Fix file_copy(overwrite = TRUE) EPERM when destination is owned by another user

[Copilot]

file_copy(overwrite = TRUE) fails with [EPERM] and corrupts the destination when the file is owned by a different user — even if the current user has write permission via group membership. The underlying uv_fs_copyfile calls fchmod() internally to mirror the source's permissions, and fchmod() requires ownership, not just write access.

Changes

• src/file.cc: Before calling uv_fs_copyfile when overwrite = TRUE, unlink the destination first. The resulting fresh file is owned by the current user, so fchmod() succeeds. ENOENT from the unlink is silently ignored; other errors are propagated.
• tests/testthat/test-copy.R: Add test asserting file_copy(overwrite = TRUE) correctly replaces an existing file's content.
• NEWS.md: Document the fix.

This matches the behavior of cp and mirrors the existing approach in link_copy.

Original prompt

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This section details on the original issue you should resolve

<issue_title>file_copy with overwrite = TRUE permission problem</issue_title>
<issue_description>I ran into strange behavior with file_copy.

When another user with the same group create file (with rw-rw-rw- permissions) then

library(fs)

file_info("tmp-tst-copy.txt")$permissions # rw-rw-rw-
file_access("tmp-tst-copy.txt", "write") # TRUE

file_copy("tmp-tst-origin.txt", "tmp-tst-copy.txt", overwrite = TRUE)
Error: [EPERM] Failed to copy 'tmp-tst-origin.txt' to 'tmp-tst-copy.txt': operation not permitted

But actually file is removed (so runing command second time works as the destination file does not exists).

There is no problem with file_delete("tmp-tst-copy.txt").
There is no problem with system command cp tmp-tst-origin.txt tmp-tst-copy.txt

Session Info

> sessioninfo::session_info()
─ Session info ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 setting  value
 version  R version 4.5.0 (2025-04-11)
 os       Red Hat Enterprise Linux 8.10 (Ootpa)
 system   x86_64, linux-gnu
 ui       RStudio
 language (EN)
 collate  en_US.UTF-8
 ctype    en_US.UTF-8
 tz       Europe/Warsaw
 date     2025-12-02
 rstudio  2025.05.0+496.pro5 Mariposa Orchid (server)
 pandoc   NA
 quarto   1.6.42 @ /usr/lib/rstudio-server/bin/quarto/bin/quarto

─ Packages ─────────────────────────────────────────────
 package     * version date (UTC) lib source
 cli           3.6.5   2025-04-23 [1] RSPM (R 4.5.0)
 crayon        1.5.3   2024-06-20 [1] RSPM (R 4.5.0)
 fs          * 1.6.6   2025-04-12 [1] RSPM (R 4.5.0)
 glue          1.8.0   2024-09-30 [1] RSPM (R 4.5.0)
 lifecycle     1.0.4   2023-11-07 [1] RSPM (R 4.5.0)
 magrittr      2.0.3   2022-03-30 [1] RSPM (R 4.5.0)
 pillar        1.10.2  2025-04-05 [1] RSPM (R 4.5.0)
 pkgconfig     2.0.3   2019-09-22 [1] RSPM (R 4.5.0)
 pkgload       1.4.0   2024-06-28 [1] RSPM (R 4.5.0)
 rlang         1.1.6   2025-04-11 [1] RSPM (R 4.5.0)
 rstudioapi    0.17.1  2024-10-22 [1] RSPM (R 4.5.0)
 sessioninfo   1.2.3   2025-02-05 [1] RSPM (R 4.5.0)
 tibble        3.2.1   2023-03-20 [1] RSPM (R 4.5.0)
 utf8          1.2.5   2025-05-01 [1] RSPM (R 4.5.0)
 vctrs         0.6.5   2023-12-01 [1] RSPM (R 4.5.0)

 [1] /opt/R/libraries/4.5.0_2025-05
 [2] /opt/R/4.5.0/lib/R/library
 * ── Packages attached to the search path.

───────────────────────────────────────────────────

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes file_copy with overwrite = TRUE permission problem #499

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