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fix(sandbox): stop the dev-server probe leaking a keep-alive connection every tick #6830

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fix(sandbox): stop the dev-server probe leaking a keep-alive connection every tick #6830

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6 changes: 5 additions & 1 deletion packages/sandbox/daemon-go/internal/probe/probe.go
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Original file line number Diff line number Diff line change
Expand Up @@ -168,10 +168,14 @@ func itoa(n int) string {
}

func head(port int) (status int, isHtml, up bool) {
// A fresh Transport is built (and discarded) on every tick, so keep-alive
// would leak an idle connection + its readLoop goroutine per tick for the
// life of the daemon instead of being reused.
client := &http.Client{
Timeout: HeadTimeout,
Transport: &http.Transport{
DialContext: DialLoopback,
DialContext: DialLoopback,
DisableKeepAlives: true,
},
}
req, err := http.NewRequest("HEAD", "http://loopback:"+itoa(port)+"/", nil)
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49 changes: 49 additions & 0 deletions packages/sandbox/daemon-go/internal/probe/probe_test.go
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Original file line number Diff line number Diff line change
@@ -0,0 +1,49 @@
package probe

import (
"net/http"
"net/http/httptest"
"runtime"
"strconv"
"testing"
"time"
)

// TestHeadDoesNotLeakKeepAliveGoroutine guards against regressing to a
// per-tick Transport that leaves an idle keep-alive connection (and its
// readLoop goroutine) open forever, since head() is called repeatedly for
// the life of the daemon.
func TestHeadDoesNotLeakKeepAliveGoroutine(t *testing.T) {
srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.WriteHeader(200)
}))
defer srv.Close()

port, err := strconv.Atoi(srv.URL[len("http://127.0.0.1:"):])

@cubic-dev-ai cubic-dev-ai Bot Sep 1, 2026

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P3: On IPv6-only hosts, this port extraction fails because httptest.NewServer can return a URL such as http://[::1]:<port>. Parse srv.URL with net/url and net.SplitHostPort so the regression test works on every loopback address supported by DialLoopback.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At packages/sandbox/daemon-go/internal/probe/probe_test.go, line 22:

<comment>On IPv6-only hosts, this port extraction fails because `httptest.NewServer` can return a URL such as `http://[::1]:<port>`. Parse `srv.URL` with `net/url` and `net.SplitHostPort` so the regression test works on every loopback address supported by `DialLoopback`.</comment>

<file context>
@@ -0,0 +1,49 @@
+	}))
+	defer srv.Close()
+
+	port, err := strconv.Atoi(srv.URL[len("http://127.0.0.1:"):])
+	if err != nil {
+		t.Fatalf("parse port from %q: %v", srv.URL, err)
</file context>

if err != nil {
t.Fatalf("parse port from %q: %v", srv.URL, err)
}

before := runtime.NumGoroutine()

@cubic-dev-ai cubic-dev-ai Bot Sep 1, 2026

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P3: This asserts on the global runtime.NumGoroutine(), so unrelated goroutines (parallel tests, race-detector/GC wakeups, runtime background activity) can transiently push the count above before+5, making the test flaky even when nothing leaks. The threshold and 2s wait reduce but do not remove that risk. Tighten it by asserting the specific readLoop goroutine count (e.g. report via the leak that this guards), or widen the tolerance and note the flakiness.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At packages/sandbox/daemon-go/internal/probe/probe_test.go, line 27:

<comment>This asserts on the global `runtime.NumGoroutine()`, so unrelated goroutines (parallel tests, race-detector/GC wakeups, runtime background activity) can transiently push the count above `before+5`, making the test flaky even when nothing leaks. The threshold and 2s wait reduce but do not remove that risk. Tighten it by asserting the specific readLoop goroutine count (e.g. report via the leak that this guards), or widen the tolerance and note the flakiness.</comment>

<file context>
@@ -0,0 +1,49 @@
+		t.Fatalf("parse port from %q: %v", srv.URL, err)
+	}
+
+	before := runtime.NumGoroutine()
+
+	for i := 0; i < 20; i++ {
</file context>


for i := 0; i < 20; i++ {
status, _, up := head(port)
if !up || status != 200 {
t.Fatalf("head() = status=%d up=%v, want 200/true", status, up)
}
}

// Idle keep-alive readLoop goroutines don't exit synchronously; give them
// a moment, then assert we haven't accumulated one per call.
deadline := time.Now().Add(2 * time.Second)
for {
if runtime.NumGoroutine() <= before+5 {
return
}
if time.Now().After(deadline) {
t.Fatalf("goroutine count grew from %d to %d after 20 head() calls — keep-alive connections are leaking",
before, runtime.NumGoroutine())
}
time.Sleep(10 * time.Millisecond)
}
}
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