git-authenticate: Ensure the target is a descendant of the introductory commit.

Fixes a bug whereby authentication of a commit *not* descending from the
introductory commit could succeed, provided the commit verifies the
authorization invariant.

In the example below, A is a common ancestor of the introductory commit
I and of commit X.  Authentication of X would succeed, even though it is
not a descendant of I, as long as X is authorized according to the
'.guix-authorizations' in A:

   X   	 I
    \   /
      A

This is because, 'authenticate-repository' would not check whether X
descends from I, and the call (commit-difference X I) would return X.

In practice that only affects forks because it means that ancestors of
the introductory commit already contain a '.guix-authorizations' file.

* guix/git-authenticate.scm (authenticate-repository): Add call to
'commit-descendant?'.
* tests/channels.scm ("authenticate-channel, not a descendant of introductory commit"):
New test.
* tests/git-authenticate.scm ("authenticate-repository, target not a descendant of intro"):
New test.
* tests/guix-git-authenticate.sh: Expect earlier test to fail since
9549f0283a78fe36f2d4ff2a04ef8ad6b0c02604 is not a descendant of
$intro_commit.  Add new test targeting an ancestor of the introductory
commit, and another test targeting the v1.2.0 commit.
* doc/guix.texi (Specifying Channel Authorizations): Add a sentence.

1 files changed, 3 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 86dbe9f201..c116be8907 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -5448,7 +5448,9 @@ commit of a channel that should be authenticated.  The first time a
 channel is fetched with @command{guix pull} or @command{guix
 time-machine}, the command looks up the introductory commit and verifies
 that it is signed by the specified OpenPGP key.  From then on, it
-authenticates commits according to the rule above.
+authenticates commits according to the rule above.  Authentication fails
+if the target commit is neither a descendant nor an ancestor of the
+introductory commit.
 
 Additionally, your channel must provide all the OpenPGP keys that were
 ever mentioned in @file{.guix-authorizations}, stored as @file{.key}