From e33e96cd9123a0892b2a89faebf5e30b0f2a6664 Mon Sep 17 00:00:00 2001
From: Andrew Wilkins <axw@elastic.co>
Date: Mon, 15 Feb 2021 11:43:08 +0800
Subject: [PATCH] sampling: require a default policy

---
 x-pack/apm-server/sampling/config.go         | 10 ++++++++++
 x-pack/apm-server/sampling/config_test.go    |  6 +++++-
 x-pack/apm-server/sampling/processor_test.go |  3 +++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/x-pack/apm-server/sampling/config.go b/x-pack/apm-server/sampling/config.go
index ce713d93ee0..9ba7794f2f9 100644
--- a/x-pack/apm-server/sampling/config.go
+++ b/x-pack/apm-server/sampling/config.go
@@ -46,6 +46,9 @@ type LocalSamplingConfig struct {
 	// Policies holds local tail-sampling policies. Policies are matched in the
 	// order provided. Policies should therefore be ordered from most to least
 	// specific.
+	//
+	// Policies must include at least one policy that matches all traces, to ensure
+	// that dropping non-matching traces is intentional.
 	Policies []Policy
 
 	// IngestRateDecayFactor holds the ingest rate decay factor, used for calculating
@@ -173,10 +176,17 @@ func (config LocalSamplingConfig) validate() error {
 	if len(config.Policies) == 0 {
 		return errors.New("Policies unspecified")
 	}
+	var anyDefaultPolicy bool
 	for i, policy := range config.Policies {
 		if err := policy.validate(); err != nil {
 			return errors.Wrapf(err, "Policy %d invalid", i)
 		}
+		if policy.PolicyCriteria == (PolicyCriteria{}) {
+			anyDefaultPolicy = true
+		}
+	}
+	if !anyDefaultPolicy {
+		return errors.New("Policies does not contain a default (empty criteria) policy")
 	}
 	if config.IngestRateDecayFactor <= 0 || config.IngestRateDecayFactor > 1 {
 		return errors.New("IngestRateDecayFactor unspecified or out of range (0,1]")
diff --git a/x-pack/apm-server/sampling/config_test.go b/x-pack/apm-server/sampling/config_test.go
index 7519dbd06b2..9db7225416f 100644
--- a/x-pack/apm-server/sampling/config_test.go
+++ b/x-pack/apm-server/sampling/config_test.go
@@ -38,7 +38,11 @@ func TestNewProcessorConfigInvalid(t *testing.T) {
 	config.MaxDynamicServices = 1
 
 	assertInvalidConfigError("invalid local sampling config: Policies unspecified")
-	config.Policies = []sampling.Policy{{}}
+	config.Policies = []sampling.Policy{{
+		PolicyCriteria: sampling.PolicyCriteria{ServiceName: "foo"},
+	}}
+	assertInvalidConfigError("invalid local sampling config: Policies does not contain a default (empty criteria) policy")
+	config.Policies[0].PolicyCriteria = sampling.PolicyCriteria{}
 	for _, invalid := range []float64{-1, 1.0, 2.0} {
 		config.Policies[0].SampleRate = invalid
 		assertInvalidConfigError("invalid local sampling config: Policy 0 invalid: SampleRate unspecified or out of range [0,1)")
diff --git a/x-pack/apm-server/sampling/processor_test.go b/x-pack/apm-server/sampling/processor_test.go
index ba412ba8ad3..71422558c05 100644
--- a/x-pack/apm-server/sampling/processor_test.go
+++ b/x-pack/apm-server/sampling/processor_test.go
@@ -292,6 +292,9 @@ func TestProcessLocalTailSamplingPolicyOrder(t *testing.T) {
 	}, {
 		PolicyCriteria: sampling.PolicyCriteria{ServiceName: "service_name"},
 		SampleRate:     0.1,
+	}, {
+		PolicyCriteria: sampling.PolicyCriteria{},
+		SampleRate:     0,
 	}}
 	config.FlushInterval = 10 * time.Millisecond
 	published := make(chan string)