filesystem: Improve docs, make some promises about overwrites and file caches.

Note that SDL_FlushIO() doesn't make promises about file data sync but that
is intended to be changed in the IOStream code in a later commit.

Fixes #10886.

include/SDL3/SDL_filesystem.h

@@ -289,7 +289,10 @@ extern SDL_DECLSPEC bool SDLCALL SDL_EnumerateDirectory(const char *path, SDL_En
 /**
  * Remove a file or an empty directory.
  *
- * \param path the path of the directory to enumerate.
+ * Directories that are not empty will fail; this function will not recursely
+ * delete directory trees.
+ *
+ * \param path the path to remove from the filesystem.
  * \returns true on success or false on failure; call SDL_GetError() for more
  *          information.
  *
@@ -300,6 +303,17 @@ extern SDL_DECLSPEC bool SDLCALL SDL_RemovePath(const char *path);
 /**
  * Rename a file or directory.
  *
+ * If the file at `newpath` already exists, it will replaced.
+ *
+ * Note that this will not copy files across filesystems/drives/volumes, as
+ * that is a much more complicated (and possibly time-consuming) operation.
+ *
+ * Which is to say, if this function fails, SDL_CopyFile() to a temporary
+ * file in the same directory as `newpath`, then SDL_RenamePath() from the
+ * temporary file to `newpath` and SDL_RemovePath() on `oldpath` might work
+ * for files. Renaming a non-empty directory across filesystems is
+ * dramatically more complex, however.
+ *
  * \param oldpath the old path.
  * \param newpath the new path.
  * \returns true on success or false on failure; call SDL_GetError() for more
@@ -312,6 +326,35 @@ extern SDL_DECLSPEC bool SDLCALL SDL_RenamePath(const char *oldpath, const char
 /**
  * Copy a file.
  *
+ * If the file at `newpath` already exists, it will be overwritten with the
+ * contents of the file at `oldpath`.
+ *
+ * This function will block until the copy is complete, which might be a
+ * significant time for large files on slow disks. On some platforms, the
+ * copy can be handed off to the OS itself, but on others SDL might just open
+ * both paths, and read from one and write to the other.
+ *
+ * Note that this is not an atomic operation! If something tries to read from
+ * `newpath` while the copy is in progress, it will see an incomplete copy
+ * of the data, and if the calling thread terminates (or the power goes out)
+ * during the copy, `oldpath`'s previous contents will be gone, replaced with
+ * an incomplete copy of the data. To avoid this risk, it is recommended that
+ * the app copy to a temporary file in the same directory as `newpath`, and
+ * if the copy is successful, use SDL_RenamePath() to replace `newpath` with
+ * the temporary file. This will ensure that reads of `newpath` will either
+ * see a complete copy of the data, or it will see the pre-copy state of
+ * `newpath`.
+ *
+ * This function attempts to synchronize the newly-copied data to disk before
+ * returning, if the platform allows it, so that the renaming trick will not
+ * have a problem in a system crash or power failure, where the file could
+ * be renamed but the contents never made it from the system file cache to
+ * the physical disk.
+ *
+ * If the copy fails for any reason, the state of `newpath` is undefined. It
+ * might be half a copy, it might be the untouched data of what was already
+ * there, or it might be a zero-byte file, etc.
+ *
  * \param oldpath the old path.
  * \param newpath the new path.
  * \returns true on success or false on failure; call SDL_GetError() for more