backends: Fix crash when interrupting during interactive prompt for values' (#36448)

* backends: Convert null values to empty objects before coercion

* backends: add test case for interrupt crash

* Add changelog

.changes/backported/BUG FIXES-20250206-145217.yaml

@@ -0,0 +1,5 @@
+kind: BUG FIXES
+body: 'backends: Fix crash when interrupting during interactive prompt for values'
+time: 2025-02-06T14:52:17.033964+01:00
+custom:
+    Issue: "36448"

internal/backend/backendbase/base.go

@@ -57,6 +57,13 @@ func (b Base) ConfigSchema() *configschema.Block {
 func (b Base) PrepareConfig(configVal cty.Value) (cty.Value, tfdiags.Diagnostics) {
 	var diags tfdiags.Diagnostics
 
+	if configVal.IsNull() {
+		// We expect the backend configuration to be an object, so if it's
+		// null for some reason (e.g. because of an interrupt), we'll turn
+		// it into an empty object so that we can still coerce it
+		configVal = cty.EmptyObjectVal
+	}
+
 	schema := b.Schema
 
 	v, err := schema.CoerceValue(configVal)

internal/backend/backendbase/base_test.go

@@ -214,3 +214,41 @@ func TestBase_deprecatedArg(t *testing.T) {
 		}
 	})
 }
+
+func TestBase_nullCrash(t *testing.T) {
+	// This test ensures that we don't crash while applying defaults to
+	// a null value
+
+	b := Base{
+		Schema: &configschema.Block{
+			Attributes: map[string]*configschema.Attribute{
+				"foo": {
+					Type:     cty.String,
+					Required: true,
+				},
+			},
+		},
+		SDKLikeDefaults: SDKLikeDefaults{
+			"foo": {
+				Fallback: "fallback",
+			},
+		},
+	}
+
+	t.Run("error", func(t *testing.T) {
+		// We pass an explicit null value here to simulate an interrupt
+		_, gotDiags := b.PrepareConfig(cty.NullVal(cty.Object(map[string]cty.Type{
+			"foo": cty.String,
+		})))
+		var wantDiags tfdiags.Diagnostics
+		wantDiags = wantDiags.Append(
+			&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Invalid backend configuration",
+				Detail:   "The backend configuration is incorrect: attribute \"foo\" is required.",
+			})
+		if diff := cmp.Diff(wantDiags.ForRPC(), gotDiags.ForRPC()); diff != "" {
+			t.Errorf("wrong diagnostics\n%s", diff)
+		}
+	})
+}