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, 15 insertions, 2 deletions

diff --git a/tests/guix-git-authenticate.sh b/tests/guix-git-authenticate.sh
index 8ebbea398b..2b90d8a4af 100644
--- a/tests/guix-git-authenticate.sh
+++ b/tests/guix-git-authenticate.sh
@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2020 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2020, 2022 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of GNU Guix.
 #
@@ -34,10 +34,18 @@ intro_signer="BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA"
 
 cache_key="test-$$"
 
-guix git authenticate "$intro_commit" "$intro_signer"	\
+# This must fail because the end commit is not a descendant of $intro_commit.
+! guix git authenticate "$intro_commit" "$intro_signer"	\
      --cache-key="$cache_key" --stats			\
      --end=9549f0283a78fe36f2d4ff2a04ef8ad6b0c02604
 
+# The v1.2.0 commit is a descendant of $intro_commit and it satisfies the
+# authorization invariant.
+v1_2_0_commit="a099685659b4bfa6b3218f84953cbb7ff9e88063"
+guix git authenticate "$intro_commit" "$intro_signer"	\
+     --cache-key="$cache_key" --stats			\
+     --end="$v1_2_0_commit"
+
 rm "$XDG_CACHE_HOME/guix/authentication/$cache_key"
 
 # Commit and signer of the 'v1.0.0' tag.
@@ -45,6 +53,11 @@ v1_0_0_commit="6298c3ffd9654d3231a6f25390b056483e8f407c"
 v1_0_0_signer="3CE4 6455 8A84 FDC6 9DB4  0CFB 090B 1199 3D9A EBB5" # civodul
 v1_0_1_commit="d68de958b60426798ed62797ff7c96c327a672ac"
 
+# This should succeed because v1.0.0 is an ancestor of $intro_commit.
+guix git authenticate "$intro_commit" "$intro_signer"	\
+     --cache-key="$cache_key" --stats			\
+     --end="$v1_0_0_commit"
+
 # This should fail because these commits lack '.guix-authorizations'.
 ! guix git authenticate "$v1_0_0_commit" "$v1_0_0_signer" \
        --cache-key="$cache_key" --end="$v1_0_1_commit"