fix: promotion over non-existent file

When we use "git diff" as the git tool, we have some logic to resolve
symlinks since "git diff" doesn't handle them. This logic doesn't take
into account the case where the target of a symlink doesn't exist, which
happens when the file you are promoting doesn't exist yet.

This can easily be fixed by taking into account this case.

Signed-off-by: Ali Caglayan <[email protected]>

Author: Alizter

doc/changes/fixed/12615.md

@@ -0,0 +1,2 @@
+- Fix a bug where dune was unable to promote over non-existant files in certain
+  cases (#12615, fixes #8075, @Alizter)

src/promote/print_diff.ml

@@ -13,8 +13,12 @@ let resolve_link_for_git path =
           "Unable to resolve symlink %s. Max recursion depth exceeded"
           (Path.to_string path)
       ]
-  | Error (Unix_error _) ->
-    User_error.raise [ Pp.textf "Unable to resolve symlink %s" (Path.to_string path) ]
+  | Error (Unix_error (ENOENT, _, _)) -> path
+  | Error (Unix_error err) ->
+    User_error.raise
+      [ Pp.textf "Unable to resolve symlink %s" (Path.to_string path)
+      ; Unix_error.Detailed.pp err
+      ]
 ;;
 
 module Diff = struct

test/blackbox-tests/test-cases/promote/symlink-to-nonexistent.t

@@ -23,12 +23,12 @@ detection will now be automatic so a dune-workspace file was included above.
 
 This should fail initially but not with the "Unable to resolve symlink" error.
   $ dune build @bench
-  File "dune", lines 4-7, characters 0-55:
-  4 | (rule
-  5 |  (alias bench)
-  6 |  (action
-  7 |   (diff promoted x.gen)))
-  Error: Unable to resolve symlink _build/default/promoted
+  File "promoted", line 1, characters 0-0:
+  ------ /dev/null
+  ++++++ x.gen
+  File "/dev/null", line 1, characters 0-1:
+  +|toto
+  No newline at the end of x.gen
   [1]
 
 Promotion should work