embedded: offscreen platform (Linux/Windows EGL) behind -Dembedded-offscreen

[lawrencecchen]

Summary

Adds an offscreen embedded platform so one fork serves both the cmux macOS app and the cmux GPUI app (Linux and Windows render terminals offscreen and composite the frames themselves). Everything is additive and behind a new build option, -Dembedded-offscreen (default false).

C API (include/ghostty.h)

• GHOSTTY_PLATFORM_OFFSCREEN = 6 (tag 3 is already the embedder-owned opengl platform in this fork).
• ghostty_platform_offscreen_s { uint32_t width; uint32_t height; double scale; }, new member offscreen of ghostty_platform_u. The union stays 40 bytes, so ghostty_surface_config_s does not change.
• typedef void (*ghostty_frame_callback_cb)(void* userdata, const void* frame);
• ghostty_offscreen_frame_s { width, height, stride, const uint8_t* data }: RGBA8, rows bottom-up.
• void ghostty_surface_set_frame_callback(ghostty_surface_t, ghostty_frame_callback_cb, void*): CPU readback for each frame.
• ghostty_dmabuf_frame_s { fd, fourcc, num_planes, stride, offset, modifier_hi, modifier_lo, width, height }
• void ghostty_surface_set_dmabuf_callback(ghostty_surface_t, ghostty_frame_callback_cb, void*): Linux only. Exports each frame as a dmabuf (the callback owns the fd). If the export fails, the frame goes to the CPU readback path.

How it works

• Ghostty owns a surfaceless EGL context (desktop GL 4.3 core on EGL_PLATFORM_SURFACELESS_MESA). Every offscreen surface drawn on the app thread shares this one context. This includes the cmux2 fix: one context per surface left older surfaces drawing with GL object names from a different context.
• Offscreen surfaces draw on the app thread. renderer/Thread.zig now decides per surface (apprt.Surface.mustDrawFromAppThread) instead of only by the comptime must_draw_from_app_thread. For these surfaces it emits GHOSTTY_ACTION_RENDER. The embedder then calls ghostty_surface_draw, which delivers the frame before it returns. Tokened renderer-thread draws are refused for these surfaces.
• Rendering does not depend on IO. Offscreen surfaces work with io_mode = GHOSTTY_SURFACE_IO_MANUAL_MIRROR, and nothing in the offscreen path touches termio or assumes Ghostty owns a PTY.
• Linux links eglGetPlatformDisplay and the other EGL calls directly, so the consumer passes -lEGL. Windows loads Mesa's libEGL.dll at runtime, first from cmux_mesa\ next to the exe and then from the default search path. ANGLE cannot work here because it provides GL ES only.
• With the option set on non-Darwin targets, the combined ghostty-internal.a is built PIC, so it can link into a PIE (Rust) executable.

No behavior change by default

Without -Dembedded-offscreen:

• Platform.Offscreen is void, and tag 6 fails with UnsupportedPlatform.
• No EGL code is analyzed and no EGL symbols are referenced.
• macOS/Metal, the existing opengl platform, and GTK keep their current code paths.

Metal rejects tag 6 explicitly. The two setters are always exported, so the header and the library stay in sync.

Also includes a one-line pre-existing fix: fontClusterQueryJson used CoreText-only fields, so libghostty on main did not compile for Linux (freetype).

Testing

• zig build -Demit-xcframework=true -Dxcframework-target=universal -Doptimize=ReleaseFast (macOS default config): OK. The archive has no EGL references.
• zig build -Dapp-runtime=none -Dtarget=x86_64-linux-gnu -Doptimize=ReleaseFast with and without -Dembedded-offscreen=true: OK. Also aarch64-linux-gnu with the option. Without the option the .so has no EGL symbols. With it, the .a has no absolute 32-bit relocations outside debug sections (PIC).
• EglContext.zig compiles on its own for x86_64-windows-gnu, x86_64-linux-gnu, and aarch64-linux-gnu.
• zig build test with filters (embedded, renderer, Thread, OpenGL, Metal, apprt, surface, App, config, font, ghostty.h, offscreen, ...): 678 pass with the option off, 679 pass with it on.
  • The one failure, renderer.opengl.Frame ... OpenGL acknowledges only after successful present, also fails on main without this change.
• New tests:
  • The tag and struct ABI match ghostty.h, and the union size does not change.
  • Platform.init accepts the tag with the option and rejects it without the option.
  • Initial geometry and scale come from the platform, and zero values keep the defaults.
  • Only offscreen surfaces report app-thread drawing.
  • Callback registration and clearing.

Not verified

• Linux and Windows runtime rendering in this fork. The same code was verified in cmux2-gpui on the cmux2 branch, but not after this port.
• Windows cross-compile of the whole libghostty. main does not currently build for x86_64-windows-gnu with Zig 0.16 (termio/Exec.zig cImport of sys/ioctl.h, pty.zig HPCON, among others), and these errors are not related to this PR.
• The full unfiltered zig build test run: some termio tests kill the test process group in this environment, and the Xcode test step is blocked locally.

Follow-ups for the consumer (cmux2 crates/ghostty), not in this PR

• GHOSTTY_PLATFORM_OFFSCREEN is 6, not 3.
• ghostty_surface_config_s in this fork is larger: the union is 40 bytes, and the config adds io_mode, io_write_cb, and other fields. The Rust ABI test values must be regenerated.
• GHOSTTY_ACTION_RENDER and other action indices differ from upstream.
• ghostty_surface_new_with_argv is not included. Use manual-mirror IO, or land argv in a separate PR.
• Build with -Dapp-runtime=none -Dembedded-offscreen=true.

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Summary by cubic

Adds an offscreen rendering platform for embedded Ghostty, gated behind a new -Dembedded-offscreen build option (default off). This lets a single fork serve both the macOS app and Linux/Windows apps that render terminals offscreen and composite frames themselves.

• Adds GHOSTTY_PLATFORM_OFFSCREEN (6) and ghostty_platform_offscreen_s with initial size and scale; the config struct ABI stays unchanged.
• Offscreen surfaces deliver each frame as an RGBA8 CPU readback via ghostty_surface_set_frame_callback, or on Linux as a dmabuf via ghostty_surface_set_dmabuf_callback, falling back to CPU readback when export fails.
• Offscreen surfaces draw on the app thread through a shared surfaceless EGL context; the renderer thread emits a RENDER action instead of drawing.
• With the option on non-Darwin targets, the combined static library is built PIC for consumer executables.
• Without the option, tag 6 fails with UnsupportedPlatform, no EGL code is compiled, and existing OpenGL/Metal/GTK paths are unchanged.
• Also fixes fontClusterQueryJson to compile with freetype (non-CoreText) backends by returning empty PostScript name and URL.

Bug Fixes

• Multi-plane or invalid dmabuf exports now close every exported fd and fall back to CPU readback instead of leaking fds per frame.
• Frames are skipped when the offscreen context is not current on the drawing thread; the context is now also set current before GL object teardown.
• Initial scale values are validated (finite, clamped to >= 1), and the C ABI layout of all offscreen structs is fully checked against ghostty.h.

^{Written for commit 285c256. Summary will update on new commits.}

Summary by CodeRabbit

• New Features
  • Added an optional offscreen platform for embedded applications, configurable with an initial width, height, and scale.
  • Offscreen surfaces can deliver rendered frames as CPU-readable pixel data or, on supported Linux systems, via dma-buf. If dma-buf delivery fails, pixel-data delivery is used when available.
  • Offscreen rendering requires a build configured for this platform and the OpenGL renderer; unsupported configurations cannot create offscreen surfaces.

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 300e38b9-8c56-40e0-ac08-53606d281058

📥 Commits

Reviewing files that changed from the base of the PR and between d4f861a and 285c256.

📒 Files selected for processing (5)

• include/ghostty.h
• src/apprt/embedded.zig
• src/renderer.zig
• src/renderer/OpenGL.zig
• src/renderer/opengl/EglContext.zig

📝 Walkthrough

Walkthrough

This change adds a build-gated embedded offscreen platform. It configures a surfaceless EGL context for OpenGL rendering, routes offscreen drawing to the app thread, and delivers frames through dma-buf callbacks or CPU readback callbacks.

Changes

Embedded Offscreen Rendering

Layer / File(s)	Summary
Offscreen platform contract and build enablement include/ghostty.h, src/apprt/embedded.zig, src/build/Config.zig, src/build/GhosttyLib.zig, src/build_config.zig	Adds the offscreen platform configuration and callback APIs, the build option and PIC setup, surface initialization, and related tests. Font identity lookups also skip CoreText calls when that backend is unavailable.
Surfaceless EGL context and dma-buf export src/renderer/opengl/EglContext.zig	Adds EGL loading and context lifecycle support, plus texture export to a dma-buf frame.
App-thread rendering and frame delivery src/renderer/Thread.zig, src/renderer/Metal.zig, src/renderer/OpenGL.zig	Routes offscreen drawing to the app thread and adds OpenGL presentation through dma-buf export or CPU readback. Metal rejects offscreen surfaces.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Embedder
  participant Thread
  participant OpenGL
  participant EglContext
  Thread->>OpenGL: Draw offscreen surface on app thread
  OpenGL->>EglContext: Make EGL context current
  OpenGL->>EglContext: Export rendered texture as dma-buf
  EglContext-->>OpenGL: Return dma-buf frame or export error
  OpenGL-->>Embedder: Deliver dma-buf frame or CPU-readback frame

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Suggested reviewers: mitchellh

Merge Risk: 🟡 Moderate · up to d4f86

Offscreen rendering can fail on invalid scale input or a failed context switch, and multi-plane dma-buf delivery can leak descriptors and produce incomplete frames. Resolve these paths before merging.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to d4f86

The optional mode changes how rendering resources and terminal frames are shared. Rendering does not stop when its required context cannot be established, and exported-buffer ownership relies on an unchecked single-plane assumption. Exposure is limited to applications that enable and use the feature; wider exploitation has not been established.

Retained concerns

• Medium · security · observed: The new offscreen path does not enforce its current-context precondition across drawing and cleanup. A failed context switch is logged by drawFrameStart, but the generic frame continues into presentation or rendering. Offscreen cleanup also continues GL deletion after reacquisition fails, while threadEnter returns without reacquiring the context for offscreen surfaces. Operations can therefore run against a prior host context or without a current context. Actual interference or disclosure is driver-dependent and was not demonstrated; later-frame retries do not contain the failed frame.
• Low · reliability · inferred: The public dma-buf contract represents one RGBA plane, but the exporter neither enforces a single-plane result nor represents every returned plane. It forwards num_planes while retaining only the first fd, stride, offset, and modifier. If a driver returns multiple planes, consumers receive incomplete metadata and any additional distinct descriptors lack an ownership or cleanup path, potentially accumulating process-wide resource leaks. The RGBA8 target is counterevidence, but single-plane behavior on deployed drivers remains unverified.

Security review details

Security Blast Radius

• inferred — The supported exposure is the enabled embedding process: terminal pixels reach its registered native callbacks, and shared-context failures can affect rendering resources on the drawing thread. Descriptor accumulation could affect process-wide availability. The inspected paths do not establish cross-tenant authority, a remotely selectable callback, or broader service exposure.

Security Findings and Attack Paths

• inferred — The principal security-relevant failure path is an unsuccessful offscreen context switch followed by continued GL operations using Ghostty resource names. If another host context remains current, those names need not identify Ghostty's objects. This supports a context-isolation concern, not a verified disclosure or attacker-driven exploit.

Trust Boundaries and Controls

• observed — The new data boundary is renderer-to-embedder delivery of terminal pixels or a GPU buffer descriptor. Registration uses native function pointers and userdata supplied by the embedder. Controls include default-off compilation, disabled-platform rejection, documented CPU borrowing, explicit fd-close responsibility, and offscreen app-thread routing; no authentication boundary is added by these APIs.

Resilience and Maintainability Implications

• observed — Frame callbacks execute inside the renderer's draw-mutex lifetime. Reentering the same draw path attempts to acquire that mutex again. Existing app-action lifetime handling retains an outer allocation during reentrant actions but explicitly allows synchronous teardown of renderer and callback contents; it does not establish that arbitrary destruction from a frame callback is safe.

Hardening Proposals

• proposed — Make successful context establishment a prerequisite for the entire frame and GPU teardown. On failure, abort or defer GPU work rather than continuing with an unproven object namespace.
• proposed — Enforce the advertised single-plane export before descriptor delivery, fall back to CPU delivery for unsupported layouts, and account for every exported descriptor that is not transferred.
• proposed — Define and enforce callback reentrancy and destruction rules. Also distinguish dma-buf descriptor ownership from content lifetime: require consumption before reuse, or provide an explicit release mechanism if asynchronous consumers may retain frames.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the main change: an embedded offscreen platform for Linux and Windows, gated behind -Dembedded-offscreen.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 3

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @src/apprt/embedded.zig:
- Around line 1469-1484: Update applyOffscreenInitialGeometry to accept
config.scale only when it is finite and positive, then clamp accepted values to
a minimum of 1 before assigning both content_scale axes. Keep invalid scales
from overwriting the existing content scale, matching updateContentScale’s
finite-input policy.

Review comments at @src/renderer/OpenGL.zig:
- Around line 524-541: Update drawFrameStart to propagate offscreenMakeCurrent
failures instead of logging and returning from a void hook; make the hook
fallible and have drawFrame abort before surface sizing, frame setup, drawing,
or presentation when it fails. Preserve the existing error context and prevent
subsequent GL calls from running with the wrong or no current context.

Review comments at @src/renderer/opengl/EglContext.zig:
- Around line 390-416: In exportTexture, validate the DMA-BUF export before
constructing the single-plane result: accept only num_planes equal to one with a
valid fds[0]. Otherwise close every nonnegative fd in fds and return an error so
the caller can use CPU readback.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 822ab90b-c8d3-4e20-9d51-36097171df00

📥 Commits

Reviewing files that changed from the base of the PR and between dcb1d69 and d4f861a.

📒 Files selected for processing (9)

• include/ghostty.h
• src/apprt/embedded.zig
• src/build/Config.zig
• src/build/GhosttyLib.zig
• src/build_config.zig
• src/renderer/Metal.zig
• src/renderer/OpenGL.zig
• src/renderer/Thread.zig
• src/renderer/opengl/EglContext.zig

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[lawrencecchen]

Independent review of PR #252 (at d4f861a) and fixes

Findings

Sev	Location (d4f861a)	Finding	Status
High	src/renderer/opengl/EglContext.zig:407	exportTexture returned only plane 0. When the driver exports more than one plane, the fds for planes 1-3 leaked on every frame, and the embedder got a frame that did not describe the whole image. A negative fds[0] was also passed through.	Fixed in fac3a25
Medium	src/renderer/OpenGL.zig:530	If eglMakeCurrent failed, drawFrameStart only logged. The generic renderer then made GL calls with the host's context or with no context.	Fixed in d11eec8
Medium	src/renderer/OpenGL.zig:268	Teardown made the offscreen context current in Offscreen.deinit. That runs from api.deinit(), after generic.deinit already deleted the swap chain and shader GL objects. If the host had another context current, those deletes went to the host's context.	Fixed in d11eec8 (prepareDeinit)
Medium	src/renderer/OpenGL.zig (no check)	GL objects belong to the context of the thread that ran surfaceInit. A draw on a different thread made a new context and silently drew with object names from another context.	Fixed in d11eec8: draws from the wrong thread now fail and log an error
Low	src/apprt/embedded.zig:1478	A NaN or infinite initial scale got into content_scale. A NaN failed > 0, but +inf passed.	Fixed in 30b183f (finite values only, clamped to >= 1, same as updateContentScale)
Low	src/apprt/embedded.zig:745	The ABI test compared struct sizes and only one field offset per struct.	Fixed in 30b183f: the test now checks size, alignment, and the offset and size of every field of the platform struct and both frame structs, including the renderer's DmabufFrame
Low	src/renderer/OpenGL.zig:128	The dmabuf fallback logged a warning on every frame on drivers without the extension (llvmpipe).	Fixed in d11eec8 (logs once)
Low	include/ghostty.h:1422,1441	The header did not document reentrancy, which thread may call the setters, buffer reuse, or the EGL context lifetime.	Fixed in 285c256
Low	libghostty-internal.so built with the option	The .so has undefined egl* symbols but no DT_NEEDED libEGL.so, so the executable must load libEGL. The static .a (the documented consumer) is not affected.	Documented in the header. Not changed: a cross-build from macOS has no libEGL to link. Loading EGL at runtime on Linux (as on Windows) would remove this requirement.

Verified with no defect found

• Default macOS: drawsFromAppThread, isOffscreen, and the requestDrawWithPresentation guard all reduce to compile-time false. Tag 6 is still rejected (now with UnsupportedPlatform). The universal ReleaseFast xcframework has no egl symbols. The only new exports are the two setters. ghostty_platform_u is still 40 bytes with 8-byte alignment. No action, config, or enum value changes. cmux feat-cmux-next uses only ghostty_platform_u(macos:), (ios:), and platform_tag, so the change is source- and ABI-compatible.
• Linux x86_64 without the option: no egl* symbols in the .so or the .a. With the option (x86_64 and aarch64): the .a has the 10 expected undefined egl* symbols.
• Threading: the renderer thread makes no GL calls (threadEnter returns early, and updateFrame, changeConfig, and setFontGrid change only CPU state under draw_mutex). The app-thread draw holds draw_mutex. Size and callbacks are read and written only on the app thread. RENDER is sent under the surface lease. The glad table and the EGL context are thread-local.
• Windows: EglContext.zig with init, exportTexture, and deinit referenced builds for x86_64-windows-gnu, x86_64-linux-gnu, and aarch64-linux-gnu.

Tests

• New test: takeSinglePlaneFd with real pipe fds. It checks that a multi-plane export closes all fds, an invalid fd is rejected, and a single-plane export keeps plane 0 open. I confirmed that the test runs by making it fail once on purpose.
• Filtered zig build test (embedded, renderer, Thread, OpenGL, Metal, apprt, surface, App, config, font, ghostty.h, offscreen, dmabuf): 588 pass / 8 skip with the option off, 589 pass / 7 skip with it on. The only failure in both is the existing renderer.opengl.Frame ... successful present test. This PR does not touch opengl/Frame.zig.
• The old callback-registration test only checks that a setter stores a pointer. It passes trivially.

Not verified

• Linux and Windows runtime rendering, dmabuf export on real drivers, and Mesa EGL_PLATFORM_SURFACELESS_MESA on Windows. Nothing in this repo runs an EGL draw.
• A process that mixes opengl-platform and offscreen surfaces on the app thread. finalizeSurfaceInit for an opengl surface unloads the thread's glad table, and offscreen draws do not reload it.
• A full Windows build of libghostty (main does not build for Windows with Zig 0.16).

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