Add git-sha to rule variables, add test for the variable.

Author: toots

src/dune_lang/pform.ml

@@ -138,6 +138,7 @@ module Var = struct
     | Pkg of Pkg.t
     | Oxcaml_supported
     | Dune_warnings
+    | Git_sha
 
   let compare : t -> t -> Ordering.t = Poly.compare
 
@@ -193,7 +194,8 @@ module Var = struct
        | Os os -> Os.to_dyn os
        | Pkg pkg -> Pkg.to_dyn pkg
        | Oxcaml_supported -> variant "Oxcaml_supported" []
-       | Dune_warnings -> variant "Dune_warnings" [])
+       | Dune_warnings -> variant "Dune_warnings" []
+       | Git_sha -> variant "Git_sha" [])
   ;;
 
   let of_opam_global_variable_name name =
@@ -541,6 +543,7 @@ let encode_to_latest_dune_lang_version t =
        | Pkg pkg -> Some (Var.Pkg.encode_to_latest_dune_lang_version pkg)
        | Oxcaml_supported -> Some "oxcaml_supported"
        | Dune_warnings -> Some "dune-warnings"
+       | Git_sha -> Some "git-sha"
      with
      | None -> Pform_was_deleted
      | Some name -> Success { name; payload = None })
@@ -762,6 +765,7 @@ module Env = struct
         ; ( "oxcaml_supported"
           , since ~what:Oxcaml.syntax ~version:(0, 1) Var.Oxcaml_supported )
         ; "dune-warnings", since ~version:(3, 21) Var.Dune_warnings
+        ; "git-sha", since ~version:(3, 24) Var.Git_sha
         ]
       in
       String.Map.of_list_exn

src/dune_lang/pform.mli

@@ -92,6 +92,7 @@ module Var : sig
     | Pkg of Pkg.t
     | Oxcaml_supported
     | Dune_warnings
+    | Git_sha
 
   val compare : t -> t -> Ordering.t
   val to_dyn : t -> Dyn.t

src/dune_rules/expander.ml

@@ -599,6 +599,16 @@ let expand_pform_var (context : Context.t) ~dir ~source (var : Pform.Var.t) =
            let+ scope = scope in
            let dune_version = Dune_project.dune_version (Scope.project scope) in
            Value.L.strings (Ocaml_flags.dune_warnings ~dune_version ~profile:Dev)))
+  | Git_sha ->
+    (let open Memo.O in
+     let+ sha =
+       Source_tree.nearest_vcs Path.Source.root
+       >>= function
+       | None -> Memo.return None
+       | Some vcs -> Vcs.git_sha_short vcs
+     in
+     string (Option.value sha ~default:""))
+    |> static
 ;;
 
 let ocaml_config_macro source macro_invocation context =

src/dune_vcs/vcs.ml

@@ -169,6 +169,17 @@ let commit_id =
          Some res)
 ;;
 
+let git_sha_short =
+  Staged.unstage
+  @@ make_fun
+       "vcs-git-sha-short"
+       ~git:(fun t ->
+         match run_git t [ "rev-parse"; "--short"; "HEAD" ] with
+         | exception _ -> Fiber.return None
+         | fiber -> fiber)
+       ~hg:(fun _ -> Fiber.return None)
+;;
+
 let files =
   let run_zero_separated_hg t args =
     Process.run_capture_zero_separated

src/dune_vcs/vcs.mli

@@ -29,6 +29,9 @@ val describe : t -> string option Memo.t
 (** String uniquely identifying the current head commit *)
 val commit_id : t -> string option Memo.t
 
+(** Short git SHA of the current head commit, or [None] if unavailable *)
+val git_sha_short : t -> string option Memo.t
+
 (** List of files committed in the repo *)
 val files : t -> Path.Source.t list Memo.t
 

test/blackbox-tests/test-cases/git-sha.t

@@ -0,0 +1,43 @@
+Test the %{git-sha} variable.
+
+Without a git repository, %{git-sha} expands to the empty string.
+
+  $ make_dune_project 3.24
+  $ cat > dune << EOF
+  > (rule
+  >  (target sha.txt)
+  >  (action (with-stdout-to sha.txt (echo "%{git-sha}"))))
+  > EOF
+  $ dune build sha.txt
+  $ cat _build/default/sha.txt
+
+In a git repository with a commit, %{git-sha} expands to the short git SHA.
+
+  $ git init --quiet
+  $ git config user.email "test@test.com"
+  $ git config user.name "Test"
+  $ git add .
+  $ git commit -q -m "initial"
+  $ rm -f _build/default/sha.txt
+  $ dune build sha.txt
+  $ [ "$(cat _build/default/sha.txt)" = "$(git rev-parse --short HEAD)" ] && echo "sha matches"
+  sha matches
+
+In a git repository with no commits, %{git-sha} expands to the empty string.
+
+  $ rm -rf .git
+  $ git init --quiet
+  $ rm -f _build/default/sha.txt
+  $ dune build sha.txt
+  $ cat _build/default/sha.txt
+
+Version check: %{git-sha} requires dune >= 3.24.
+
+  $ make_dune_project 3.23
+  $ dune build sha.txt
+  File "dune", line 3, characters 40-50:
+  3 |  (action (with-stdout-to sha.txt (echo "%{git-sha}"))))
+                                              ^^^^^^^^^^
+  Error: %{git-sha} is only available since version 3.24 of the dune language.
+  Please update your dune-project file to have (lang dune 3.24).
+  [1]