ui: Improve pager selection logic when less is not installed.

* guix/ui.scm (find-available-pager): New procedure.
(call-with-paginated-output-port): Use it.
* guix/utils.scm (call-with-environment-variables): Allow clearing of
specified environment variables.
* tests/ui.scm (make-empty-file, assert-equals-find-available-pager):
New procedures.
("find-available-pager, GUIX_PAGER takes precedence")
("find-available-pager, PAGER takes precedence")
("find-available-pager, 'less' takes precedence")
("find-available-pager, 'more' takes precedence")
("find-available-pager, no pager"): New tests.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

guix/ui.scm

@@ -17,6 +17,7 @@
 ;;; Copyright © 2020 Arun Isaac <arunisaac@systemreboot.net>
 ;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2018 Steve Sprang <scs@stevesprang.com>
+;;; Copyright © 2022 Taiju HIGASHI <higashi@taiju.info>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -1672,11 +1673,18 @@ return the underlying port.  Otherwise return #f."
     (_
      #f)))
 
+(define (find-available-pager)
+  "Return the program name of an available pager or the empty string if none is
+available."
+  (or (getenv "GUIX_PAGER")
+      (getenv "PAGER")
+      (which "less")
+      (which "more")
+      ""))
+
 (define* (call-with-paginated-output-port proc
                                           #:key (less-options "FrX"))
-  (let ((pager-command-line (or (getenv "GUIX_PAGER")
-                                (getenv "PAGER")
-                                "less")))
+  (let ((pager-command-line (find-available-pager)))
     ;; Setting PAGER to the empty string conventionally disables paging.
     (if (and (not (string-null? pager-command-line))
              (isatty?* (current-output-port)))

guix/utils.scm

@@ -13,6 +13,7 @@
 ;;; Copyright © 2021 Chris Marusich <cmmarusich@gmail.com>
 ;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
 ;;; Copyright © 2018 Steve Sprang <scs@stevesprang.com>
+;;; Copyright © 2022 Taiju HIGASHI <higashi@taiju.info>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -158,6 +159,8 @@
     (dynamic-wind
       (lambda ()
         (for-each (match-lambda
+                    ((variable #false)
+                     (unsetenv variable))
                     ((variable value)
                      (setenv variable value)))
                   variables))

tests/ui.scm

@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2022 Taiju HIGASHI <higashi@taiju.info>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -24,6 +25,7 @@
   #:use-module (guix derivations)
   #:use-module ((gnu packages) #:select (specification->package))
   #:use-module (guix tests)
+  #:use-module (guix utils)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-19)
@@ -292,4 +294,70 @@ Second line" 24))
          (>0 (package-relevance libb2
                                 (map rx '("crypto" "library")))))))
 
+(define (make-empty-file directory file)
+  ;; Create FILE in DIRECTORY.
+  (close-port (open-output-file (in-vicinity directory file))))
+
+(define (assert-equals-find-available-pager expected)
+  ;; Use 'with-paginated-output-port' and return true if it invoked EXPECTED.
+  (define used-command "")
+  (mock ((ice-9 popen) open-pipe*
+         (lambda (mode command . args)
+           (unless (string-null? used-command)
+             (error "open-pipe* should only be called once"))
+           (set! used-command command)
+           (%make-void-port "")))
+        (mock ((ice-9 popen) close-pipe (const 'ok))
+              (mock ((guix colors) isatty?* (const #t))
+                    (with-paginated-output-port port 'ok)
+                    (string=? expected used-command)))))
+
+
+(test-assert "find-available-pager, GUIX_PAGER takes precedence"
+  (call-with-temporary-directory
+   (lambda (dir)
+     (with-environment-variables `(("PATH" ,dir)
+                                   ("GUIX_PAGER" "guix-pager")
+                                   ("PAGER" "pager"))
+       (make-empty-file dir "less")
+       (make-empty-file dir "more")
+       (assert-equals-find-available-pager "guix-pager")))))
+
+(test-assert "find-available-pager, PAGER takes precedence"
+  (call-with-temporary-directory
+   (lambda (dir)
+     (with-environment-variables `(("PATH" ,dir)
+                                   ("GUIX_PAGER" #false)
+                                   ("PAGER" "pager"))
+       (make-empty-file dir "less")
+       (make-empty-file dir "more")
+       (assert-equals-find-available-pager "pager")))))
+
+(test-assert "find-available-pager, 'less' takes precedence"
+  (call-with-temporary-directory
+   (lambda (dir)
+     (with-environment-variables `(("PATH" ,dir)
+                                   ("GUIX_PAGER" #false)
+                                   ("PAGER" #false))
+       (make-empty-file dir "less")
+       (make-empty-file dir "more")
+       (assert-equals-find-available-pager (in-vicinity dir "less"))))))
+
+(test-assert "find-available-pager, 'more' takes precedence"
+  (call-with-temporary-directory
+   (lambda (dir)
+     (with-environment-variables `(("PATH" ,dir)
+                                   ("GUIX_PAGER" #false)
+                                   ("PAGER" #false))
+       (make-empty-file dir "more")
+       (assert-equals-find-available-pager (in-vicinity dir "more"))))))
+
+(test-assert "find-available-pager, no pager"
+  (call-with-temporary-directory
+   (lambda (dir)
+     (with-environment-variables `(("PATH" ,dir)
+                                   ("GUIX_PAGER" #false)
+                                   ("PAGER" #false))
+       (assert-equals-find-available-pager "")))))
+
 (test-end "ui")