build: use build options to configure terminal C ABI mode (#8945)

Fixes various issues:

- C ABI detection was faulty, which caused some Zig programs to use the
C ABI mode and some C programs not to. Let's be explicit.

- Unit tests now tests C ABI mode.

- Build binary no longer rebuilds on any terminal change (a regression).

- Zig programs can choose to depend on the C ABI version of the terminal
lib by using the `ghostty-vt-c` module.

9 files changed, 72 insertions, 24 deletions

diff --git a/build.zig b/build.zig
index 008fc849e..62fa77511 100644
--- a/build.zig
+++ b/build.zig
@@ -255,6 +255,15 @@ pub fn build(b: *std.Build) !void {
         });
         const mod_vt_test_run = b.addRunArtifact(mod_vt_test);
         test_lib_vt_step.dependOn(&mod_vt_test_run.step);
+
+        const mod_vt_c_test = b.addTest(.{
+            .root_module = mod.vt_c,
+            .target = config.target,
+            .optimize = config.optimize,
+            .filters = test_filters,
+        });
+        const mod_vt_c_test_run = b.addRunArtifact(mod_vt_c_test);
+        test_lib_vt_step.dependOn(&mod_vt_c_test_run.step);
     }
 
     // Tests
diff --git a/src/build/Config.zig b/src/build/Config.zig
index 474674d3a..0b7dae14d 100644
--- a/src/build/Config.zig
+++ b/src/build/Config.zig
@@ -498,6 +498,7 @@ pub fn terminalOptions(self: *const Config) TerminalBuildOptions {
         .artifact = .ghostty,
         .simd = self.simd,
         .oniguruma = true,
+        .c_abi = false,
         .slow_runtime_safety = switch (self.optimize) {
             .Debug => true,
             .ReleaseSafe,
diff --git a/src/build/GhosttyLibVt.zig b/src/build/GhosttyLibVt.zig
index 0029d6756..80f2bf9ad 100644
--- a/src/build/GhosttyLibVt.zig
+++ b/src/build/GhosttyLibVt.zig
@@ -26,7 +26,7 @@ pub fn initShared(
     const target = zig.vt.resolved_target.?;
     const lib = b.addSharedLibrary(.{
         .name = "ghostty-vt",
-        .root_module = zig.vt,
+        .root_module = zig.vt_c,
     });
     lib.installHeader(
         b.path("include/ghostty/vt.h"),
diff --git a/src/build/GhosttyZig.zig b/src/build/GhosttyZig.zig
index f175eb957..a8d2726bc 100644
--- a/src/build/GhosttyZig.zig
+++ b/src/build/GhosttyZig.zig
@@ -5,18 +5,17 @@ const GhosttyZig = @This();
 const std = @import("std");
 const Config = @import("Config.zig");
 const SharedDeps = @import("SharedDeps.zig");
+const TerminalBuildOptions = @import("../terminal/build_options.zig").Options;
 
+/// The `_c`-suffixed modules are built with the C ABI enabled.
 vt: *std.Build.Module,
+vt_c: *std.Build.Module,
 
 pub fn init(
     b: *std.Build,
     cfg: *const Config,
     deps: *const SharedDeps,
 ) !GhosttyZig {
-    // General build options
-    const general_options = b.addOptions();
-    try cfg.addOptions(general_options);
-
     // Terminal module build options
     var vt_options = cfg.terminalOptions();
     vt_options.artifact = .lib;
@@ -25,7 +24,41 @@ pub fn init(
     // conditionally do this.
     vt_options.oniguruma = false;
 
-    const vt = b.addModule("ghostty-vt", .{
+    return .{
+        .vt = try initVt(
+            "ghostty-vt",
+            b,
+            cfg,
+            deps,
+            vt_options,
+        ),
+
+        .vt_c = try initVt(
+            "ghostty-vt-c",
+            b,
+            cfg,
+            deps,
+            options: {
+                var dup = vt_options;
+                dup.c_abi = true;
+                break :options dup;
+            },
+        ),
+    };
+}
+
+fn initVt(
+    name: []const u8,
+    b: *std.Build,
+    cfg: *const Config,
+    deps: *const SharedDeps,
+    vt_options: TerminalBuildOptions,
+) !*std.Build.Module {
+    // General build options
+    const general_options = b.addOptions();
+    try cfg.addOptions(general_options);
+
+    const vt = b.addModule(name, .{
         .root_source_file = b.path("src/lib_vt.zig"),
         .target = cfg.target,
         .optimize = cfg.optimize,
@@ -45,5 +78,5 @@ pub fn init(
         try SharedDeps.addSimd(b, vt, null);
     }
 
-    return .{ .vt = vt };
+    return vt;
 }
diff --git a/src/lib_vt.zig b/src/lib_vt.zig
index 37ab7ae68..b7ef9459a 100644
--- a/src/lib_vt.zig
+++ b/src/lib_vt.zig
@@ -7,6 +7,7 @@
 //! by thousands of users for years. However, the API itself (functions,
 //! types, etc.) may change without warning. We're working on stabilizing
 //! this in the future.
+const lib = @This();
 
 // The public API below reproduces a lot of terminal/main.zig but
 // is separate because (1) we need our root file to be in `src/`
@@ -68,7 +69,7 @@ pub const Attribute = terminal.Attribute;
 comptime {
     // If we're building the C library (vs. the Zig module) then
     // we want to reference the C API so that it gets exported.
-    if (terminal.is_c_lib) {
+    if (@import("root") == lib) {
         const c = terminal.c_api;
         @export(&c.osc_new, .{ .name = "ghostty_osc_new" });
         @export(&c.osc_free, .{ .name = "ghostty_osc_free" });
@@ -81,8 +82,8 @@ comptime {
 
 test {
     _ = terminal;
-
-    // Tests always test the C API and shared C functions
-    _ = terminal.c_api;
     _ = @import("lib/main.zig");
+    if (comptime terminal.options.c_abi) {
+        _ = terminal.c_api;
+    }
 }
diff --git a/src/terminal/build_options.zig b/src/terminal/build_options.zig
index 2085e2243..e209a56fa 100644
--- a/src/terminal/build_options.zig
+++ b/src/terminal/build_options.zig
@@ -1,8 +1,6 @@
 const std = @import("std");
 
-/// True if we're building the C library libghostty-vt.
-pub const is_c_lib = @import("root") == @import("../lib_vt.zig");
-
+/// Options set by Zig build.zig and exposed via `terminal_options`.
 pub const Options = struct {
     /// The target artifact to build. This will gate some functionality.
     artifact: Artifact,
@@ -26,6 +24,10 @@ pub const Options = struct {
     /// generally be disabled in production builds.
     slow_runtime_safety: bool,
 
+    /// Force C ABI mode on or off. If not set, then it will be set based on
+    /// Options.
+    c_abi: bool,
+
     /// Add the required build options for the terminal module.
     pub fn add(
         self: Options,
@@ -34,6 +36,7 @@ pub const Options = struct {
     ) void {
         const opts = b.addOptions();
         opts.addOption(Artifact, "artifact", self.artifact);
+        opts.addOption(bool, "c_abi", self.c_abi);
         opts.addOption(bool, "oniguruma", self.oniguruma);
         opts.addOption(bool, "simd", self.simd);
         opts.addOption(bool, "slow_runtime_safety", self.slow_runtime_safety);
diff --git a/src/terminal/c/osc.zig b/src/terminal/c/osc.zig
index c04626b69..d1998f4e1 100644
--- a/src/terminal/c/osc.zig
+++ b/src/terminal/c/osc.zig
@@ -73,9 +73,9 @@ test "command type" {
     ));
     defer free(p);
 
-    p.next('0');
-    p.next(';');
-    p.next('a');
-    const cmd = p.end(0);
+    next(p, '0');
+    next(p, ';');
+    next(p, 'a');
+    const cmd = end(p, 0);
     try testing.expectEqual(.change_window_title, commandType(cmd));
 }
diff --git a/src/terminal/main.zig b/src/terminal/main.zig
index 7403ff309..6875ba89d 100644
--- a/src/terminal/main.zig
+++ b/src/terminal/main.zig
@@ -1,5 +1,4 @@
 const builtin = @import("builtin");
-const build_options = @import("terminal_options");
 
 const charsets = @import("charsets.zig");
 const sanitize = @import("sanitize.zig");
@@ -21,7 +20,7 @@ pub const page = @import("page.zig");
 pub const parse_table = @import("parse_table.zig");
 pub const search = @import("search.zig");
 pub const size = @import("size.zig");
-pub const tmux = if (build_options.tmux_control_mode) @import("tmux.zig") else struct {};
+pub const tmux = if (options.tmux_control_mode) @import("tmux.zig") else struct {};
 pub const x11_color = @import("x11_color.zig");
 
 pub const Charset = charsets.Charset;
@@ -62,9 +61,11 @@ pub const Attribute = sgr.Attribute;
 
 pub const isSafePaste = sanitize.isSafePaste;
 
+pub const Options = @import("build_options.zig").Options;
+pub const options = @import("terminal_options");
+
 /// This is set to true when we're building the C library.
-pub const is_c_lib = @import("build_options.zig").is_c_lib;
-pub const c_api = if (is_c_lib) @import("c/main.zig") else void;
+pub const c_api = if (options.c_abi) @import("c/main.zig") else void;
 
 test {
     @import("std").testing.refAllDecls(@This());
diff --git a/src/terminal/osc.zig b/src/terminal/osc.zig
index 71d2f8598..20b22d1ef 100644
--- a/src/terminal/osc.zig
+++ b/src/terminal/osc.zig
@@ -7,11 +7,11 @@ const osc = @This();
 
 const std = @import("std");
 const builtin = @import("builtin");
+const build_options = @import("terminal_options");
 const mem = std.mem;
 const assert = std.debug.assert;
 const Allocator = mem.Allocator;
 const LibEnum = @import("../lib/enum.zig").Enum;
-const is_c_lib = @import("build_options.zig").is_c_lib;
 const RGB = @import("color.zig").RGB;
 const kitty_color = @import("kitty/color.zig");
 const osc_color = @import("osc/color.zig");
@@ -175,7 +175,7 @@ pub const Command = union(Key) {
     conemu_guimacro: []const u8,
 
     pub const Key = LibEnum(
-        if (is_c_lib) .c else .zig,
+        if (build_options.c_abi) .c else .zig,
         // NOTE: Order matters, see LibEnum documentation.
         &.{
             "invalid",