From 71cc0f167537667a878ec5995cf8167c69c362df Mon Sep 17 00:00:00 2001
From: Matthew Stevenson <52979934+matthewstevenson88@users.noreply.github.com>
Date: Thu, 28 Dec 2023 14:33:59 -0800
Subject: [PATCH] Revert "alts: Queue ALTS handshakes once limit is reached
 rather than dropping. (#6884)" (#6903)

This reverts commit adc76852e0971c76dd470e5e629360aeea412d55.
---
 credentials/alts/alts_test.go                        |  2 +-
 credentials/alts/internal/handshaker/handshaker.go   | 10 ++++++----
 .../alts/internal/handshaker/handshaker_test.go      | 12 ++++++------
 3 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/credentials/alts/alts_test.go b/credentials/alts/alts_test.go
index d693541f15e3..20062fe77539 100644
--- a/credentials/alts/alts_test.go
+++ b/credentials/alts/alts_test.go
@@ -397,7 +397,7 @@ func establishAltsConnection(t *testing.T, handshakerAddress, serverAddress stri
 		if err == nil {
 			break
 		}
-		if code := status.Code(err); code == codes.Unavailable || code == codes.DeadlineExceeded {
+		if code := status.Code(err); code == codes.Unavailable {
 			// The server is not ready yet. Try again.
 			continue
 		}
diff --git a/credentials/alts/internal/handshaker/handshaker.go b/credentials/alts/internal/handshaker/handshaker.go
index 6c867dd85015..64890797f30d 100644
--- a/credentials/alts/internal/handshaker/handshaker.go
+++ b/credentials/alts/internal/handshaker/handshaker.go
@@ -61,6 +61,8 @@ var (
 	// control number of concurrent created (but not closed) handshakes.
 	clientHandshakes = semaphore.NewWeighted(int64(envconfig.ALTSMaxConcurrentHandshakes))
 	serverHandshakes = semaphore.NewWeighted(int64(envconfig.ALTSMaxConcurrentHandshakes))
+	// errDropped occurs when maxPendingHandshakes is reached.
+	errDropped = errors.New("maximum number of concurrent ALTS handshakes is reached")
 	// errOutOfBound occurs when the handshake service returns a consumed
 	// bytes value larger than the buffer that was passed to it originally.
 	errOutOfBound = errors.New("handshaker service consumed bytes value is out-of-bound")
@@ -154,8 +156,8 @@ func NewServerHandshaker(ctx context.Context, conn *grpc.ClientConn, c net.Conn,
 // ClientHandshake starts and completes a client ALTS handshake for GCP. Once
 // done, ClientHandshake returns a secure connection.
 func (h *altsHandshaker) ClientHandshake(ctx context.Context) (net.Conn, credentials.AuthInfo, error) {
-	if err := clientHandshakes.Acquire(ctx, 1); err != nil {
-		return nil, nil, err
+	if !clientHandshakes.TryAcquire(1) {
+		return nil, nil, errDropped
 	}
 	defer clientHandshakes.Release(1)
 
@@ -207,8 +209,8 @@ func (h *altsHandshaker) ClientHandshake(ctx context.Context) (net.Conn, credent
 // ServerHandshake starts and completes a server ALTS handshake for GCP. Once
 // done, ServerHandshake returns a secure connection.
 func (h *altsHandshaker) ServerHandshake(ctx context.Context) (net.Conn, credentials.AuthInfo, error) {
-	if err := serverHandshakes.Acquire(ctx, 1); err != nil {
-		return nil, nil, err
+	if !serverHandshakes.TryAcquire(1) {
+		return nil, nil, errDropped
 	}
 	defer serverHandshakes.Release(1)
 
diff --git a/credentials/alts/internal/handshaker/handshaker_test.go b/credentials/alts/internal/handshaker/handshaker_test.go
index 9f877c48dcf3..956e0fbd1c20 100644
--- a/credentials/alts/internal/handshaker/handshaker_test.go
+++ b/credentials/alts/internal/handshaker/handshaker_test.go
@@ -193,10 +193,10 @@ func (s) TestClientHandshake(t *testing.T) {
 			}()
 		}
 
-		// Ensure that there are no errors.
+		// Ensure all errors are expected.
 		for i := 0; i < testCase.numberOfHandshakes; i++ {
-			if err := <-errc; err != nil {
-				t.Errorf("ClientHandshake() = _, %v, want _, <nil>", err)
+			if err := <-errc; err != nil && err != errDropped {
+				t.Errorf("ClientHandshake() = _, %v, want _, <nil> or %v", err, errDropped)
 			}
 		}
 
@@ -250,10 +250,10 @@ func (s) TestServerHandshake(t *testing.T) {
 			}()
 		}
 
-		// Ensure that there are no errors.
+		// Ensure all errors are expected.
 		for i := 0; i < testCase.numberOfHandshakes; i++ {
-			if err := <-errc; err != nil {
-				t.Errorf("ServerHandshake() = _, %v, want _, <nil>", err)
+			if err := <-errc; err != nil && err != errDropped {
+				t.Errorf("ServerHandshake() = _, %v, want _, <nil> or %v", err, errDropped)
 			}
 		}