fix(macos-container): preserve working builder dns

bot_bottle/backend/macos_container/util.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 import json
 import os
+import ipaddress
 import platform
 import shutil
 import subprocess
@@ -36,7 +37,10 @@ def require_container() -> None:
 
 
 def dns_server() -> str:
-    return os.environ.get("BOT_BOTTLE_MACOS_CONTAINER_DNS", _DEFAULT_DNS)
+    override = os.environ.get("BOT_BOTTLE_MACOS_CONTAINER_DNS", "").strip()
+    if override:
+        return override
+    return _host_ipv4_dns() or _DEFAULT_DNS
 
 
 def build_image(ref: str, context: str, *, dockerfile: str = "") -> None:
@@ -55,8 +59,16 @@ def build_image(ref: str, context: str, *, dockerfile: str = "") -> None:
 
 def _ensure_builder_dns() -> None:
     dns = dns_server()
-    if _builder_has_dns(dns):
+    status = _builder_status()
+    override = os.environ.get("BOT_BOTTLE_MACOS_CONTAINER_DNS", "").strip()
+    if _builder_running(status) and _builder_resolves_build_hosts():
+        if override and not _builder_has_dns(status, dns):
+            _restart_builder_with_dns(dns)
         return
+    _restart_builder_with_dns(dns)
+
+
+def _restart_builder_with_dns(dns: str) -> None:
     subprocess.run(
         [_CONTAINER, "builder", "stop"],
         stdout=subprocess.DEVNULL,
@@ -69,7 +81,55 @@ def _ensure_builder_dns() -> None:
     )
 
 
-def _builder_has_dns(dns: str) -> bool:
+def _host_ipv4_dns() -> str:
+    if not is_macos():
+        return ""
+    result = subprocess.run(
+        ["scutil", "--dns"],
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+    if result.returncode != 0:
+        return ""
+    blocks: list[list[str]] = []
+    current: list[str] = []
+    for line in result.stdout.splitlines():
+        if line.startswith("resolver #") and current:
+            blocks.append(current)
+            current = []
+        current.append(line)
+    if current:
+        blocks.append(current)
+    for direct_only in (True, False):
+        for block in blocks:
+            text = "\n".join(block)
+            if direct_only and "Directly Reachable Address" not in text:
+                continue
+            for line in block:
+                if "nameserver[" not in line or ":" not in line:
+                    continue
+                candidate = line.split(":", 1)[1].strip()
+                if _usable_ipv4(candidate):
+                    return candidate
+    return ""
+
+
+def _usable_ipv4(value: str) -> bool:
+    try:
+        address = ipaddress.ip_address(value)
+    except ValueError:
+        return False
+    return (
+        address.version == 4
+        and not address.is_loopback
+        and not address.is_link_local
+        and not address.is_multicast
+        and not address.is_unspecified
+    )
+
+
+def _builder_status() -> list[dict[str, object]]:
     result = subprocess.run(
         [_CONTAINER, "builder", "status", "--format", "json"],
         capture_output=True,
@@ -77,18 +137,29 @@ def _builder_has_dns(dns: str) -> bool:
         check=False,
     )
     if result.returncode != 0:
-        return False
+        return []
     try:
         data = json.loads(result.stdout or "[]")
     except json.JSONDecodeError:
-        return False
-    entries = data if isinstance(data, list) else [data]
-    for entry in entries:
-        if not isinstance(entry, dict):
-            continue
-        status = entry.get("status")
-        if isinstance(status, dict) and status.get("state") != "running":
-            continue
+        return []
+    if isinstance(data, list):
+        return [entry for entry in data if isinstance(entry, dict)]
+    if isinstance(data, dict):
+        return [data]
+    return []
+
+
+def _builder_running(status: list[dict[str, object]]) -> bool:
+    for entry in status:
+        entry_status = entry.get("status")
+        if isinstance(entry_status, dict) and entry_status.get("state") == "running":
+            return True
+    return False
+
+
+def _builder_dns_nameservers(status: list[dict[str, object]]) -> list[str]:
+    out: list[str] = []
+    for entry in status:
         config = entry.get("configuration")
         config_dns = config.get("dns") if isinstance(config, dict) else None
         nameservers = (
@@ -96,9 +167,24 @@ def _builder_has_dns(dns: str) -> bool:
             if isinstance(config_dns, dict)
             else None
         )
-        if isinstance(nameservers, list) and dns in nameservers:
-            return True
-    return False
+        if not isinstance(nameservers, list):
+            continue
+        out.extend(name for name in nameservers if isinstance(name, str))
+    return out
+
+
+def _builder_has_dns(status: list[dict[str, object]], dns: str) -> bool:
+    return dns in _builder_dns_nameservers(status)
+
+
+def _builder_resolves_build_hosts() -> bool:
+    result = subprocess.run(
+        [_CONTAINER, "exec", "buildkit", "getent", "hosts", "deb.debian.org"],
+        stdout=subprocess.DEVNULL,
+        stderr=subprocess.DEVNULL,
+        check=False,
+    )
+    return result.returncode == 0
 
 
 def image_exists(ref: str) -> bool:

docs/prds/prd-new-macos-container-backend.md

@@ -171,9 +171,13 @@ delivery design lands.
   and verify that egress cannot bypass the sidecar. They also preflight
   Apple Container BuildKit DNS because image builds must resolve
   package mirrors before a launch smoke can be meaningful. The backend
-  starts/restarts the Apple Container builder with the configured DNS
-  server before image builds so BuildKit `RUN` steps inherit a working
-  resolver.
+  probes the running builder before image builds and leaves it alone
+  when its current resolver works. If the probe fails, or if the
+  operator explicitly sets `BOT_BOTTLE_MACOS_CONTAINER_DNS`, the backend
+  restarts the Apple Container builder with the configured DNS server.
+  Without an explicit override, that server is discovered from the
+  host's directly reachable IPv4 resolver before falling back to a
+  public resolver.
 
 ## References
 

tests/unit/test_macos_container_util.py

@@ -28,6 +28,27 @@ class TestMacosContainerAvailability(unittest.TestCase):
 
 
 class TestMacosContainerCommands(unittest.TestCase):
+    def test_dns_server_prefers_direct_host_ipv4_resolver(self):
+        scutil = util.subprocess.CompletedProcess(
+            args=[],
+            returncode=0,
+            stdout="""
+resolver #1
+  nameserver[0] : 100.100.100.100
+  reach    : 0x00000003 (Reachable,Transient Connection)
+
+resolver #2
+  nameserver[0] : 2600:4041:5c43:b900::1
+  nameserver[1] : 192.168.1.1
+  reach    : 0x00020002 (Reachable,Directly Reachable Address)
+""",
+            stderr="",
+        )
+        with patch.object(util.os, "environ", {}), \
+             patch.object(util.platform, "system", return_value="Darwin"), \
+             patch.object(util.subprocess, "run", return_value=scutil):
+            self.assertEqual("192.168.1.1", util.dns_server())
+
     def test_build_image(self):
         status = util.subprocess.CompletedProcess(
             args=[],
@@ -81,6 +102,64 @@ class TestMacosContainerCommands(unittest.TestCase):
             calls[-1],
         )
 
+    def test_build_image_leaves_working_builder_with_different_dns_alone(self):
+        status = util.subprocess.CompletedProcess(
+            args=[],
+            returncode=0,
+            stdout=(
+                '[{"status":{"state":"running"},'
+                '"configuration":{"dns":{"nameservers":["8.8.8.8"]}}}]'
+            ),
+            stderr="",
+        )
+        probe = util.subprocess.CompletedProcess(
+            args=[], returncode=0, stdout="", stderr="",
+        )
+        build = util.subprocess.CompletedProcess(
+            args=[], returncode=0, stdout="", stderr="",
+        )
+        with patch.object(util, "dns_server", return_value="192.168.1.1"), \
+             patch.object(util.os, "environ", {}), \
+             patch.object(util.subprocess, "run", side_effect=[status, probe, build]) as run:
+            util.build_image("bot-bottle-agent:latest", "/repo")
+        calls = [c.args[0] for c in run.call_args_list]
+        self.assertNotIn(["container", "builder", "stop"], calls)
+        self.assertNotIn(
+            ["container", "builder", "start", "--dns", "192.168.1.1"],
+            calls,
+        )
+
+    def test_build_image_restarts_builder_when_dns_probe_fails(self):
+        status = util.subprocess.CompletedProcess(
+            args=[],
+            returncode=0,
+            stdout=(
+                '[{"status":{"state":"running"},'
+                '"configuration":{"dns":{"nameservers":["8.8.8.8"]}}}]'
+            ),
+            stderr="",
+        )
+        failed_probe = util.subprocess.CompletedProcess(
+            args=[], returncode=2, stdout="", stderr="",
+        )
+        ok = util.subprocess.CompletedProcess(
+            args=[], returncode=0, stdout="", stderr="",
+        )
+        with patch.object(util, "dns_server", return_value="192.168.1.1"), \
+             patch.object(util.os, "environ", {}), \
+             patch.object(
+                 util.subprocess,
+                 "run",
+                 side_effect=[status, failed_probe, ok, ok, ok],
+             ) as run:
+            util.build_image("bot-bottle-agent:latest", "/repo")
+        calls = [c.args[0] for c in run.call_args_list]
+        self.assertIn(["container", "builder", "stop"], calls)
+        self.assertIn(
+            ["container", "builder", "start", "--dns", "192.168.1.1"],
+            calls,
+        )
+
     def test_container_exists_parses_quiet_list(self):
         completed = util.subprocess.CompletedProcess(
             args=[], returncode=0, stdout="bot-bottle-a\nother\n", stderr="",