update fuzzing for build system changes

lib/std/Build/Fuzz.zig

@@ -135,6 +135,14 @@ pub fn start(fuzz: *Fuzz) void {
             fatal("unable to spawn coverage task: {t}", .{err});
     }
 
+    for (fuzz.run_steps) |run| {
+        if (run.fuzz_tests.items.len > 1) {
+            // Multiple fuzzWorkerRuns currently cause race-conditions
+            // since they use the same Run step. See #30969
+            fatal("--fuzz not yet implemented for multiple tests", .{});
+        }
+    }
+
     for (fuzz.run_steps) |run| {
         for (run.fuzz_tests.items) |unit_test_index| {
             assert(run.rebuilt_executable != null);

lib/std/Build/WebServer.zig

@@ -608,7 +608,10 @@ fn buildClientWasm(ws: *WebServer, arena: Allocator, optimize: std.builtin.Optim
     defer body_buffer.deinit(gpa);
 
     while (true) {
-        const header = try stdout.takeStruct(Header, .little);
+        const header = stdout.takeStruct(Header, .little) catch |e| switch (e) {
+            error.ReadFailed => return error.ReadFailed,
+            error.EndOfStream => break,
+        };
         body_buffer.clearRetainingCapacity();
         try stdout.appendExact(gpa, &body_buffer, header.bytes_len);
         const body = body_buffer.items;