serving a compress enabled public folder without +w permissions results in a 404

Original bug gofiber/fiber#2940

The problem is in fs.go in the following function: openIndexFile()

func (h *fsHandler) openIndexFile(ctx *RequestCtx, dirPath string, mustCompress bool, fileEncoding string) (*fsFile, error) {
    for _, indexName := range h.indexNames {
        indexFilePath := dirPath + "/" + indexName
        ff, err := h.openFSFile(indexFilePath, mustCompress, fileEncoding)
        if err == nil {
            return ff, nil
        }
        if !errors.Is(err, fs.ErrNotExist) {
            return nil, fmt.Errorf("cannot open file %q: %w", indexFilePath, err)
        }
    }

    if !h.generateIndexPages {
        return nil, fmt.Errorf("cannot access directory without index page. Directory %q", dirPath)
    }

    return h.createDirIndex(ctx, dirPath, mustCompress, fileEncoding)
}

Because nowhere is checked if h.openFSFile() returns an error because the compressed file can't be written to the filesystem. The solution would be to check for the error like in handleRequest() and retry without compression:

func (h *fsHandler) handleRequest(ctx *RequestCtx) {
...
                var err error
        ff, err = h.openFSFile(filePath, mustCompress, fileEncoding)
        if mustCompress && err == errNoCreatePermission {
            ctx.Logger().Printf("insufficient permissions for saving compressed file for %q. Serving uncompressed file. "+
                "Allow write access to the directory with this file in order to improve fasthttp performance", filePath)
            mustCompress = false
            ff, err = h.openFSFile(filePath, mustCompress, fileEncoding)
        }
...
}

valyala / fasthttp

serving a compress enabled public folder without +w permissions results in a 404 #1766