config: Fix handling of invalid OS env config overrides

Fixes #8709

config/defaultConfigProvider.go

@@ -169,6 +169,9 @@ func (c *defaultConfigProvider) Set(k string, v interface{}) {
 	}
 
 	key, m := c.getNestedKeyAndMap(k, true)
+	if m == nil {
+		return
+	}
 
 	if existing, found := m[key]; found {
 		if p1, ok := existing.(maps.Params); ok {
@@ -289,6 +292,9 @@ func (c *defaultConfigProvider) Merge(k string, v interface{}) {
 	}
 
 	key, m := c.getNestedKeyAndMap(k, true)
+	if m == nil {
+		return
+	}
 
 	if existing, found := m[key]; found {
 		if p1, ok := existing.(maps.Params); ok {
@@ -422,7 +428,12 @@ func (c *defaultConfigProvider) getNestedKeyAndMap(key string, create bool) (str
 				return "", nil
 			}
 		}
-		current = next.(maps.Params)
+		var ok bool
+		current, ok = next.(maps.Params)
+		if !ok {
+			// E.g. a string, not a map that we can store values in.
+			return "", nil
+		}
 	}
 	return parts[len(parts)-1], current
 }

hugolib/config_test.go

@@ -576,6 +576,7 @@ anchor = "smart"
 quality = 75 
 `
 
+	newB := func(t testing.TB) *sitesBuilder {
 		b := newTestSitesBuilder(t).WithConfigFile("toml", baseConfig)
 
 		b.WithSourceFile("themes/mytheme/config.toml", `
@@ -595,6 +596,13 @@ theme_param="themevalue2"
 
 `)
 
+		return b
+	}
+
+	c.Run("Variations", func(c *qt.C) {
+
+		b := newB(c)
+
 		b.WithEnviron(
 			"HUGO_ENVIRONMENT", "test",
 			"HUGO_NEW", "new", // key not in config.toml
@@ -655,4 +663,23 @@ theme_param="themevalue2"
 		c.Assert(ofBase.MediaType, qt.Equals, media.TextType)
 		c.Assert(ofTheme.MediaType, qt.Equals, media.TextType)
 
+	})
+
+	// Issue #8709
+	c.Run("Set in string", func(c *qt.C) {
+		b := newB(c)
+
+		b.WithEnviron(
+			// imaging.anchor is a string, and it's not possible
+			// to set a child attribute.
+			"HUGO_IMAGING_ANCHOR_FOO", "top",
+		)
+
+		b.Build(BuildCfg{})
+
+		cfg := b.H.Cfg
+		c.Assert(cfg.Get("imaging.anchor"), qt.Equals, "smart")
+
+	})
+
 }