[PATCH v2 06/10] push doc: correct lies about how push refspecs work

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

There's complex rules governing whether a push is allowed to take
place depending on whether we're pushing to refs/heads/*, refs/tags/*
or refs/not-that/*. See is_branch() in refs.c, and the various
assertions in refs/files-backend.c. (e.g. "trying to write non-commit
object %s to branch '%s'").

This documentation has never been quite correct, but went downhill
after dbfeddb12e ("push: require force for refs under refs/tags/",
2012-11-29) when we started claiming that <dst> couldn't be a tag
object, which is incorrect. After some of the logic in that patch was
changed in 256b9d70a4 ("push: fix "refs/tags/ hierarchy cannot be
updated without --force"", 2013-01-16) the docs weren't updated, and
we've had some version of documentation that confused whether <src>
was a tag or not with whether <dst> would accept either an annotated
tag object or the commit it points to.

This makes the intro somewhat more verbose & complex, perhaps we
should have a shorter description here and split the full complexity
into a dedicated section. Very few users will find themselves needing
to e.g. push blobs or trees to refs/custom-namespace/* (or blobs or
trees at all), and that could be covered separately as an advanced
topic.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/git-push.txt     | 30 ++++++++++++++++++++++--------
 Documentation/gitrevisions.txt |  7 ++++---
 2 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/Documentation/git-push.txt b/Documentation/git-push.txt
index 55277a9781..fe654482dc 100644
--- a/Documentation/git-push.txt
+++ b/Documentation/git-push.txt
@@ -60,8 +60,9 @@ OPTIONS[[OPTIONS]]
 	by a colon `:`, followed by the destination ref <dst>.
 +
 The <src> is often the name of the branch you would want to push, but
-it can be any arbitrary "SHA-1 expression", such as `master~4` or
-`HEAD` (see linkgit:gitrevisions[7]).
+it can be any arbitrary expression to a commit, such as `master~4` or
+`HEAD` (see linkgit:gitrevisions[7]). It can also refer to tag
+objects, trees or blobs if the <dst> is outside of `refs/heads/*`.
 +
 The <dst> tells which ref on the remote side is updated with this
 push. Arbitrary expressions cannot be used here, an actual ref must
@@ -74,12 +75,25 @@ without any `<refspec>` on the command line.  Otherwise, missing
 `:<dst>` means to update the same ref as the `<src>`.
 +
 The object referenced by <src> is used to update the <dst> reference
-on the remote side.  By default this is only allowed if <dst> is not
-a tag (annotated or lightweight), and then only if it can fast-forward
-<dst>.  By having the optional leading `+`, you can tell Git to update
-the <dst> ref even if it is not allowed by default (e.g., it is not a
-fast-forward.)  This does *not* attempt to merge <src> into <dst>.  See
-EXAMPLES below for details.
+on the remote side. Whether this is allowed depends on where in
+`refs/*` the <dst> reference lives. The `refs/heads/*` namespace will
+only accept commit objects, and then only they can be
+fast-forwarded. The `refs/tags/*` namespace will accept any kind of
+object, and any changes to them and others types of objects will be
+rejected. Finally, it's possible to push any type of object to any
+namespace outside of `refs/{tags,heads}/*`, but these will be treated
+as branches for the purposes of whether `--force` is required, even in
+the case where a tag object is pushed. That tag object will be
+overwritten by another tag object (or commit!) without `--force` if
+the new tag happens to point to a commit that's a fast-forward of the
+commit it replaces.
++
+By having the optional leading `+` to a refspec (or using `--force`
+command line option) you can tell Git to update the <dst> ref even if
+it is not allowed by its respective namespace clobbering rules (e.g.,
+it is not a fast-forward. in the case of `refs/heads/*` updates) This
+does *not* attempt to merge <src> into <dst>.  See EXAMPLES below for
+details.
 +
 `tag <tag>` means the same as `refs/tags/<tag>:refs/tags/<tag>`.
 +
diff --git a/Documentation/gitrevisions.txt b/Documentation/gitrevisions.txt
index 1f6cceaefb..d407b7dee1 100644
--- a/Documentation/gitrevisions.txt
+++ b/Documentation/gitrevisions.txt
@@ -19,9 +19,10 @@ walk the revision graph (such as linkgit:git-log[1]), all commits which are
 reachable from that commit. For commands that walk the revision graph one can
 also specify a range of revisions explicitly.
 
-In addition, some Git commands (such as linkgit:git-show[1]) also take
-revision parameters which denote other objects than commits, e.g. blobs
-("files") or trees ("directories of files").
+In addition, some Git commands (such as linkgit:git-show[1] and
+linkgit:git-push[1]) can also take revision parameters which denote
+other objects than commits, e.g. blobs ("files") or trees
+("directories of files").
 
 include::revisions.txt[]
 
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