build system: capture stderr and report it later

Instead of dumping directly to stderr. This prevents processes running
simultaneously from racing their stderr against each other.

For now it only reports at the end, but an improvement would be to
report as soon as a failed step occurs.

1 files changed, 3 insertions, 5 deletions

diff --git a/lib/build_runner.zig b/lib/build_runner.zig
index 87a9226bf1..6ba583c59d 100644
--- a/lib/build_runner.zig
+++ b/lib/build_runner.zig
@@ -318,8 +318,8 @@ fn runStepNames(b: *std.Build, step_names: []const []const u8) !void {
             .success => continue,
             .failure => {
                 any_failed = true;
-                std.debug.print("{s}: {s}\n", .{
-                    s.name, @errorName(s.result.err_code),
+                std.debug.print("{s}: {s}\n{s}", .{
+                    s.name, @errorName(s.result.err_code), s.result.stderr,
                 });
             },
         }
@@ -404,9 +404,7 @@ fn workerMakeOneStep(
     // *Build object in install header steps that might be able to be removed
     // by passing the *Build object through the make() functions.
     s.make() catch |err| {
-        s.result = .{
-            .err_code = err,
-        };
+        s.result.err_code = err;
         @atomicStore(Step.State, &s.state, .failure, .SeqCst);
         return;
     };