fix: binary corruption during parallel promotion/install

Fixes ocaml#6668

When performing multiple promotions in parallel (e.g. for a .bc and an .exe)
then sharing a single global buf means that one of them could get corrupted
if the buffer is overwritten by the other build.

Do not use global variables: allocate a new per-file buffer in 'parse' instead!

`cd test && dune build @unit-tests/artifact_substitution/runtest @blackbox-tests/test-cases/versioncorruption` passes now

Signed-off-by: Edwin Török <edvin.torok@citrix.com>

Author: edwintorok

CHANGES.md

@@ -1,6 +1,9 @@
 Unreleased
 ----------
 
+- Fix binary corruption when installing or promoting in parallel (#6669, fixes
+  #6668, @edwintorok)
+
 - Fix the parsing of alerts. They will now show up in diagnostics correctly.
   (#6678, @rginberg)
 

src/dune_rules/artifact_substitution.ml

@@ -415,8 +415,6 @@ end
 
 let buf_len = max_len
 
-let buf = Bytes.create buf_len
-
 type mode =
   | Test
   | Copy of
@@ -469,6 +467,7 @@ output the replacement        |                                             |
     v} *)
 let parse ~input ~mode =
   let open Fiber.O in
+  let buf = Bytes.create buf_len in
   let rec loop scanner_state ~beginning_of_data ~pos ~end_of_data ~status =
     let scanner_state = Scanner.run scanner_state ~buf ~pos ~end_of_data in
     let placeholder_start =

test/blackbox-tests/test-cases/version-corruption.t

@@ -95,8 +95,6 @@ shared buffer):
   > EOF
 
   $ rm -f gen_lifecycle.bc gen_lifecycle.exe && dune clean && dune build && ./gen_lifecycle.exe >/dev/null
-  Segmentation fault
-  [139]
   $ cp _build/default/gen_lifecycle.exe gen_lifecycle.old
 
   $ dune install -j16 --prefix=./_install 2>/dev/null
@@ -115,8 +113,10 @@ shared buffer):
   - evaluates to: "v0.0.1"
 
   $ ./compare.sh gen_lifecycle.old ./gen_lifecycle.exe
-  28362
+  100
 
   $ ./gen_lifecycle.exe
-  Segmentation fault
-  [139]
+  0.0.1
+  0.0.1
+  0.0.1
+  0.0.1