Build.WebServer: update concurrency API usage

lib/std/Build/Step.zig

@@ -542,6 +542,7 @@ fn zigProcessUpdate(s: *Step, zp: *ZigProcess, watch: bool, web_server: ?*Build.
     const stdout = &stdout_reader.interface;
 
     var body_buffer: std.ArrayList(u8) = .empty;
+    defer body_buffer.deinit(gpa);
 
     while (true) {
         const Header = std.zig.Server.Message.Header;

lib/std/Build/WebServer.zig

@@ -588,11 +588,12 @@ fn buildClientWasm(ws: *WebServer, arena: Allocator, optimize: std.builtin.Optim
     });
     defer child.kill(io);
 
-    var poller = Io.poll(gpa, enum { stdout, stderr }, .{
-        .stdout = child.stdout.?,
-        .stderr = child.stderr.?,
-    });
-    defer poller.deinit();
+    var stderr_task = try io.concurrent(readStreamAlloc, .{ gpa, io, child.stderr.?, .unlimited });
+    defer if (stderr_task.cancel(io)) |slice| gpa.free(slice) else |_| {};
+
+    var stdout_buffer: [512]u8 = undefined;
+    var stdout_reader: Io.File.Reader = .initStreaming(child.stdout.?, io, &stdout_buffer);
+    const stdout = &stdout_reader.interface;
 
     try child.stdin.?.writeStreamingAll(io, @ptrCast(@as([]const std.zig.Client.Message.Header, &.{
         .{ .tag = .update, .bytes_len = 0 },
@@ -600,16 +601,17 @@ fn buildClientWasm(ws: *WebServer, arena: Allocator, optimize: std.builtin.Optim
     })));
 
     const Header = std.zig.Server.Message.Header;
+
     var result: ?Cache.Path = null;
     var result_error_bundle = std.zig.ErrorBundle.empty;
+    var body_buffer: std.ArrayList(u8) = .empty;
+    defer body_buffer.deinit(gpa);
 
-    const stdout = poller.reader(.stdout);
-
-    poll: while (true) {
-        while (stdout.buffered().len < @sizeOf(Header)) if (!(try poller.poll())) break :poll;
-        const header = stdout.takeStruct(Header, .little) catch unreachable;
-        while (stdout.buffered().len < header.bytes_len) if (!try poller.poll()) break :poll;
-        const body = stdout.take(header.bytes_len) catch unreachable;
+    while (true) {
+        const header = try stdout.takeStruct(Header, .little);
+        body_buffer.clearRetainingCapacity();
+        try stdout.appendExact(gpa, &body_buffer, header.bytes_len);
+        const body = body_buffer.items;
 
         switch (header.tag) {
             .zig_version => {
@@ -636,7 +638,7 @@ fn buildClientWasm(ws: *WebServer, arena: Allocator, optimize: std.builtin.Optim
         }
     }
 
-    const stderr_contents = try poller.toOwnedSlice(.stderr);
+    const stderr_contents = try stderr_task.await(io);
     if (stderr_contents.len > 0) {
         std.debug.print("{s}", .{stderr_contents});
     }
@@ -697,6 +699,14 @@ fn buildClientWasm(ws: *WebServer, arena: Allocator, optimize: std.builtin.Optim
     return base_path.join(arena, bin_name);
 }
 
+fn readStreamAlloc(gpa: Allocator, io: Io, file: Io.File, limit: Io.Limit) ![]u8 {
+    var file_reader: Io.File.Reader = .initStreaming(file, io, &.{});
+    return file_reader.interface.allocRemaining(gpa, limit) catch |err| switch (err) {
+        error.ReadFailed => return file_reader.err.?,
+        else => |e| return e,
+    };
+}
+
 pub fn updateTimeReportCompile(ws: *WebServer, opts: struct {
     compile: *Build.Step.Compile,