bug#26075: [PATCH -v3 1/2] build: union: Add create-all-directories? parameter to union-build

[Huang Ying]

* guix/build/union.scm (union-build): Add create-all-directories? keyword
  parameter.
---
 guix/build/union.scm | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/guix/build/union.scm b/guix/build/union.scm
index 6640b5652..b852bc66e 100644
--- a/guix/build/union.scm
+++ b/guix/build/union.scm
@@ -73,9 +73,12 @@ identical, #f otherwise."
                                   (loop)))))))))))))
 
 (define* (union-build output inputs
-                      #:key (log-port (current-error-port)))
-  "Build in the OUTPUT directory a symlink tree that is the union of all
-the INPUTS."
+                      #:key (log-port (current-error-port))
+                      (create-all-directories? #f))
+  "Build in the OUTPUT directory a symlink tree that is the union of all the
+INPUTS.  As a special case, if CREATE-ALL-DIRECTORIES?, creates the
+subdirectories in the output directory to make sure the caller can modify them
+later."
 
   (define (symlink* input output)
     (format log-port "`~a' ~~> `~a'~%" input output)
@@ -104,8 +107,11 @@ the INPUTS."
   (define (union output inputs)
     (match inputs
       ((input)
-       ;; There's only one input, so just make a link.
-       (symlink* input output))
+       ;; There's only one input, so just make a link unless
+       ;; create-all-directories?.
+       (if (and create-all-directories? (file-is-directory? input))
+           (union-of-directories output inputs)
+           (symlink* input output)))
       (_
        (call-with-values (lambda () (partition file-is-directory? inputs))
          (match-lambda*
-- 
2.12.0