Improve the behaviour of dune promote when files are missing

Fixes #1772

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

CHANGES.md

@@ -99,6 +99,9 @@ unreleased
   `.merlin` with `S`-directives pointed to original source locations and thus
   allowing merlin to see those.
 
+- Improve the behavior of `dune promote` when the files to be promoted have been
+  deleted. (#1775, fixes #1772, @diml)
+
 - unstable-fmt: preserve comments (#1766, @emillon)
 
 1.6.2 (05/12/2018)

src/promotion.ml

@@ -19,10 +19,17 @@ module File = struct
   let register t = db := t :: !db
 
   let promote { src; dst } =
-    Errors.print_to_console (Format.sprintf "Promoting %s to %s.@."
-      (Path.to_string_maybe_quoted src)
-      (Path.to_string_maybe_quoted dst));
-    Io.copy_file ~src ~dst ()
+    let src_exists = Path.exists src in
+    Errors.print_to_console
+      (Format.sprintf
+         (if src_exists then
+            "Promoting %s to %s.@."
+          else
+            "Skipping promotion of %s to %s as the file is missing.@.")
+         (Path.to_string_maybe_quoted src)
+         (Path.to_string_maybe_quoted dst));
+    if src_exists then
+      Io.copy_file ~src ~dst ()
 end
 
 let clear_cache () =

test/blackbox-tests/test-cases/promote/run.t

@@ -74,24 +74,5 @@ Reproduction case for #1772
   [1]
   $ rm -f _build/default/x.gen
   $ dune promote
-  Promoting _build/default/x.gen to x.
-  Error: exception Sys_error("_build/default/x.gen: No such file or directory")
-  Backtrace:
-  Raised by primitive operation at file "stdlib.ml", line 390, characters 28-54
-  Called from file "src/stdune/io.ml", line 74, characters 17-37
-  Called from file "src/promotion.ml", line 86, characters 6-31
-  Called from file "map.ml", line 295, characters 20-25
-  Called from file "src/promotion.ml", line 93, characters 4-44
-  Called from file "src/promotion.ml", line 124, characters 11-41
-  Called from file "vendor/cmdliner/src/cmdliner_term.ml", line 25, characters 19-24
-  Called from file "vendor/cmdliner/src/cmdliner.ml", line 116, characters 32-39
-  Called from file "vendor/cmdliner/src/cmdliner.ml", line 146, characters 18-36
-  Called from file "vendor/cmdliner/src/cmdliner.ml", line 261, characters 22-48
-  Called from file "bin/main.ml", line 177, characters 10-51
-
-  I must not segfault.  Uncertainty is the mind-killer.  Exceptions are
-  the little-death that brings total obliteration.  I will fully express
-  my cases.  Execution will pass over me and through me.  And when it
-  has gone past, I will unwind the stack along its path.  Where the
-  cases are handled there will be nothing.  Only I will remain.
-  [1]
+  Skipping promotion of _build/default/x.gen to x as the file is missing.
+  Promoting _build/default/y.gen to y.