guix build: Add '--with-commit'.

* guix/git.scm (<git-checkout>)[commit]: New field.
(git-checkout-compiler): Honor it.
* guix/scripts/build.scm (evaluate-git-replacement-specs): Add 'proc'
parameter and honor it.
(transform-package-source-branch)[replace]: New procedure.
Adjust 'evaluate-git-replacement-specs' accordingly.
(transform-package-source-commit): New procedure.
(%transformations, %transformation-options)
(show-transformation-options-help): Add 'with-commit'.
* tests/guix-build-branch.sh: Add test.
* doc/guix.texi (Package Transformation Options): Document it.

doc/guix.texi

@@ -6478,6 +6478,11 @@ integration (CI).
 Checkouts are kept in a cache under @file{~/.cache/guix/checkouts} to speed up
 consecutive accesses to the same repository.  You may want to clean it up once
 in a while to save disk space.
+
+@item --with-commit=@var{package}=@var{commit}
+This is similar to @code{--with-branch}, except that it builds from
+@var{commit} rather than the tip of a branch.  @var{commit} must be a valid
+Git commit SHA1 identifier.
 @end table
 
 @node Additional Build Options

guix/git.scm

@@ -198,12 +198,13 @@ (define (dot-git? file stat)
 ;;; Checkouts.
 ;;;
 
-;; Representation of the "latest" checkout of a branch.
+;; Representation of the "latest" checkout of a branch or a specific commit.
 (define-record-type* <git-checkout>
   git-checkout make-git-checkout
   git-checkout?
   (url     git-checkout-url)
-  (branch  git-checkout-branch (default "master")))
+  (branch  git-checkout-branch (default "master"))
+  (commit  git-checkout-commit (default #f)))
 
 (define latest-repository-commit*
   (store-lift latest-repository-commit))
@@ -213,7 +214,9 @@ (define-gexp-compiler (git-checkout-compiler (checkout <git-checkout>)
   ;; "Compile" CHECKOUT by updating the local checkout and adding it to the
   ;; store.
   (match checkout
-    (($ <git-checkout> url branch)
+    (($ <git-checkout> url branch commit)
      (latest-repository-commit* url
-                                #:ref `(branch . ,branch)
+                                #:ref (if commit
+                                          `(commit . ,commit)
+                                          `(branch . ,branch))
                                 #:log-port (current-error-port)))))

guix/scripts/build.scm

@@ -272,16 +272,17 @@ (define (replacement-pair old new)
           (rewrite obj)
           obj))))
 
-(define (evaluate-git-replacement-specs specs)
+(define (evaluate-git-replacement-specs specs proc)
   "Parse SPECS, a list of strings like \"guile=stable-2.2\", and return a list
-of package pairs.  Raise an error if an element of SPECS uses invalid syntax,
-or if a package it refers to could not be found."
+of package pairs, where (PROC PACKAGE URL BRANCH-OR-COMMIT) returns the
+replacement package.  Raise an error if an element of SPECS uses invalid
+syntax, or if a package it refers to could not be found."
   (define not-equal
     (char-set-complement (char-set #\=)))
 
   (map (lambda (spec)
          (match (string-tokenize spec not-equal)
-           ((name branch)
+           ((name branch-or-commit)
             (let* ((old    (specification->package name))
                    (source (package-source old))
                    (url    (cond ((and (origin? source)
@@ -293,11 +294,7 @@ (define not-equal
                                   (leave (G_ "the source of ~a is not a Git \
 reference~%")
                                          (package-full-name old))))))
-              (cons old
-                    (package
-                      (inherit old)
-                      (version (string-append "git." branch))
-                      (source (git-checkout (url url) (branch branch)))))))
+              (cons old (proc old url branch-or-commit))))
            (x
             (leave (G_ "invalid replacement specification: ~s~%") spec))))
        specs))
@@ -307,7 +304,36 @@ (define (transform-package-source-branch replacement-specs)
 dependencies according to REPLACEMENT-SPECS.  REPLACEMENT-SPECS is a list of
 strings like \"guile-next=stable-3.0\" meaning that packages are built using
 'guile-next' from the latest commit on its 'stable-3.0' branch."
-  (let* ((replacements (evaluate-git-replacement-specs replacement-specs))
+  (define (replace old url branch)
+    (package
+      (inherit old)
+      (version (string-append "git." branch))
+      (source (git-checkout (url url) (branch branch)))))
+
+  (let* ((replacements (evaluate-git-replacement-specs replacement-specs
+                                                       replace))
+         (rewrite      (package-input-rewriting replacements)))
+    (lambda (store obj)
+      (if (package? obj)
+          (rewrite obj)
+          obj))))
+
+(define (transform-package-source-commit replacement-specs)
+  "Return a procedure that, when passed a package, replaces its direct
+dependencies according to REPLACEMENT-SPECS.  REPLACEMENT-SPECS is a list of
+strings like \"guile-next=cabba9e\" meaning that packages are built using
+'guile-next' from commit 'cabba9e'."
+  (define (replace old url commit)
+    (package
+      (inherit old)
+      (version (string-append "git."
+                              (if (< (string-length commit) 7)
+                                  commit
+                                  (string-take commit 7))))
+      (source (git-checkout (url url) (commit commit)))))
+
+  (let* ((replacements (evaluate-git-replacement-specs replacement-specs
+                                                       replace))
          (rewrite      (package-input-rewriting replacements)))
     (lambda (store obj)
       (if (package? obj)
@@ -322,7 +348,8 @@ (define %transformations
   `((with-source . ,transform-package-source)
     (with-input  . ,transform-package-inputs)
     (with-graft  . ,transform-package-inputs/graft)
-    (with-branch . ,transform-package-source-branch)))
+    (with-branch . ,transform-package-source-branch)
+    (with-commit . ,transform-package-source-commit)))
 
 (define %transformation-options
   ;; The command-line interface to the above transformations.
@@ -338,7 +365,9 @@ (define %transformation-options
           (option '("with-graft") #t #f
                   (parser 'with-graft))
           (option '("with-branch") #t #f
-                  (parser 'with-branch)))))
+                  (parser 'with-branch))
+          (option '("with-commit") #t #f
+                  (parser 'with-commit)))))
 
 (define (show-transformation-options-help)
   (display (G_ "
@@ -350,9 +379,12 @@ (define (show-transformation-options-help)
   (display (G_ "
       --with-graft=PACKAGE=REPLACEMENT
                          graft REPLACEMENT on packages that refer to PACKAGE"))
-    (display (G_ "
+  (display (G_ "
       --with-branch=PACKAGE=BRANCH
-                         build PACKAGE from the latest commit of BRANCH")))
+                         build PACKAGE from the latest commit of BRANCH"))
+  (display (G_ "
+      --with-commit=PACKAGE=COMMIT
+                         build PACKAGE from COMMIT")))
 
 
 (define (options->transformation opts)

tests/guix-build-branch.sh

@@ -46,3 +46,8 @@ orig_drv="`guix build guix -d`"
 latest_drv="`guix build guix --with-branch=guile-gcrypt=master -d`"
 guix gc -R "$latest_drv" | grep guile-gcrypt-git.master
 test "$orig_drv" != "$latest_drv"
+
+v0_1_0_drv="`guix build guix --with-commit=guile-gcrypt=9e3eacdec1d -d`"
+guix gc -R "$v0_1_0_drv" | grep guile-gcrypt-git.9e3eacd
+test "$v0_1_0_drv" != "$latest_drv"
+test "$v0_1_0_drv" != "$orig_drv"