balancer: introduce env flag for case-sensitive registry (#8837)

Part of #5288

Follow up of PR: #6647 

This PR introduces the
`GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES` environment
variable to transition balancer registries to be case-sensitive.
 
 **Default Behavior**: 
    *   The env var is disabled by default.
* The registry remains case-insensitive (legacy behavior) and a warning
is logged if a balancer name contains uppercase letters, notifying users
of the upcoming change.

RELEASE NOTES:
* balancer: grpc will log a warning if balancer registries are used with
uppercase letter(s).

Author: Pranjali-2501

balancer/balancer.go

@@ -33,6 +33,7 @@ import (
 	estats "google.golang.org/grpc/experimental/stats"
 	"google.golang.org/grpc/grpclog"
 	"google.golang.org/grpc/internal"
+	"google.golang.org/grpc/internal/envconfig"
 	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/resolver"
 	"google.golang.org/grpc/serviceconfig"
@@ -46,21 +47,21 @@ var (
 )
 
 // Register registers the balancer builder to the balancer map. b.Name
-// (lowercased) will be used as the name registered with this builder.  If the
-// Builder implements ConfigParser, ParseConfig will be called when new service
+// will be used as the name registered with this builder. If the Builder
+// implements ConfigParser, ParseConfig will be called when new service
 // configs are received by the resolver, and the result will be provided to the
 // Balancer in UpdateClientConnState.
 //
 // NOTE: this function must only be called during initialization time (i.e. in
 // an init() function), and is not thread-safe. If multiple Balancers are
 // registered with the same name, the one registered last will take effect.
 func Register(b Builder) {
-	name := strings.ToLower(b.Name())
-	if name != b.Name() {
-		// TODO: Skip the use of strings.ToLower() to index the map after v1.59
-		// is released to switch to case sensitive balancer registry. Also,
-		// remove this warning and update the docstrings for Register and Get.
-		logger.Warningf("Balancer registered with name %q. grpc-go will be switching to case sensitive balancer registries soon", b.Name())
+	name := b.Name()
+	if !envconfig.CaseSensitiveBalancerRegistries {
+		name = strings.ToLower(name)
+		if name != b.Name() {
+			logger.Warningf("Balancer registered with name %q. grpc-go will be switching to case sensitive balancer registries soon. After 2 releases, we will enable the env var by default.", b.Name())
+		}
 	}
 	m[name] = b
 }
@@ -78,16 +79,17 @@ func init() {
 }
 
 // Get returns the resolver builder registered with the given name.
-// Note that the compare is done in a case-insensitive fashion.
+// Note that the compare is done in a case-sensitive fashion.
 // If no builder is register with the name, nil will be returned.
 func Get(name string) Builder {
-	if strings.ToLower(name) != name {
-		// TODO: Skip the use of strings.ToLower() to index the map after v1.59
-		// is released to switch to case sensitive balancer registry. Also,
-		// remove this warning and update the docstrings for Register and Get.
-		logger.Warningf("Balancer retrieved for name %q. grpc-go will be switching to case sensitive balancer registries soon", name)
+	if !envconfig.CaseSensitiveBalancerRegistries {
+		lowerName := strings.ToLower(name)
+		if lowerName != name {
+			logger.Warningf("Balancer retrieved for name %q. grpc-go will be switching to case sensitive balancer registries soon. After 2 releases, we will enable the env var by default.", name)
+		}
+		name = lowerName
 	}
-	if b, ok := m[strings.ToLower(name)]; ok {
+	if b, ok := m[name]; ok {
 		return b
 	}
 	return nil

internal/envconfig/envconfig.go

@@ -78,6 +78,14 @@ var (
 	//   - The DNS resolver is being used.
 	EnableDefaultPortForProxyTarget = boolFromEnv("GRPC_EXPERIMENTAL_ENABLE_DEFAULT_PORT_FOR_PROXY_TARGET", true)
 
+	// CaseSensitiveBalancerRegistries is set if the balancer registry should be
+	// case-sensitive. This is disabled by default, but can be enabled by setting
+	// the env variable "GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES"
+	// to "true".
+	//
+	// TODO: After 2 releases, we will enable the env var by default.
+	CaseSensitiveBalancerRegistries = boolFromEnv("GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES", false)
+
 	// XDSAuthorityRewrite indicates whether xDS authority rewriting is enabled.
 	// This feature is defined in gRFC A81 and is enabled by setting the
 	// environment variable GRPC_EXPERIMENTAL_XDS_AUTHORITY_REWRITE to "true".