Merge branch 'ab/fetch-tags-noclobber'

The rules used by "git push" and "git fetch" to determine if a ref
can or cannot be updated were inconsistent; specifically, fetching
to update existing tags were allowed even though tags are supposed
to be unmoving anchoring points.  "git fetch" was taught to forbid
updates to existing tags without the "--force" option.

* ab/fetch-tags-noclobber:
  fetch: stop clobbering existing tags without --force
  fetch: document local ref updates with/without --force
  push doc: correct lies about how push refspecs work
  push doc: move mention of "tag <tag>" later in the prose
  push doc: remove confusing mention of remote merger
  fetch tests: add a test for clobbering tag behavior
  push tests: use spaces in interpolated string
  push tests: make use of unused $1 in test description
  fetch: change "branch" to "reference" in --force -h output

4 files changed, 94 insertions, 24 deletions

diff --git a/Documentation/fetch-options.txt b/Documentation/fetch-options.txt
index 8bc36af4b1..fa0a3151b3 100644
--- a/Documentation/fetch-options.txt
+++ b/Documentation/fetch-options.txt
@@ -68,11 +68,16 @@ endif::git-pull[]
 
 -f::
 --force::
-	When 'git fetch' is used with `<rbranch>:<lbranch>`
-	refspec, it refuses to update the local branch
-	`<lbranch>` unless the remote branch `<rbranch>` it
-	fetches is a descendant of `<lbranch>`.  This option
-	overrides that check.
+	When 'git fetch' is used with `<src>:<dst>` refspec it may
+	refuse to update the local branch as discussed
+ifdef::git-pull[]
+	in the `<refspec>` part of the linkgit:git-fetch[1]
+	documentation.
+endif::git-pull[]
+ifndef::git-pull[]
+	in the `<refspec>` part below.
+endif::git-pull[]
+	This option overrides that check.
 
 -k::
 --keep::
diff --git a/Documentation/git-push.txt b/Documentation/git-push.txt
index 55277a9781..f345bd30fc 100644
--- a/Documentation/git-push.txt
+++ b/Documentation/git-push.txt
@@ -74,22 +74,57 @@ 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.
-+
-`tag <tag>` means the same as `refs/tags/<tag>:refs/tags/<tag>`.
-+
-Pushing an empty <src> allows you to delete the <dst> ref from
-the remote repository.
+on the remote side. Whether this is allowed depends on where in
+`refs/*` the <dst> reference lives as described in detail below, in
+those sections "update" means any modifications except deletes, which
+as noted after the next few sections are treated differently.
++
+The `refs/heads/*` namespace will only accept commit objects, and
+updates only if they can be fast-forwarded.
++
+The `refs/tags/*` namespace will accept any kind of object (as
+commits, trees and blobs can be tagged), and any updates to them will
+be rejected.
++
+It's possible to push any type of object to any namespace outside of
+`refs/{tags,heads}/*`. In the case of tags and commits, these will be
+treated as if they were the commits inside `refs/heads/*` for the
+purposes of whether the update is allowed.
++
+I.e. a fast-forward of commits and tags outside `refs/{tags,heads}/*`
+is allowed, even in cases where what's being fast-forwarded is not a
+commit, but a tag object which happens to point to a new commit which
+is a fast-forward of the commit the last tag (or commit) it's
+replacing. Replacing a tag with an entirely different tag is also
+allowed, if it points to the same commit, as well as pushing a peeled
+tag, i.e. pushing the commit that existing tag object points to, or a
+new tag object which an existing commit points to.
++
+Tree and blob objects outside of `refs/{tags,heads}/*` will be treated
+the same way as if they were inside `refs/tags/*`, any update of them
+will be rejected.
++
+All of the rules described above about what's not allowed as an update
+can be overridden by adding an the optional leading `+` to a refspec
+(or using `--force` command line option). The only exception to this
+is that no amount of forcing will make the `refs/heads/*` namespace
+accept a non-commit object. Hooks and configuration can also override
+or amend these rules, see e.g. `receive.denyNonFastForwards` in
+linkgit:git-config[1] and`pre-receive` and `update` in
+linkgit:githooks[5].
++
+Pushing an empty <src> allows you to delete the <dst> ref from the
+remote repository. Deletions are always accepted without a leading `+`
+in the refspec (or `--force`), except when forbidden by configuration
+or hooks. See `receive.denyDeletes` in linkgit:git-config[1] and
+`pre-receive` and `update` in linkgit:githooks[5].
 +
 The special refspec `:` (or `+:` to allow non-fast-forward updates)
 directs Git to push "matching" branches: for every branch that exists on
 the local side, the remote side is updated if a branch of the same name
 already exists on the remote side.
++
+`tag <tag>` means the same as `refs/tags/<tag>:refs/tags/<tag>`.
 
 --all::
 	Push all branches (i.e. refs under `refs/heads/`); cannot be
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[]
 
diff --git a/Documentation/pull-fetch-param.txt b/Documentation/pull-fetch-param.txt
index f1fb08dc68..293c6b967d 100644
--- a/Documentation/pull-fetch-param.txt
+++ b/Documentation/pull-fetch-param.txt
@@ -33,11 +33,40 @@ name.
 it requests fetching everything up to the given tag.
 +
 The remote ref that matches <src>
-is fetched, and if <dst> is not an empty string, the local
-ref that matches it is fast-forwarded using <src>.
-If the optional plus `+` is used, the local ref
-is updated even if it does not result in a fast-forward
-update.
+is fetched, and if <dst> is not an empty string, an attempt
+is made to update the local ref that matches it.
++
+Whether that update is allowed without `--force` depends on the ref
+namespace it's being fetched to, the type of object being fetched, and
+whether the update is considered to be a fast-forward. Generally, the
+same rules apply for fetching as when pushing, see the `<refspec>...`
+section of linkgit:git-push[1] for what those are. Exceptions to those
+rules particular to 'git fetch' are noted below.
++
+Until Git version 2.20, and unlike when pushing with
+linkgit:git-push[1], any updates to `refs/tags/*` would be accepted
+without `+` in the refspec (or `--force`). The receiving promiscuously
+considered all tag updates from a remote to be forced fetches.  Since
+Git version 2.20, fetching to update `refs/tags/*` work the same way
+as when pushing. I.e. any updates will be rejected without `+` in the
+refspec (or `--force`).
++
+Unlike when pushing with linkgit:git-push[1], any updates outside of
+`refs/{tags,heads}/*` will be accepted without `+` in the refspec (or
+`--force`), whether that's swapping e.g. a tree object for a blob, or
+a commit for another commit that's doesn't have the previous commit as
+an ancestor etc.
++
+Unlike when pushing with linkgit:git-push[1], there is no
+configuration which'll amend these rules, and nothing like a
+`pre-fetch` hook analogous to the `pre-receive` hook.
++
+As with pushing with linkgit:git-push[1], all of the rules described
+above about what's not allowed as an update can be overridden by
+adding an the optional leading `+` to a refspec (or using `--force`
+command line option). The only exception to this is that no amount of
+forcing will make the `refs/heads/*` namespace accept a non-commit
+object.
 +
 [NOTE]
 When the remote branch you want to fetch is known to