Three different approaches to making it possible to cover logging outputs in code tests

makelogginglibmockable.go

// VERSION 1
err := c.cache.Set(sizeOnDiskCacheKey, []byte(sizeStrWithUnitSuffix))
if err != nil {
    // impossible to mock as logging.Error is unoverridable func def
    logging.Error("unable to store disk size in cache: %w", err)
}

// VERSION 2

// define local overridable log error wrapper
var logConnError = func(format string, args ...interface{}) {
    logging.Error(format, args...)
}
err := c.cache.Set(sizeOnDiskCacheKey, []byte(sizeStrWithUnitSuffix))
if err != nil {
    logConnError("unable to store disk size in cache: %w", err)
}

// VERSION 3
// define the same overridable log error wrapper but in a separate
// package called "log" which allows for similar calling symantics
err := c.cache.Set(sizeOnDiskCacheKey, []byte(sizeStrWithUnitSuffix))
if err != nil {
    // calling same wrapper as above except the
    // wrapping func declared in separate package
    log.Error("unable to store disk size in cache: %w", err)
}