common: fs: Amend IsFile check in FileOpen / (Write/Append)StringToFile

This check was preventing files with the Write or Append file access modes from being created, as per the documented behavior in FileAccessMode.
This amends the check to test for the existence of a filesystem object prior to checking whether it is a regular file.
Thanks to liushuyu for pointing out that removing the check altogether would not guard against attempting to open non-regular files such as directories, symlinks, FIFO (pipes), sockets, block devices, or character devices.
The documentation has also been updated for these functions to clarify that a file refers to a regular file.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/common/fs/fs.cpp b/src/common/fs/fs.cpp
index d3159e908c..9089cad67b 100644
--- a/src/common/fs/fs.cpp
+++ b/src/common/fs/fs.cpp
@@ -135,8 +135,9 @@ std::shared_ptr<IOFile> FileOpen(const fs::path& path, FileAccessMode mode, File
         return nullptr;
     }
 
-    if (!IsFile(path)) {
-        LOG_ERROR(Common_Filesystem, "Filesystem object at path={} is not a file",
+    if (Exists(path) && !IsFile(path)) {
+        LOG_ERROR(Common_Filesystem,
+                  "Filesystem object at path={} exists and is not a regular file",
                   PathToUTF8String(path));
         return nullptr;
     }