Modification time of an element with an open workspace propagates into consumer element's sandbox

Start by reproducing the issue with the supplied project, comparing the build sandbox mtime for a.txt with and without bst workspace open a.bst. Trace how workspace artifacts are staged into b.bst's sandbox. Done when the workspace and non-workspace builds both preserve the magic timestamp.

When an element has a dependency with an open workspace, that dependency's artifacts get staged with their real mtimes instead of the magic timestamp. This can cause differences between builds with and without the workspace.

Steps to reproduce

A small project to reproduce the issue:

.
├── a.txt
├── elements
│   ├── a.bst
│   ├── alpine.bst
│   └── b.bst
└── project.conf

The context of a.txt are irrelevant, it can be left empty. elements/a.bst carries a.txt:

kind: import

sources:
  - kind: local
    path: a.txt

config:
  target: /

elements/b.bst has a.bst and alpine.bst as build dependencies:

kind: manual

build-depends:
  - alpine.bst
  - a.bst

kind: import
description: |

    Alpine Linux base runtime

sources:
- kind: tar

  # This is a post doctored, trimmed down system image
  # of the Alpine linux distribution.
  #
  url: alpine:integration-tests-base.v1.x86_64.tar.xz
  ref: 3eb559250ba82b64a68d86d0636a6b127aa5f6d25d3601a79f79214dc9703639

# Unique project name
name: bug-repro

# Required BuildStream version
min-version: 2.7

# Subdirectory where elements are stored
element-path: elements

aliases:
  alpine: https://bst-integration-test-images.ams3.cdn.digitaloceanspaces.com/

When no workspace is open, building b.bst, opening its build sandbox, and checking the mtime of a.txt shows that all is well, and modification time is normalised:

$ bst build b.bst
...
$ bst shell --build b.bst
...
[20440600@b.bst:/buildstream/repro/b.bst]$ stat /a.txt 
  File: /a.txt
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 31h/49d Inode: 2           Links: 1
Access: (0644/-rw-r--r--)  Uid: (1876721912/ UNKNOWN)   Gid: (1174200513/ UNKNOWN)
Access: 2011-11-11 11:11:11.000000000
Modify: 2011-11-11 11:11:11.000000000
Change: 2011-11-11 11:11:11.000000000

If a.bst's workspace is opened first, the modification time is no longer the magic timestamp but the actual a.bst's build time instead:

$ bst workspace open a.bst
...
$ bst build b.bst
...
$ bst shell --build b.bst
...
[e1f40f6c@b.bst:/buildstream/repro/b.bst]$ stat /a.txt
  File: /a.txt
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 31h/49d Inode: 3           Links: 1
Access: (0644/-rw-r--r--)  Uid: (1876721912/ UNKNOWN)   Gid: (1174200513/ UNKNOWN)
Access: 2011-11-11 11:11:11.000000000
Modify: 2026-08-25 11:06:30.000000000
Change: 2011-11-11 11:11:11.000000000