[ruby-core:124297] [Ruby Bug#21790] `Socket.getaddrinfo` hangs after `fork()` on macOS 26.1 (Tahoe) for IPv4-only hosts

17 Dec 2025

      Issue #21790 has been updated by adamoffat (Adam Moffat).

I saw that this was added in 3.4.0: https://github.com/ruby/ruby/pull/10864

Seen here: (https://github.com/ruby/ruby/releases/tag/v3_4_0_preview2)

But I also tested this using 3.4.1 and it was still an issue.

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Bug #21790: `Socket.getaddrinfo` hangs after `fork()` on macOS 26.1 (Tahoe) for IPv4-only hosts
https://bugs.ruby-lang.org/issues/21790#change-115795

* Author: adamoffat (Adam Moffat)
* Status: Open
* ruby -v: 3.3.8
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Ruby's `Socket.getaddrinfo` hangs indefinitely in forked child processes on macOS 26.1 (Tahoe) when resolving IPv4-only hostnames. This is a regression that does not occur on macOS 15.x (Sonoma) or earlier.

**Ruby version:**
ruby 3.3.8 (2025-04-09 revision b200bad6cd) [arm64-darwin24]
Also confirmed this affects Ruby 3.2.6 and 3.4.1.

**Reproducible script:**
``` ruby
require "socket"
require "timeout"

puts "Ruby #{RUBY_VERSION} on #{RUBY_PLATFORM}"
Socket.getaddrinfo("api.segment.io", 443, nil, :STREAM)
puts "Parent: DNS completed"

pid = fork do
  puts "Child: Attempting DNS resolution..."
  begin
    Timeout.timeout(90) do
      Socket.getaddrinfo("api.segment.io", 443, nil, :STREAM)
    end
    puts "Child: SUCCESS"
    exit 0
  rescue Timeout::Error
      puts "Child: FAILED - hung for 90 seconds"
      exit 1
  end
end

Process.wait(pid)
```

**Note:** Remove the `Timeout.timeout(90)` wrapper to observe the hang indefinitely. The timeout is included only to allow the script to exit for testing purposes.

**Result of reproduce process:**
```
Ruby 3.3.8 on arm64-darwin24
Parent: DNS completed
Child: Attempting DNS resolution...
Child: FAILED - hung for 90 seconds
```

The child process hangs with one thread consuming 100% CPU.

Expected result: The child process should complete DNS resolution successfully, as it does on macOS 15.x and earlier.

**Analysis:**
Stack trace shows:
Main thread: Blocked in `wait_getaddrinfo` → `_pthread_cond_wait`
DNS thread: Spinning in `_gai_nat64_second_pass` → `nw_path_access_agent_cache` → `_os_log_preferences_refresh` → `SIGSEGV`

The crash occurs in macOS's NAT64 synthesis code path. Ruby's signal handler catches the `SIGSEGV` but cannot recover, causing the DNS thread to spin.

**Key observations:**
- Only affects IPv4-only hosts. Hosts with IPv6 (like google.com) work correctly.
- Using `AF_INET` instead of `AF_UNSPEC` works. `Socket.getaddrinfo("api.segment.io", 443, Socket::AF_INET, :STREAM)` succeeds.
- Python is not affected. Python calls `getaddrinfo()` synchronously without a background thread.
- Parent must do DNS before fork. If the parent has not called getaddrinfo(), the child works correctly.

**Workaround:**
- Use `resolv-replace` to bypass the native DNS resolver: `require "resolv-replace"`

**Impact:**
This breaks all Ruby applications using pre-forking worker models (Resque, Unicorn, Puma, Sidekiq, Passenger) on macOS Tahoe.

**Apple Bug Report:** 
Filed with Apple as Feedback Assistant #FB21364061

---Files--------------------------------
stack_trace.txt (66.6 KB)
ruby_dns_fork_bug.rb (1.02 KB)

-- 
https://bugs.ruby-lang.org/