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

NEWS.md

@@ -8,6 +8,10 @@
 * `dir_exists()` follows relative symlinks in non-current directories
   (@heavywatal, #395).
 
+* `file_copy(overwrite = TRUE)` no longer fails with `[EPERM]` when the
+  destination file is owned by a different user but the current user has write
+  permission via group membership (#487).
+
 # fs 1.6.6
 
 * No changes.

src/file.cc

@@ -368,6 +368,20 @@ fs_copyfile_(SEXP path_sxp, SEXP new_path_sxp, SEXP overwrite_sxp) {
     uv_fs_t req;
     const char* p = CHAR(STRING_ELT(path_sxp, i));
     const char* n = CHAR(STRING_ELT(new_path_sxp, i));
+
+    // When overwriting, unlink the destination first so that fchmod() does not
+    // fail with EPERM when the destination is owned by a different user (but
+    // the current user has write permission via group membership).
+    if (overwrite) {
+      uv_fs_t unlink_req;
+      uv_fs_unlink(uv_default_loop(), &unlink_req, n, NULL);
+      // Ignore ENOENT (file didn't exist); propagate other errors.
+      if (unlink_req.result < 0 && unlink_req.result != UV_ENOENT) {
+        stop_for_error(unlink_req, "Failed to remove '%s'", n);
+      }
+      uv_fs_req_cleanup(&unlink_req);
+    }
+
     uv_fs_copyfile(
         uv_default_loop(),
         &req,

tests/testthat/test-copy.R

@@ -31,6 +31,12 @@ describe("file_copy", {
       expect_equal(readLines("bar2"), readLines("bar"))
     })
   })
+  it("can overwrite an existing file", {
+    with_dir_tree(list("foo" = "original", "bar" = "new content"), {
+      file_copy("bar", "foo", overwrite = TRUE)
+      expect_equal(readLines("foo"), "new content")
+    })
+  })
   it("errors on missing input", {
     expect_error(file_copy(NA, "foo2"), class = "invalid_argument")
     expect_error(file_copy("foo", NA), class = "invalid_argument")