From a220cce77eb81ce57f34e4ed83e723575953c272 Mon Sep 17 00:00:00 2001
From: GnomeZworc <nicolas.boufidjeline@g3e.fr>
Date: Wed, 9 Sep 2026 21:57:51 +0200
Subject: [PATCH] f-46: config: fail loudly on an unreadable config file #46

Signed-off-by: GnomeZworc <nicolas.boufidjeline@g3e.fr>
---
 internal/config/agent/dhcp_test.go | 26 ++++++++++++++++++++++++++
 internal/config/agent/struct.go    |  8 +++++++-
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/internal/config/agent/dhcp_test.go b/internal/config/agent/dhcp_test.go
index 6796e56..50fed1c 100644
--- a/internal/config/agent/dhcp_test.go
+++ b/internal/config/agent/dhcp_test.go
@@ -54,3 +54,29 @@ func TestLoadConfig_ReadsTheTwoBackend(t *testing.T) {
 		t.Errorf("backend = %q, want two", cfg.DHCP.Backend)
 	}
 }
+
+func TestLoadConfig_MalformedFileIsReported(t *testing.T) {
+	path := writeConfig(t, "--\n\ndhcp:\n  backend: two\n")
+
+	if _, err := LoadConfig(path); err == nil {
+		t.Fatal("an unparseable config must be reported: silently falling back to defaults would run the wrong dhcp backend")
+	}
+}
+
+func TestLoadConfig_TabIndentedFileIsReported(t *testing.T) {
+	path := writeConfig(t, "dhcp:\n\tbackend: two\n")
+
+	if _, err := LoadConfig(path); err == nil {
+		t.Fatal("yaml forbids tabs for indentation: that must surface, not be swallowed")
+	}
+}
+
+func TestLoadConfig_MissingFileStillFallsBackToDefaults(t *testing.T) {
+	cfg, err := LoadConfig(filepath.Join(t.TempDir(), "absent.yml"))
+	if err != nil {
+		t.Fatalf("an absent file remains valid, only an unreadable one is an error: %v", err)
+	}
+	if cfg.DHCP.Backend != BackendDnsmasq {
+		t.Errorf("backend = %q, want %q", cfg.DHCP.Backend, BackendDnsmasq)
+	}
+}
diff --git a/internal/config/agent/struct.go b/internal/config/agent/struct.go
index d4afea1..0f84a09 100644
--- a/internal/config/agent/struct.go
+++ b/internal/config/agent/struct.go
@@ -1,6 +1,10 @@
 package configuration
 
 import (
+	"errors"
+	"fmt"
+	"os"
+
 	"github.com/spf13/viper"
 )
 
@@ -86,7 +90,9 @@ func LoadConfig(path string) (*Config, error) {
 	v.SetDefault("logger.level", "info")
 	v.SetDefault("logger.debug", false)
 
-	v.ReadInConfig()
+	if err := v.ReadInConfig(); err != nil && !errors.Is(err, os.ErrNotExist) {
+		return nil, fmt.Errorf("read %s: %w", path, err)
+	}
 
 	var cfg Config
 	if err := v.Unmarshal(&cfg); err != nil {