update" invokes "git fetch", it had the
same problem.

The command "git remote prune", on the other hand, disregarded the
setting of remote.<name>.tagopt, and so its behavior was inconsistent
with that of the other commands.

So the old behavior made it too easy to lose tags.  To fix this
problem, change "fetch --prune" to prune references based only on
refspecs specified explicitly by the user, either on the command line
or via remote.<name>.fetch.  Thus, tags are no longer made subject to
pruning by the --tags option or the remote.<name>.tagopt setting.

However, tags *are* still subject to pruning if they are fetched as
part of a refspec, and that is good.  For example:

* On the command line,

      git fetch --prune 'refs/tags/*:refs/tags/*'

  causes tags, and only tags, to be fetched and pruned, and is
  therefore a simple way for the user to get the equivalent of the old
  behavior of "--prune --tag".

* For a remote that was configured with the "--mirror" option, the
  configuration is set to include

      [remote "name"]
              fetch = +refs/*:refs/*

  , which causes tags to be subject to pruning along with all other
  references.  This is the behavior that will typically be desired for
  a mirror.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 5 deletions

diff --git a/Documentation/fetch-options.txt b/Documentation/fetch-options.txt
index 921f6be8d4..12b1d92a2f 100644
--- a/Documentation/fetch-options.txt
+++ b/Documentation/fetch-options.txt
@@ -41,8 +41,14 @@ ifndef::git-pull[]
 
 -p::
 --prune::
-	After fetching, remove any remote-tracking branches which
-	no longer exist	on the remote.
+	After fetching, remove any remote-tracking references that no
+	longer exist on the remote.  Tags are not subject to pruning
+	if they are fetched only because of the default tag
+	auto-following or due to a --tags option.  However, if tags
+	are fetched due to an explicit refspec (either on the command
+	line or in the remote configuration, for example if the remote
+	was cloned with the --mirror option), then they are also
+	subject to pruning.
 endif::git-pull[]
 
 ifdef::git-pull[]
@@ -61,9 +67,12 @@ endif::git-pull[]
 ifndef::git-pull[]
 -t::
 --tags::
-	Request that all tags be fetched from the remote in addition
-	to whatever else is being fetched.  Its effect is similar to
-	that of the refspec `refs/tags/*:refs/tags/*`.
+	Fetch all tags from the remote (i.e., fetch remote tags
+	`refs/tags/*` into local tags with the same name), in addition
+	to whatever else would otherwise be fetched.  Using this
+	option alone does not subject tags to pruning, even if --prune
+	is used (though tags may be pruned anyway if they are also the
+	destination of an explicit refspec; see '--prune').
 
 --recurse-submodules[=yes|on-demand|no]::
 	This option controls if and under what conditions new commits of