Serve grpc.health.v1.Health on the build session so BuildKit does not drop it

lib/proto/health.proto

@@ -0,0 +1,22 @@
+syntax = "proto3";
+
+package grpc.health.v1;
+
+message HealthCheckRequest {
+  string service = 1;
+}
+
+message HealthCheckResponse {
+  enum ServingStatus {
+    UNKNOWN = 0;
+    SERVING = 1;
+    NOT_SERVING = 2;
+    SERVICE_UNKNOWN = 3;
+  }
+  ServingStatus status = 1;
+}
+
+service Health {
+  rpc Check(HealthCheckRequest) returns (HealthCheckResponse);
+  rpc Watch(HealthCheckRequest) returns (stream HealthCheckResponse);
+}

lib/session.js

@@ -51,6 +51,26 @@ function withSession(docker, auth, handler) {
       },
     });
 
+    // BuildKit health-checks the session's gRPC connection (grpc.health.v1.Health/Check)
+    // and tears the session down after a couple of consecutive failed checks
+    // (moby/buildkit session/grpc.go). Without a Health service the check returns
+    // UNIMPLEMENTED, so BuildKit drops the session before the build's first vertex and
+    // the build stalls (typically at "load metadata for <base image>"). Serve SERVING so
+    // the session stays up for the lifetime of the build.
+    const healthPkg = protoLoader.loadSync(
+      path.resolve(__dirname, "proto", "health.proto")
+    );
+    const healthService = grpc.loadPackageDefinition(healthPkg);
+
+    server.addService(healthService.grpc.health.v1.Health.service, {
+      Check(call, callback) {
+        callback(null, { status: "SERVING" });
+      },
+      Watch(call) {
+        call.write({ status: "SERVING" });
+      },
+    });
+
     function done() {
       server.forceShutdown();
       socket.end();