Crash on io thread with "attempt to use null value" after large pty writes

[cataggar]

Issue Description

I'm embedding libghostty in an app. The app crashes in src/datastruct/segmented_pool.zig . It does not appear to be limited to solely the way I use it.

Expected Behavior

The SegmentedPool does not hand out in-use values after growth.

Actual Behavior

The SegmentedPool hands out in-use values after growth.

Reproduction Steps

Disclosure: Copilot used to create these tests.

src/datastruct/segmented_pool.zig

test "SegmentedPool: growth does not hand out a checked out value" {
    var pool: SegmentedPool(u8, 2) = .{};
    defer pool.deinit(testing.allocator);

    // Walk the head off of slot 0 so that growth has an order to preserve.
    const v1 = try pool.get();
    const v2 = try pool.get();
    pool.put(); // returns v1, the oldest
    try testing.expectEqual(v1, try pool.get());
    // Checked out, oldest first: v2, v1

    // Grow. Everything handed out so far is still checked out, so the two
    // values this hands out have to be brand new ones.
    const v3 = try pool.getGrow(testing.allocator);
    const v4 = try pool.get();
    try testing.expect(v3 != v1 and v3 != v2);
    try testing.expect(v4 != v1 and v4 != v2 and v4 != v3);
    try testing.expectError(error.OutOfValues, pool.get());
    // Checked out, oldest first: v2, v1, v3, v4

    // One value comes back. Values are put back in the order they were
    // handed out, so that is v2, and v2 is the only value free to hand out.
    pool.put();
    const reused = try pool.get();
    try testing.expectEqual(v2, reused);
}

The above is from:

• datastruct: add failing tests for SegmentedPool growth order cataggar/ghostty#17

Ghostty Logs

No response

Ghostty Version

Ghostty 1.3.2-main-+08f039fbb

Version
  - version: 1.3.2-main-+08f039fbb
  - channel: tip
Build Config
  - Zig version   : 0.16.0
  - build mode    : .ReleaseFast
  - app runtime   : .none
  - font engine   : .coretext
  - renderer      : renderer.generic.Renderer(renderer.Metal)
  - libxev        : kqueue

OS Version Information

Mac Tahoe 26.6

(Linux only) Display Server

None

(Linux only) Desktop Environment/Window Manager

No response

Minimal Ghostty Configuration

Empty.  ~/.config/ghostty/config

Additional Relevant Configuration

No response

I acknowledge that:

• I have reviewed the FAQ and confirm that my issue is NOT among them.
• I have searched the Ghostty repository (both open and closed Discussions and Issues) and confirm this is not a duplicate of an existing issue or discussion.
• I have checked the "Preview" tab on all text fields to ensure that everything looks right, and have wrapped all configuration and code in code blocks with a group of three backticks (```) on separate lines.

[mitchellh]

Confirmed. Amazed we never hit this in Ghostty. Working on it.

[mitchellh]

Actually, this begs the question... how did you hit this?

[mitchellh]

Okay wait, also just to clarify, this isn't libghostty, this is ghostty-internal (our internal only library). libghostty-vt does not use segmentedpool.

[mitchellh]

#13659