Fix bug with merging vault agent configs that set template_config (#29680)

* fix bug with merging configs with TemplateConfig and add test

* add changelog

changelog/29680.txt

@@ -0,0 +1,4 @@
+```release-note:bug
+agent: Fixed an issue where giving the agent multiple config files could cause the merged config to be incorrect
+when `template_config` is set in one of the config files.
+```

command/agent/config/config.go

@@ -181,6 +181,9 @@ type ExecConfig struct {
 func NewConfig() *Config {
 	return &Config{
 		SharedConfig: new(configutil.SharedConfig),
+		TemplateConfig: &TemplateConfig{
+			MaxConnectionsPerHost: DefaultTemplateConfigMaxConnsPerHost,
+		},
 	}
 }
 
@@ -248,8 +251,13 @@ func (c *Config) Merge(c2 *Config) *Config {
 		result.DisableKeepAlivesTemplating = c2.DisableKeepAlivesTemplating
 	}
 
+	// Instead of checking if TemplateConfig is not nil, we compare against the default value
+	// that is set in NewConfig to determine if a TemplateConfig was specified in the config
+	defaultConfig := TemplateConfig{
+		MaxConnectionsPerHost: DefaultTemplateConfigMaxConnsPerHost,
+	}
 	result.TemplateConfig = c.TemplateConfig
-	if c2.TemplateConfig != nil {
+	if *c2.TemplateConfig != defaultConfig {
 		result.TemplateConfig = c2.TemplateConfig
 	}
 
@@ -1104,9 +1112,6 @@ func parseTemplateConfig(result *Config, list *ast.ObjectList) error {
 
 	templateConfigList := list.Filter(name)
 	if len(templateConfigList.Items) == 0 {
-		result.TemplateConfig = &TemplateConfig{
-			MaxConnectionsPerHost: DefaultTemplateConfigMaxConnsPerHost,
-		}
 		return nil
 	}
 

command/agent/config/config_test.go

@@ -1100,6 +1100,55 @@ func TestLoadConfigFile_TemplateConfig(t *testing.T) {
 	}
 }
 
+// TestLoadConfigFile_TemplateConfig_MergeConfigs tests that in the case of multiple config files,
+// a provided TemplateConfig in one file is properly preserved in the merged config
+func TestLoadConfigFile_TemplateConfig_MergeConfigs(t *testing.T) {
+	configPaths := []string{
+		"./test-fixtures/config-template_config.hcl",
+		"./test-fixtures/config-template-min-no-auth.hcl",
+	}
+
+	expected := &Config{
+		SharedConfig: &configutil.SharedConfig{},
+		Vault: &Vault{
+			Address: "http://127.0.0.1:1111",
+			Retry: &Retry{
+				NumRetries: 5,
+			},
+		},
+		TemplateConfig: &TemplateConfig{
+			ExitOnRetryFailure:    true,
+			StaticSecretRenderInt: 1 * time.Minute,
+			MaxConnectionsPerHost: 100,
+			LeaseRenewalThreshold: FloatPtr(0.8),
+		},
+		Templates: []*ctconfig.TemplateConfig{
+			{
+				Source:      pointerutil.StringPtr("/path/on/disk/to/template.ctmpl"),
+				Destination: pointerutil.StringPtr("/path/on/disk/where/template/will/render.txt"),
+			},
+			{
+				Source:      pointerutil.StringPtr("/path/on/disk/to/template2.ctmpl"),
+				Destination: pointerutil.StringPtr("/path/on/disk/where/template/will/render2.txt"),
+			},
+		},
+	}
+
+	cfg := NewConfig()
+	for _, path := range configPaths {
+		configFromPath, err := LoadConfigFile(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		cfg = cfg.Merge(configFromPath)
+	}
+
+	cfg.Prune()
+	if diff := deep.Equal(cfg, expected); diff != nil {
+		t.Fatal(diff)
+	}
+}
+
 // TestLoadConfigFile_Template tests template definitions in Vault Agent
 func TestLoadConfigFile_Template(t *testing.T) {
 	testCases := map[string]struct {

command/agent/config/test-fixtures/config-template-min-no-auth.hcl

@@ -0,0 +1,7 @@
+# Copyright (c) HashiCorp, Inc.
+# SPDX-License-Identifier: BUSL-1.1
+
+template {
+  source      = "/path/on/disk/to/template2.ctmpl"
+  destination = "/path/on/disk/where/template/will/render2.txt"
+}