lib-vt: enable freestanding wasm builds (#9301)

This makes `libghostty-vt` build for freestanding wasm targets (aka a
browser) and produce a `ghostty-vt.wasm` file. This exports the same C
API that libghostty-vt does.

This commit specifically makes the changes necessary for the build to
build properly and for us to run the build in CI. We don't yet actually
try using it...

.github/workflows/test.yml

@@ -204,6 +204,7 @@ jobs:
             aarch64-linux,
             x86_64-linux,
             x86_64-windows,
+            wasm32-freestanding,
           ]
     runs-on: namespace-profile-ghostty-sm
     needs: test

build.zig

@@ -101,10 +101,19 @@ pub fn build(b: *std.Build) !void {
     );
 
     // libghostty-vt
-    const libghostty_vt_shared = try buildpkg.GhosttyLibVt.initShared(
+    const libghostty_vt_shared = shared: {
-        b,
+        if (config.target.result.cpu.arch.isWasm()) {
-        &mod,
+            break :shared try buildpkg.GhosttyLibVt.initWasm(
-    );
+                b,
+                &mod,
+            );
+        }
+
+        break :shared try buildpkg.GhosttyLibVt.initShared(
+            b,
+            &mod,
+        );
+    };
     libghostty_vt_shared.install(libvt_step);
     libghostty_vt_shared.install(b.getInstallStep());
 

src/build/Config.zig

@@ -173,7 +173,13 @@ pub fn init(b: *std.Build, appVersion: []const u8) !Config {
         bool,
         "simd",
         "Build with SIMD-accelerated code paths. Results in significant performance improvements.",
-    ) orelse true;
+    ) orelse simd: {
+        // We can't build our SIMD dependencies for Wasm. Note that we may
+        // still use SIMD features in the Wasm-builds.
+        if (target.result.cpu.arch.isWasm()) break :simd false;
+
+        break :simd true;
+    };
 
     config.wayland = b.option(
         bool,

src/build/GhosttyLibVt.zig

@@ -1,6 +1,7 @@
 const GhosttyLibVt = @This();
 
 const std = @import("std");
+const assert = std.debug.assert;
 const RunStep = std.Build.Step.Run;
 const Config = @import("Config.zig");
 const GhosttyZig = @import("GhosttyZig.zig");
@@ -17,7 +18,35 @@ artifact: *std.Build.Step.InstallArtifact,
 /// The final library file
 output: std.Build.LazyPath,
 dsym: ?std.Build.LazyPath,
-pkg_config: std.Build.LazyPath,
+pkg_config: ?std.Build.LazyPath,
+
+pub fn initWasm(
+    b: *std.Build,
+    zig: *const GhosttyZig,
+) !GhosttyLibVt {
+    const target = zig.vt.resolved_target.?;
+    assert(target.result.cpu.arch.isWasm());
+
+    const exe = b.addExecutable(.{
+        .name = "ghostty-vt",
+        .root_module = zig.vt_c,
+        .version = std.SemanticVersion{ .major = 0, .minor = 1, .patch = 0 },
+    });
+
+    // Allow exported symbols to actually be exported.
+    exe.rdynamic = true;
+
+    // There is no entrypoint for this wasm module.
+    exe.entry = .disabled;
+
+    return .{
+        .step = &exe.step,
+        .artifact = b.addInstallArtifact(exe, .{}),
+        .output = exe.getEmittedBin(),
+        .dsym = null,
+        .pkg_config = null,
+    };
+}
 
 pub fn initShared(
     b: *std.Build,
@@ -82,9 +111,11 @@ pub fn install(
 ) void {
     const b = step.owner;
     step.dependOn(&self.artifact.step);
-    step.dependOn(&b.addInstallFileWithDir(
+    if (self.pkg_config) |pkg_config| {
-        self.pkg_config,
+        step.dependOn(&b.addInstallFileWithDir(
-        .prefix,
+            pkg_config,
-        "share/pkgconfig/libghostty-vt.pc",
+            .prefix,
-    ).step);
+            "share/pkgconfig/libghostty-vt.pc",
+        ).step);
+    }
 }

src/lib/allocator.zig

@@ -20,11 +20,17 @@ pub fn default(c_alloc_: ?*const Allocator) std.mem.Allocator {
     // If we're given an allocator, use it.
     if (c_alloc_) |c_alloc| return c_alloc.zig();
 
+    // Tests always use the test allocator so we can detect leaks.
+    if (comptime builtin.is_test) return testing.allocator;
+
     // If we have libc, use that. We prefer libc if we have it because
     // its generally fast but also lets the embedder easily override
     // malloc/free with custom allocators like mimalloc or something.
     if (comptime builtin.link_libc) return std.heap.c_allocator;
 
+    // Wasm
+    if (comptime builtin.target.cpu.arch.isWasm()) return std.heap.wasm_allocator;
+
     // No libc, use the preferred allocator for releases which is the
     // Zig SMP allocator.
     return std.heap.smp_allocator;

src/lib_vt.zig

@@ -9,6 +9,9 @@
 //! this in the future.
 const lib = @This();
 
+const std = @import("std");
+const builtin = @import("builtin");
+
 // 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/`
 // so we can access other directories and (2) we may want to withhold
@@ -126,6 +129,26 @@ comptime {
     }
 }
 
+pub const std_options: std.Options = options: {
+    if (builtin.target.cpu.arch.isWasm()) break :options .{
+        // Wasm builds we specifically want to optimize for space with small
+        // releases so we bump up to warn. Everything else acts pretty normal.
+        .log_level = switch (builtin.mode) {
+            .Debug => .debug,
+            .ReleaseSmall => .warn,
+            else => .info,
+        },
+
+        // Wasm doesn't have access to stdio so we have a custom log function.
+        .logFn = @import("os/wasm/log.zig").log,
+    };
+
+    // For everything else we currently use defaults. Longer term I'm
+    // SURE this isn't right (e.g. we definitely want to customize the log
+    // function for the C lib at least).
+    break :options .{};
+};
+
 test {
     _ = terminal;
     _ = @import("lib/main.zig");

src/os/wasm.zig

@@ -23,93 +23,3 @@ pub const alloc = if (builtin.is_test)
     std.testing.allocator
 else
     std.heap.wasm_allocator;
-
-/// For host-owned allocations:
-/// We need to keep track of our own pointer lengths because Zig
-/// allocators usually don't do this and we need to be able to send
-/// a direct pointer back to the host system. A more appropriate thing
-/// to do would be to probably make a custom allocator that keeps track
-/// of size.
-var allocs: std.AutoHashMapUnmanaged([*]u8, usize) = .{};
-
-/// Allocate len bytes and return a pointer to the memory in the host.
-/// The data is not zeroed.
-pub export fn malloc(len: usize) ?[*]u8 {
-    return alloc_(len) catch return null;
-}
-
-fn alloc_(len: usize) ![*]u8 {
-    // Create the allocation
-    const slice = try alloc.alloc(u8, len);
-    errdefer alloc.free(slice);
-
-    // Store the size so we can deallocate later
-    try allocs.putNoClobber(alloc, slice.ptr, slice.len);
-    errdefer _ = allocs.remove(slice.ptr);
-
-    return slice.ptr;
-}
-
-/// Free an allocation from malloc.
-pub export fn free(ptr: ?[*]u8) void {
-    if (ptr) |v| {
-        if (allocs.get(v)) |len| {
-            const slice = v[0..len];
-            alloc.free(slice);
-            _ = allocs.remove(v);
-        }
-    }
-}
-
-/// Convert an allocated pointer of any type to a host-owned pointer.
-/// This pushes the responsibility to free it to the host. The returned
-/// pointer will match the pointer but is typed correctly for returning
-/// to the host.
-pub fn toHostOwned(ptr: anytype) ![*]u8 {
-    // Convert our pointer to a byte array
-    const info = @typeInfo(@TypeOf(ptr)).pointer;
-    const T = info.child;
-    const size = @sizeOf(T);
-    const casted = @as([*]u8, @ptrFromInt(@intFromPtr(ptr)));
-
-    // Store the information about it
-    try allocs.putNoClobber(alloc, casted, size);
-    errdefer _ = allocs.remove(casted);
-
-    return casted;
-}
-
-/// Returns true if the value is host owned.
-pub fn isHostOwned(ptr: anytype) bool {
-    const casted = @as([*]u8, @ptrFromInt(@intFromPtr(ptr)));
-    return allocs.contains(casted);
-}
-
-/// Convert a pointer back to a module-owned value. The caller is expected
-/// to cast or have the valid pointer for alloc calls.
-pub fn toModuleOwned(ptr: anytype) void {
-    const casted = @as([*]u8, @ptrFromInt(@intFromPtr(ptr)));
-    _ = allocs.remove(casted);
-}
-
-test "basics" {
-    const testing = std.testing;
-    const buf = malloc(32).?;
-    try testing.expect(allocs.size == 1);
-    free(buf);
-    try testing.expect(allocs.size == 0);
-}
-
-test "toHostOwned" {
-    const testing = std.testing;
-
-    const Point = struct { x: u32 = 0, y: u32 = 0 };
-    const p = try alloc.create(Point);
-    errdefer alloc.destroy(p);
-    const ptr = try toHostOwned(p);
-    try testing.expect(allocs.size == 1);
-    try testing.expect(isHostOwned(p));
-    try testing.expect(isHostOwned(ptr));
-    free(ptr);
-    try testing.expect(allocs.size == 0);
-}

src/os/wasm/log.zig

@@ -1,15 +1,12 @@
 const std = @import("std");
 const builtin = @import("builtin");
 const wasm = @import("../wasm.zig");
-const wasm_target = @import("target.zig");
 
 // Use the correct implementation
-pub const log = if (wasm_target.target) |target| switch (target) {
+pub const log = Freestanding.log;
-    .browser => Browser.log,
-} else @compileError("wasm target required");
 
-/// Browser implementation calls an extern "log" function.
+/// Freestanding implementation calls an extern "log" function.
-pub const Browser = struct {
+pub const Freestanding = struct {
     // The function std.log will call.
     pub fn log(
         comptime level: std.log.Level,

src/os/wasm/target.zig

@@ -10,7 +10,7 @@ pub const Target = enum {
 };
 
 /// Our specific target platform.
-pub const target: ?Target = if (!builtin.target.isWasm()) null else target: {
+pub const target: ?Target = if (!builtin.target.cpu.arch.isWasm()) null else target: {
     const result = @as(Target, @enumFromInt(@intFromEnum(options.wasm_target)));
     // This maybe isn't necessary but I don't know if enums without a specific
     // tag type and value are guaranteed to be the same between build.zig

src/terminal/c/key_encode.zig

@@ -123,7 +123,9 @@ pub fn encode(
                 encoder_.?.opts,
             ) catch unreachable;
 
-            out_written.* = discarding.count;
+            // Discarding always uses a u64. If we're on 32-bit systems
+            // we cast down. We should make this safer in the future.
+            out_written.* = @intCast(discarding.count);
             return .out_of_memory;
         },
     };