git: Add 'commit-descendant?'.

* guix/git.scm (commit-descendant?): New procedure.
* tests/git.scm ("commit-descendant?"): New test.

guix/git.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017, 2020 Mathieu Othacehe <m.othacehe@gmail.com>
-;;; Copyright © 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2018-2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2021 Kyle Meyer <kyle@kyleam.com>
 ;;; Copyright © 2021 Marius Bakke <marius@gnu.org>
 ;;; Copyright © 2022 Maxime Devos <maximedevos@telenet.be>
@@ -46,6 +46,7 @@ (define-module (guix git)
   #:use-module (ice-9 ftw)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
+  #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
   #:export (%repository-cache-directory
@@ -60,6 +61,7 @@ (define-module (guix git)
             latest-repository-commit
             commit-difference
             commit-relation
+            commit-descendant?
 
             remote-refs
 
@@ -623,6 +625,26 @@ (define (commit-relation old new)
               (if (set-contains? oldest new)
                   'descendant
                   'unrelated))))))
+
+(define (commit-descendant? new old)
+  "Return true if NEW is the descendant of one of OLD, a list of commits.
+
+When the expected result is likely #t, this is faster than using
+'commit-relation' since fewer commits need to be traversed."
+  (let ((old (list->setq old)))
+    (let loop ((commits (list new))
+               (visited (setq)))
+      (match commits
+        (()
+         #f)
+        (_
+         ;; Perform a breadth-first search as this is likely going to
+         ;; terminate more quickly than a depth-first search.
+         (let ((commits (remove (cut set-contains? visited <>) commits)))
+           (or (any (cut set-contains? old <>) commits)
+               (loop (append-map commit-parents commits)
+                     (fold set-insert visited commits)))))))))
+
 
 ;;
 ;;; Remote operations.

tests/git.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2019, 2020, 2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz
 ;;;
 ;;; This file is part of GNU Guix.
@@ -162,6 +162,56 @@ (define-module (test-git)
               (commit-relation master1 merge)
               (commit-relation merge master1))))))
 
+(unless (which (git-command)) (test-skip 1))
+(test-equal "commit-descendant?"
+  '((master3 master3 => #t)
+    (master1 master3 => #f)
+    (master3 master1 => #t)
+    (master2 branch1 => #f)
+    (master2 branch1 master1 => #t)
+    (branch1 master2 => #f)
+    (branch1 merge   => #f)
+    (merge branch1   => #t)
+    (master1 merge   => #f)
+    (merge master1   => #t))
+  (with-temporary-git-repository directory
+      '((add "a.txt" "A")
+        (commit "first commit")
+        (branch "hack")
+        (checkout "hack")
+        (add "1.txt" "1")
+        (commit "branch commit")
+        (checkout "master")
+        (add "b.txt" "B")
+        (commit "second commit")
+        (add "c.txt" "C")
+        (commit "third commit")
+        (merge "hack" "merge"))
+    (with-repository directory repository
+      (let ((master1 (find-commit repository "first"))
+            (master2 (find-commit repository "second"))
+            (master3 (find-commit repository "third"))
+            (branch1 (find-commit repository "branch"))
+            (merge   (find-commit repository "merge")))
+        (letrec-syntax ((verify
+                         (syntax-rules ()
+                           ((_) '())
+                           ((_ (new old ...) rest ...)
+                            (cons `(new old ... =>
+                                        ,(commit-descendant? new
+                                                             (list old ...)))
+                                  (verify rest ...))))))
+          (verify (master3 master3)
+                  (master1 master3)
+                  (master3 master1)
+                  (master2 branch1)
+                  (master2 branch1 master1)
+                  (branch1 master2)
+                  (branch1 merge)
+                  (merge branch1)
+                  (master1 merge)
+                  (merge master1)))))))
+
 (unless (which (git-command)) (test-skip 1))
 (test-equal "remote-refs"
   '("refs/heads/develop" "refs/heads/master"