feat: expand tildes ~ in config file paths to HOME (#3811)

Fixes https://github.com/ghostty-org/ghostty/issues/3328

src/config/Config.zig

@@ -4361,6 +4361,45 @@ pub const RepeatablePath = struct {
             // If it isn't absolute, we need to make it absolute relative
             // to the base.
             var buf: [std.fs.max_path_bytes]u8 = undefined;
+
+            // Check if the path starts with a tilde and expand it to the
+            // home directory on Linux/macOS. We explicitly look for "~/"
+            // because we don't support alternate users such as "~alice/"
+            if (std.mem.startsWith(u8, path, "~/")) expand: {
+                // Windows isn't supported yet
+                if (comptime builtin.os.tag == .windows) break :expand;
+
+                const expanded: []const u8 = internal_os.expandHome(
+                    path,
+                    &buf,
+                ) catch |err| {
+                    try diags.append(alloc, .{
+                        .message = try std.fmt.allocPrintZ(
+                            alloc,
+                            "error expanding home directory for path {s}: {}",
+                            .{ path, err },
+                        ),
+                    });
+
+                    // Blank this path so that we don't attempt to resolve it
+                    // again
+                    self.value.items[i] = .{ .required = "" };
+
+                    continue;
+                };
+
+                log.debug(
+                    "expanding file path from home directory: path={s}",
+                    .{expanded},
+                );
+
+                switch (self.value.items[i]) {
+                    .optional, .required => |*p| p.* = try alloc.dupeZ(u8, expanded),
+                }
+
+                continue;
+            }
+
             const abs = dir.realpath(path, &buf) catch |err| abs: {
                 if (err == error.FileNotFound) {
                     // The file doesn't exist. Try to resolve the relative path

src/os/homedir.zig

@@ -12,7 +12,7 @@ const Error = error{
 
 /// Determine the home directory for the currently executing user. This
 /// is generally an expensive process so the value should be cached.
-pub inline fn home(buf: []u8) !?[]u8 {
+pub inline fn home(buf: []u8) !?[]const u8 {
     return switch (builtin.os.tag) {
         inline .linux, .macos => try homeUnix(buf),
         .windows => try homeWindows(buf),
@@ -24,7 +24,7 @@ pub inline fn home(buf: []u8) !?[]u8 {
     };
 }
 
-fn homeUnix(buf: []u8) !?[]u8 {
+fn homeUnix(buf: []u8) !?[]const u8 {
     // First: if we have a HOME env var, then we use that.
     if (posix.getenv("HOME")) |result| {
         if (buf.len < result.len) return Error.BufferTooSmall;
@@ -77,7 +77,7 @@ fn homeUnix(buf: []u8) !?[]u8 {
     return null;
 }
 
-fn homeWindows(buf: []u8) !?[]u8 {
+fn homeWindows(buf: []u8) !?[]const u8 {
     const drive_len = blk: {
         var fba_instance = std.heap.FixedBufferAllocator.init(buf);
         const fba = fba_instance.allocator();
@@ -110,6 +110,68 @@ fn trimSpace(input: []const u8) []const u8 {
     return std.mem.trim(u8, input, " \n\t");
 }
 
+pub const ExpandError = error{
+    HomeDetectionFailed,
+    BufferTooSmall,
+};
+
+/// Expands a path that starts with a tilde (~) to the home directory of
+/// the current user.
+///
+/// Errors if `home` fails or if the size of the expanded path is larger
+/// than `buf.len`.
+pub fn expandHome(path: []const u8, buf: []u8) ExpandError![]const u8 {
+    return switch (builtin.os.tag) {
+        .linux, .macos => try expandHomeUnix(path, buf),
+        .ios => return path,
+        else => @compileError("unimplemented"),
+    };
+}
+
+fn expandHomeUnix(path: []const u8, buf: []u8) ExpandError![]const u8 {
+    if (!std.mem.startsWith(u8, path, "~/")) return path;
+    const home_dir: []const u8 = if (home(buf)) |home_|
+        home_ orelse return error.HomeDetectionFailed
+    else |_|
+        return error.HomeDetectionFailed;
+    const rest = path[1..]; // Skip the ~
+    const expanded_len = home_dir.len + rest.len;
+
+    if (expanded_len > buf.len) return Error.BufferTooSmall;
+    @memcpy(buf[home_dir.len..expanded_len], rest);
+
+    return buf[0..expanded_len];
+}
+
+test "expandHomeUnix" {
+    const testing = std.testing;
+    const allocator = testing.allocator;
+    var buf: [std.fs.max_path_bytes]u8 = undefined;
+    const home_dir = try expandHomeUnix("~/", &buf);
+    // Joining the home directory `~` with the path `/`
+    // the result should end with a separator here. (e.g. `/home/user/`)
+    try testing.expect(home_dir[home_dir.len - 1] == std.fs.path.sep);
+
+    const downloads = try expandHomeUnix("~/Downloads/shader.glsl", &buf);
+    const expected_downloads = try std.mem.concat(allocator, u8, &[_][]const u8{ home_dir, "Downloads/shader.glsl" });
+    defer allocator.free(expected_downloads);
+    try testing.expectEqualStrings(expected_downloads, downloads);
+
+    try testing.expectEqualStrings("~", try expandHomeUnix("~", &buf));
+    try testing.expectEqualStrings("~abc/", try expandHomeUnix("~abc/", &buf));
+    try testing.expectEqualStrings("/home/user", try expandHomeUnix("/home/user", &buf));
+    try testing.expectEqualStrings("", try expandHomeUnix("", &buf));
+
+    // Expect an error if the buffer is large enough to hold the home directory,
+    // but not the expanded path
+    var small_buf = try allocator.alloc(u8, home_dir.len);
+    defer allocator.free(small_buf);
+    try testing.expectError(error.BufferTooSmall, expandHomeUnix(
+        "~/Downloads",
+        small_buf[0..],
+    ));
+}
+
 test {
     const testing = std.testing;
 

src/os/main.zig

@@ -38,6 +38,7 @@ pub const freeTmpDir = file.freeTmpDir;
 pub const isFlatpak = flatpak.isFlatpak;
 pub const FlatpakHostCommand = flatpak.FlatpakHostCommand;
 pub const home = homedir.home;
+pub const expandHome = homedir.expandHome;
 pub const ensureLocale = locale.ensureLocale;
 pub const clickInterval = mouse.clickInterval;
 pub const open = openpkg.open;