fix: keep sse session alive with comment lines

DEVELOPMENT.md

@@ -36,7 +36,7 @@ and allow sufficient network access:
 
 ```
 sbx create shell --name workbench .
-sbx policy allow network --sandbox workbench '*.docker.com:443,production.cloudfront.docker.com:443,*.docker.io:443,openvsx.eclipsecontent.org:443,electronjs.org:443,*.electronjs.org:443,fonts.googleapis.com:443,github-cloud.githubusercontent.com:443,raw.githubusercontent.com:443,release-assets.githubusercontent.com:443,github.com:443,*.github.com:443,fonts.gstatic.com:443,*.lean-lang.org:443,playwright.download.prss.microsoft.com:443,nodejs.org:443,*.nodejs.com:443,deb.nodesource.com:443,*.npmjs.org:443,open-vsx.org:443,*.playwright.dev:443,checkpoint.prisma.io:443,binaries.prisma.sh:443,www.schemastore.org:443,ports.ubuntu.com:80,ports.ubuntu.com:443'
+sbx policy allow network --sandbox workbench '*.docker.com:443,production.cloudfront.docker.com:443,*.docker.io:443,openvsx.eclipsecontent.org:443,electronjs.org:443,*.electronjs.org:443,fonts.googleapis.com:443,github-cloud.githubusercontent.com:443,raw.githubusercontent.com:443,release-assets.githubusercontent.com:443,github.com:443,*.github.com:443,fonts.gstatic.com:443,*.lean-lang.org:443,playwright.download.prss.microsoft.com:443,nodejs.org:443,*.nodejs.com:443,deb.nodesource.com:443,*.npmjs.org:443,open-vsx.org:443,*.playwright.dev:443,checkpoint.prisma.io:443,binaries.prisma.sh:443,www.schemastore.org:443,ports.ubuntu.com:80,ports.ubuntu.com:443,lakecache.blob.core.windows.net:443'
 ```
 
 The sandbox can be started by running

src/lib/server/util.ts

@@ -111,6 +111,8 @@ export function sseStreamResponse(onCancel?: () => void): [Response, (msg: objec
   let close: () => void = () => {}
   let closed = false
   const encoder = new TextEncoder()
+  let keepAliveTimeout: ReturnType<typeof setInterval> | undefined = undefined
+
   const stream = new ReadableStream({
     start(controller) {
       send = msg => {
@@ -120,11 +122,20 @@ export function sseStreamResponse(onCancel?: () => void): [Response, (msg: objec
       close = () => {
         if (closed) return
         closed = true
+        clearInterval(keepAliveTimeout)
+        keepAliveTimeout = undefined
         controller.close()
       }
+      // nginx will close connections that don't send some message in 60s
+      keepAliveTimeout = setInterval(() => {
+        if (closed) return
+        controller.enqueue(encoder.encode(`:\n`))
+      }, 10_000)
     },
     cancel() {
       closed = true
+      clearInterval(keepAliveTimeout)
+      keepAliveTimeout = undefined
       if (onCancel) onCancel()
     },
   })