Emit vars even if provider data is empty from the start

We only signal from providers if the emitted data is different from the
current state. This
caused a bug where the data wasn't ever emitted if it was empty. The
provider controller now distinguishes between no data (yet) and empty
data. As a result, it's again possible to effectively disable all
providers by using a local provider with no data.

This is a regression caused by
#6114. It doesn't apply to
the main and 8.x branches due to
#6169 refactoring provider
initialisation in those.

changelog/fragments/1737735780-fix-empty-provider-data.yaml

@@ -0,0 +1,32 @@
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: bug-fix
+
+# Change summary; a 80ish characters long description of the change.
+summary: Emit vars even if provider data is empty from the start
+
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+# NOTE: This field will be rendered only for breaking-change and known-issue kinds at the moment.
+#description:
+
+# Affected component; usually one of "elastic-agent", "fleet-server", "filebeat", "metricbeat", "auditbeat", "all", etc.
+component: elastic-agent
+
+# PR URL; optional; the PR number that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+#pr: https://github.com/owner/repo/1234
+
+# Issue URL; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+#issue: https://github.com/owner/repo/1234

internal/pkg/composable/controller.go

@@ -79,12 +79,10 @@ func New(log *logger.Logger, c *config.Config, managed bool) (Controller, error)
 		if err != nil {
 			return nil, errors.New(err, fmt.Sprintf("failed to build provider '%s'", name), errors.TypeConfig, errors.M("provider", name))
 		}
-		emptyMapping, _ := transpiler.NewAST(nil)
 		contextProviders[name] = &contextProviderState{
 			// Safe for Context to be nil here because it will be filled in
 			// by (*controller).Run before the provider is started.
 			provider: provider,
-			mapping:  emptyMapping,
 		}
 	}
 
@@ -279,7 +277,10 @@ func (c *controller) generateVars(fetchContextProviders mapstr.M) []*transpiler.
 	vars := make([]*transpiler.Vars, 1)
 	mapping, _ := transpiler.NewAST(map[string]any{})
 	for name, state := range c.contextProviders {
-		_ = mapping.Insert(state.Current(), name)
+		providerMapping := state.Current()
+		if providerMapping != nil { // the provider may not have emitted any data yet
+			_ = mapping.Insert(providerMapping, name)
+		}
 	}
 	// this is ensured not to error, by how the mappings states are verified
 	vars[0] = transpiler.NewVarsFromAst("", mapping, fetchContextProviders)

internal/pkg/composable/controller_test.go

@@ -146,6 +146,16 @@ func TestProvidersDefaultDisabled(t *testing.T) {
 			},
 			want: 0,
 		},
+		{
+			name: "default disabled, local provider without data",
+			cfg: map[string]interface{}{
+				"agent.providers.initial_default": "false",
+				"providers": map[string]any{
+					"local": map[string]any{},
+				},
+			},
+			want: 1,
+		},
 		{
 			name: "default enabled",
 			cfg: map[string]interface{}{