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.

src/common/fs/file.cpp

@@ -172,7 +172,7 @@ std::string ReadStringFromFile(const std::filesystem::path& path, FileType type)
 
 size_t WriteStringToFile(const std::filesystem::path& path, FileType type,
                          std::string_view string) {
-    if (!IsFile(path)) {
+    if (Exists(path) && !IsFile(path)) {
         return 0;
     }
 
@@ -183,7 +183,7 @@ size_t WriteStringToFile(const std::filesystem::path& path, FileType type,
 
 size_t AppendStringToFile(const std::filesystem::path& path, FileType type,
                           std::string_view string) {
-    if (!IsFile(path)) {
+    if (Exists(path) && !IsFile(path)) {
         return 0;
     }
 

src/common/fs/file.h

@@ -49,7 +49,7 @@ void OpenFileStream(FileStream& file_stream, const Path& path, std::ios_base::op
 
 /**
  * Reads an entire file at path and returns a string of the contents read from the file.
- * If the filesystem object at path is not a file, this function returns an empty string.
+ * If the filesystem object at path is not a regular file, this function returns an empty string.
  *
  * @param path Filesystem path
  * @param type File type
@@ -72,7 +72,8 @@ template <typename Path>
 /**
  * Writes a string to a file at path and returns the number of characters successfully written.
  * If a file already exists at path, its contents will be erased.
- * If the filesystem object at path is not a file, this function returns 0.
+ * If a file does not exist at path, it creates and opens a new empty file for writing.
+ * If the filesystem object at path exists and is not a regular file, this function returns 0.
  *
  * @param path Filesystem path
  * @param type File type
@@ -95,7 +96,8 @@ template <typename Path>
 
 /**
  * Appends a string to a file at path and returns the number of characters successfully written.
- * If the filesystem object at path is not a file, this function returns 0.
+ * If a file does not exist at path, it creates and opens a new empty file for appending.
+ * If the filesystem object at path exists and is not a regular file, this function returns 0.
  *
  * @param path Filesystem path
  * @param type File type

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;
     }

src/common/fs/fs.h

@@ -110,8 +110,8 @@ template <typename Path1, typename Path2>
  *
  * Failures occur when:
  * - Input path is not valid
- * - Filesystem object at path is not a file
- * - The file is not opened
+ * - Filesystem object at path exists and is not a regular file
+ * - The file is not open
  *
  * @param path Filesystem path
  * @param mode File access mode