agent: remove data race in agent config (#20200)

To fix an issue displaying the current reloaded config in the 
v1/agent/self endpoint #18681 caused the agent's internal 
config struct member to be deepcopied and replaced on reload.

This is not safe because the field is not protected by a lock, nor 
should it be due to how it is accessed by the rest of the system.

This PR does the same deepcopy, but into a new field solely for 
the point of capturing the current reloaded values for display 
purposes. If there has been no reload then the original config is used.

agent/agent.go

@@ -24,18 +24,19 @@ import (
 
 	"github.com/armon/go-metrics"
 	"github.com/armon/go-metrics/prometheus"
+	"github.com/rboyer/safeio"
+	"golang.org/x/net/http2"
+	"golang.org/x/net/http2/h2c"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/keepalive"
+
 	"github.com/hashicorp/go-connlimit"
 	"github.com/hashicorp/go-hclog"
 	"github.com/hashicorp/go-memdb"
 	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/hcp-scada-provider/capability"
 	"github.com/hashicorp/raft"
 	"github.com/hashicorp/serf/serf"
-	"github.com/rboyer/safeio"
-	"golang.org/x/net/http2"
-	"golang.org/x/net/http2/h2c"
-	"google.golang.org/grpc"
-	"google.golang.org/grpc/keepalive"
 
 	"github.com/hashicorp/consul/acl"
 	"github.com/hashicorp/consul/acl/resolver"
@@ -240,6 +241,9 @@ type Agent struct {
 	// config is the agent configuration.
 	config *config.RuntimeConfig
 
+	displayOnlyConfigCopy     *config.RuntimeConfig
+	displayOnlyConfigCopyLock sync.Mutex
+
 	// Used for writing our logs
 	logger hclog.InterceptLogger
 
@@ -4438,11 +4442,22 @@ func (a *Agent) reloadConfigInternal(newCfg *config.RuntimeConfig) error {
 	a.config.EnableDebug = newCfg.EnableDebug
 
 	// update Agent config with new config
-	a.config = newCfg.DeepCopy()
+	a.displayOnlyConfigCopyLock.Lock()
+	a.displayOnlyConfigCopy = newCfg.DeepCopy()
+	a.displayOnlyConfigCopyLock.Unlock()
 
 	return nil
 }
 
+func (a *Agent) getRuntimeConfigForDisplay() *config.RuntimeConfig {
+	a.displayOnlyConfigCopyLock.Lock()
+	defer a.displayOnlyConfigCopyLock.Unlock()
+	if a.displayOnlyConfigCopy != nil {
+		return a.displayOnlyConfigCopy.DeepCopy()
+	}
+	return a.config
+}
+
 // LocalBlockingQuery performs a blocking query in a generic way against
 // local agent state that has no RPC or raft to back it. It uses `hash` parameter
 // instead of an `index`.

agent/agent_endpoint.go

@@ -11,14 +11,15 @@ import (
 	"strings"
 	"time"
 
+	"github.com/mitchellh/hashstructure"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+
 	"github.com/hashicorp/go-bexpr"
 	"github.com/hashicorp/go-hclog"
 	"github.com/hashicorp/go-memdb"
 	"github.com/hashicorp/serf/coordinate"
 	"github.com/hashicorp/serf/serf"
-	"github.com/mitchellh/hashstructure"
-	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promhttp"
 
 	"github.com/hashicorp/consul/acl"
 	cachetype "github.com/hashicorp/consul/agent/cache-types"
@@ -89,22 +90,24 @@ func (s *HTTPHandlers) AgentSelf(resp http.ResponseWriter, req *http.Request) (i
 		}
 	}
 
+	displayConfig := s.agent.getRuntimeConfigForDisplay()
+
 	var xds *XDSSelf
 	if s.agent.xdsServer != nil {
 		xds = &XDSSelf{
 			SupportedProxies: map[string][]string{
 				"envoy": xdscommon.EnvoyVersions,
 			},
 			// Prefer the TLS port. See comment on the XDSSelf struct for details.
-			Port: s.agent.config.GRPCTLSPort,
+			Port: displayConfig.GRPCTLSPort,
 			Ports: GRPCPorts{
-				Plaintext: s.agent.config.GRPCPort,
-				TLS:       s.agent.config.GRPCTLSPort,
+				Plaintext: displayConfig.GRPCPort,
+				TLS:       displayConfig.GRPCTLSPort,
 			},
 		}
 		// Fallback to standard port if TLS is not enabled.
-		if s.agent.config.GRPCTLSPort <= 0 {
-			xds.Port = s.agent.config.GRPCPort
+		if displayConfig.GRPCTLSPort <= 0 {
+			xds.Port = displayConfig.GRPCPort
 		}
 	}
 
@@ -119,22 +122,22 @@ func (s *HTTPHandlers) AgentSelf(resp http.ResponseWriter, req *http.Request) (i
 		Version           string
 		BuildDate         string
 	}{
-		Datacenter:        s.agent.config.Datacenter,
-		PrimaryDatacenter: s.agent.config.PrimaryDatacenter,
-		NodeName:          s.agent.config.NodeName,
-		NodeID:            string(s.agent.config.NodeID),
-		Partition:         s.agent.config.PartitionOrEmpty(),
-		Revision:          s.agent.config.Revision,
-		Server:            s.agent.config.ServerMode,
+		Datacenter:        displayConfig.Datacenter,
+		PrimaryDatacenter: displayConfig.PrimaryDatacenter,
+		NodeName:          displayConfig.NodeName,
+		NodeID:            string(displayConfig.NodeID),
+		Partition:         displayConfig.PartitionOrEmpty(),
+		Revision:          displayConfig.Revision,
+		Server:            displayConfig.ServerMode,
 		// We expect the ent version to be part of the reported version string, and that's now part of the metadata, not the actual version.
-		Version:   s.agent.config.VersionWithMetadata(),
-		BuildDate: s.agent.config.BuildDate.Format(time.RFC3339),
+		Version:   displayConfig.VersionWithMetadata(),
+		BuildDate: displayConfig.BuildDate.Format(time.RFC3339),
 	}
 
 	return Self{
 		Config:      config,
-		DebugConfig: s.agent.config.Sanitized(),
-		Coord:       cs[s.agent.config.SegmentName],
+		DebugConfig: displayConfig.Sanitized(),
+		Coord:       cs[displayConfig.SegmentName],
 		Member:      s.agent.AgentLocalMember(),
 		Stats:       s.agent.Stats(),
 		Meta:        s.agent.State.Metadata(),