Fix config handling with empty config entries after merge

Fixes #8701

4 files changed, 58 insertions, 5 deletions

diff --git a/common/maps/params.go b/common/maps/params.go
index e5a1bd07d..c14026df7 100644
--- a/common/maps/params.go
+++ b/common/maps/params.go
@@ -52,6 +52,24 @@ func (p Params) Set(pp Params) {
 	}
 }
 
+// IsZero returns true if p is considered empty.
+func (p Params) IsZero() bool {
+	if p == nil || len(p) == 0 {
+		return true
+	}
+
+	if len(p) > 1 {
+		return false
+	}
+
+	for k, _ := range p {
+		return k == mergeStrategyKey
+	}
+
+	return false
+
+}
+
 // Merge transfers values from pp to p for new keys.
 // This is done recursively.
 func (p Params) Merge(pp Params) {
@@ -82,12 +100,9 @@ func (p Params) merge(ps ParamsMergeStrategy, pp Params) {
 				if pv, ok := v.(Params); ok {
 					vvv.merge(ms, pv)
 				}
-
 			}
-
 		} else if !noUpdate {
 			p[k] = v
-
 		}
 
 	}
diff --git a/common/maps/params_test.go b/common/maps/params_test.go
index 8859bb86b..5c799aae1 100644
--- a/common/maps/params_test.go
+++ b/common/maps/params_test.go
@@ -156,3 +156,15 @@ func TestParamsSetAndMerge(t *testing.T) {
 	})
 
 }
+
+func TestParamsIsZero(t *testing.T) {
+	c := qt.New(t)
+
+	var nilParams Params
+
+	c.Assert(Params{}.IsZero(), qt.IsTrue)
+	c.Assert(nilParams.IsZero(), qt.IsTrue)
+	c.Assert(Params{"foo": "bar"}.IsZero(), qt.IsFalse)
+	c.Assert(Params{"_merge": "foo", "foo": "bar"}.IsZero(), qt.IsFalse)
+	c.Assert(Params{"_merge": "foo"}.IsZero(), qt.IsTrue)
+}
diff --git a/config/defaultConfigProvider.go b/config/defaultConfigProvider.go
index a5e2d09fd..80353664e 100644
--- a/config/defaultConfigProvider.go
+++ b/config/defaultConfigProvider.go
@@ -214,6 +214,7 @@ func (c *defaultConfigProvider) Merge(k string, v interface{}) {
 		if p, ok := maps.ToParamsAndPrepare(v); ok {
 			// As there may be keys in p not in root, we need to handle
 			// those as a special case.
+			var keysToDelete []string
 			for kk, vv := range p {
 				if pp, ok := vv.(maps.Params); ok {
 					if pppi, ok := c.root[kk]; ok {
@@ -261,14 +262,19 @@ func (c *defaultConfigProvider) Merge(k string, v interface{}) {
 						strategy := c.determineMergeStrategy(KeyParams{Key: "", Params: c.root}, KeyParams{Key: kk, Params: np})
 						np.SetDefaultMergeStrategy(strategy)
 						np.Merge(pp)
-						if len(np) > 0 {
-							c.root[kk] = np
+						c.root[kk] = np
+						if np.IsZero() {
+							// Just keep it until merge is done.
+							keysToDelete = append(keysToDelete, kk)
 						}
 					}
 				}
 			}
 			// Merge the rest.
 			c.root.Merge(p)
+			for _, k := range keysToDelete {
+				delete(c.root, k)
+			}
 		} else {
 			panic(fmt.Sprintf("unsupported type %T received in Merge", v))
 		}
diff --git a/config/defaultConfigProvider_test.go b/config/defaultConfigProvider_test.go
index 6752ab2e5..7ab8c049a 100644
--- a/config/defaultConfigProvider_test.go
+++ b/config/defaultConfigProvider_test.go
@@ -283,6 +283,26 @@ func TestDefaultConfigProvider(t *testing.T) {
 
 	})
 
+	// Issue #8701
+	c.Run("Prevent _merge only maps", func(c *qt.C) {
+		cfg := New()
+
+		cfg.Set("", map[string]interface{}{
+			"B": "bv",
+		})
+
+		cfg.Merge("", map[string]interface{}{
+			"c": map[string]interface{}{
+				"_merge": "shallow",
+				"d":      "dv2",
+			},
+		})
+
+		c.Assert(cfg.Get(""), qt.DeepEquals, maps.Params{
+			"b": "bv",
+		})
+	})
+
 	c.Run("IsSet", func(c *qt.C) {
 		cfg := New()