emacs: Add "Build Log" button to Package Info.

* emacs/guix-main.scm (package-build-log-file): New procedure.
* emacs/guix-ui-package.el (guix-package-build-log-file)
(guix-package-find-build-log)
(guix-package-info-insert-build-log-button): New procedures.
(guix-package-info-button-functions): Add
'guix-package-info-insert-build-log-button'.

emacs/guix-main.scm

@@ -1000,6 +1000,16 @@ GENERATIONS is a list of generation numbers."
         (format #t "The source store path: ~a~%"
                 (package-source-derivation->store-path derivation))))))
 
+(define (package-build-log-file package-id)
+  "Return the build log file of a package PACKAGE-ID.
+Return #f if the build log is not found."
+  (and-let* ((package (package-by-id package-id)))
+    (with-store store
+      (let* ((derivation (package-derivation store package))
+             (file       (derivation-file-name derivation)))
+        (or (log-file store file)
+            ((@@ (guix scripts build) log-url) store file))))))
+
 
 ;;; Executing guix commands
 

emacs/guix-ui-package.el

@@ -111,6 +111,19 @@ is found and `guix-package-list-single' is nil."
       (list (if (= 0 package-id) package-id-str package-id)
             output))))
 
+(defun guix-package-build-log-file (id)
+  "Return build log file name of a package defined by ID."
+  (guix-eval-read
+   (guix-make-guile-expression 'package-build-log-file id)))
+
+(defun guix-package-find-build-log (id)
+  "Show build log of a package defined by ID."
+  (require 'guix-build-log)
+  (let ((file (guix-package-build-log-file id)))
+    (if file
+        (guix-build-log-find-file file)
+      (message "Couldn't find the package build log."))))
+
 
 ;;; Processing package actions
 
@@ -333,7 +346,8 @@ prompt depending on `guix-operation-confirm' variable)."
   :group 'guix-package-info)
 
 (defcustom guix-package-info-button-functions
-  '(guix-package-info-insert-build-button)
+  '(guix-package-info-insert-build-button
+    guix-package-info-insert-build-log-button)
   "List of functions used to insert package buttons in Info buffer.
 Each function is called with 2 arguments: package ID and full name."
   :type '(repeat function)
@@ -598,6 +612,15 @@ PACKAGE-ID is an ID of the package which store path to show."
    (format "Build the current package")
    'id id))
 
+(defun guix-package-info-insert-build-log-button (id _name)
+  "Insert button to show build log of a package defined by ID."
+  (guix-info-insert-action-button
+   "Build Log"
+   (lambda (btn)
+     (guix-package-find-build-log (button-get btn 'id)))
+   "View build log of the current package"
+   'id id))
+
 (defun guix-package-info-show-source (entry-id package-id)
   "Show file name of a package source in the current info buffer.
 Find the file if needed (see `guix-package-info-auto-find-source').