Missing configuration file is silently ignored

[candlerb]

snmp_exporter reads --config.file or by default snmp.yml. However, these go via glob expansion. If there are no matching files, then no error is logged and the exporter will run with an empty configuration, which is not useful.

A solution would be to generate a warning when any expansion is empty, and also to abort if the final configuration has no auths or modules.

A simpler solution would be to fail if any glob expansion is empty:

--- a/config/config.go
+++ b/config/config.go
@@ -32,6 +32,9 @@ func LoadFile(paths []string, expandEnvVars bool) (*Config, error) {
                if err != nil {
                        return nil, err
                }
+               if len(files) == 0 {
+                       return nil, fmt.Errorf("No files matching '%s'", p)
+               }
                for _, f := range files {
                        content, err := os.ReadFile(f)
                        if err != nil {

However that is not backwards-compatible, e.g. if someone is relying on --config.file=/etc/snmp_exporter.d/*.yml when that directory may be legitimately empty. (Although the error could be downgraded to a warning if p contains a wildcard character?)