unix: handle src, dest same in uv_fs_copyfile()

This commit handles the case where the source and destination
are the same. This behavior was originally addressed in #2298,
but the test added in that PR doesn't validate the file size
after the operation. This commit also updates the test to check
for that case.

Refs: #2298
Refs: nodejs/node#34624
PR-URL: #2947
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>

src/unix/fs.c

@@ -1146,7 +1146,7 @@ static ssize_t uv__fs_copyfile(uv_fs_t* req) {
     goto out;
   }
 
-  dst_flags = O_WRONLY | O_CREAT | O_TRUNC;
+  dst_flags = O_WRONLY | O_CREAT;
 
   if (req->flags & UV_FS_COPYFILE_EXCL)
     dst_flags |= O_EXCL;
@@ -1165,16 +1165,26 @@ static ssize_t uv__fs_copyfile(uv_fs_t* req) {
     goto out;
   }
 
-  /* Get the destination file's mode. */
-  if (fstat(dstfd, &dst_statsbuf)) {
-    err = UV__ERR(errno);
-    goto out;
-  }
+  /* If the file is not being opened exclusively, verify that the source and
+     destination are not the same file. If they are the same, bail out early. */
+  if ((req->flags & UV_FS_COPYFILE_EXCL) == 0) {
+    /* Get the destination file's mode. */
+    if (fstat(dstfd, &dst_statsbuf)) {
+      err = UV__ERR(errno);
+      goto out;
+    }
 
-  /* Check if srcfd and dstfd refer to the same file */
-  if (src_statsbuf.st_dev == dst_statsbuf.st_dev &&
-      src_statsbuf.st_ino == dst_statsbuf.st_ino) {
-    goto out;
+    /* Check if srcfd and dstfd refer to the same file */
+    if (src_statsbuf.st_dev == dst_statsbuf.st_dev &&
+        src_statsbuf.st_ino == dst_statsbuf.st_ino) {
+      goto out;
+    }
+
+    /* Truncate the file in case the destination already existed. */
+    if (ftruncate(dstfd, 0) != 0) {
+      err = UV__ERR(errno);
+      goto out;
+    }
   }
 
   if (fchmod(dstfd, src_statsbuf.st_mode) == -1) {
@@ -2046,7 +2056,7 @@ void uv_fs_req_cleanup(uv_fs_t* req) {
 
   /* Only necessary for asychronous requests, i.e., requests with a callback.
    * Synchronous ones don't copy their arguments and have req->path and
-   * req->new_path pointing to user-owned memory.  UV_FS_MKDTEMP and 
+   * req->new_path pointing to user-owned memory.  UV_FS_MKDTEMP and
    * UV_FS_MKSTEMP are the exception to the rule, they always allocate memory.
    */
   if (req->path != NULL &&

test/test-fs-copyfile.c

@@ -125,6 +125,11 @@ TEST_IMPL(fs_copyfile) {
   r = uv_fs_copyfile(NULL, &req, src, src, 0, NULL);
   ASSERT(r == 0);
   uv_fs_req_cleanup(&req);
+  /* Verify that the src file did not get truncated. */
+  r = uv_fs_stat(NULL, &req, src, NULL);
+  ASSERT_EQ(r, 0);
+  ASSERT_EQ(req.statbuf.st_size, 12);
+  uv_fs_req_cleanup(&req);
   unlink(src);
 
   /* Copies file synchronously. Creates new file. */