ca: add some godoc and func for finding leaf signing cert

This will be used in a follow up commit.

agent/consul/leader_connect_ca.go

@@ -1149,10 +1149,8 @@ func (c *CAManager) RenewIntermediate(ctx context.Context, isPrimary bool) error
 
 	// If this is the primary, check if this is a provider that uses an intermediate cert. If
 	// it isn't, we don't need to check for a renewal.
-	if isPrimary {
-		if _, ok := provider.(ca.PrimaryUsesIntermediate); !ok {
-			return nil
-		}
+	if isPrimary && !primaryUsesIntermediate(provider) {
+		return nil
 	}
 
 	activeIntermediate, err := provider.ActiveIntermediate()
@@ -1536,3 +1534,16 @@ func (c *CAManager) checkExpired(pem string) error {
 	}
 	return nil
 }
+
+func primaryUsesIntermediate(provider ca.Provider) bool {
+	_, ok := provider.(ca.PrimaryUsesIntermediate)
+	return ok
+}
+
+func (c *CAManager) isIntermediateUsedToSignLeaf() bool {
+	if c.serverConf.Datacenter != c.serverConf.PrimaryDatacenter {
+		return true
+	}
+	provider, _ := c.getCAProvider()
+	return primaryUsesIntermediate(provider)
+}

agent/structs/connect_ca.go

@@ -86,11 +86,20 @@ type CARoot struct {
 	NotBefore time.Time
 	NotAfter  time.Time
 
-	// RootCert is the PEM-encoded public certificate.
+	// RootCert is the PEM-encoded public certificate for the root CA. The
+	// certificate is the same for all federated clusters.
 	RootCert string
 
 	// IntermediateCerts is a list of PEM-encoded intermediate certs to
-	// attach to any leaf certs signed by this CA.
+	// attach to any leaf certs signed by this CA. The list may include a
+	// certificate cross-signed by an old root CA, any subordinate CAs below the
+	// root CA, and the intermediate CA used to sign leaf certificates in the
+	// local Datacenter.
+	//
+	// If the provider which created this root uses an intermediate to sign
+	// leaf certificates (Vault provider), or this is a secondary Datacenter then
+	// the intermediate used to sign leaf certificates will be the last in the
+	// list.
 	IntermediateCerts []string
 
 	// SigningCert is the PEM-encoded signing certificate and SigningKey