std.Build: better handling of stderr of child processes

With this commit, the build runner now communicates progress towards
completion of the step graph to the terminal, while also printing the
stderr of child processes as soon as possible, without clobbering each
other, and without clobbering the CLI progress output.

1 files changed, 24 insertions, 2 deletions

diff --git a/lib/std/Build/Step.zig b/lib/std/Build/Step.zig
index 7bab2d9ae4..42698f2190 100644
--- a/lib/std/Build/Step.zig
+++ b/lib/std/Build/Step.zig
@@ -9,7 +9,7 @@ state: State,
 /// Populated only if state is success.
 result: struct {
     err_code: anyerror,
-    stderr: []u8,
+    error_msgs: std.ArrayListUnmanaged([]const u8),
 },
 /// The return addresss associated with creation of this step that can be useful
 /// to print along with debugging messages.
@@ -96,7 +96,7 @@ pub fn init(allocator: Allocator, options: Options) Step {
         .state = .precheck_unstarted,
         .result = .{
             .err_code = undefined,
-            .stderr = &.{},
+            .error_msgs = .{},
         },
         .debug_stack_trace = addresses,
     };
@@ -133,6 +133,28 @@ pub fn cast(step: *Step, comptime T: type) ?*T {
     return null;
 }
 
+/// For debugging purposes, prints identifying information about this Step.
+pub fn dump(step: *Step) void {
+    std.debug.getStderrMutex().lock();
+    defer std.debug.getStderrMutex().unlock();
+
+    const stderr = std.io.getStdErr();
+    const w = stderr.writer();
+    const tty_config = std.debug.detectTTYConfig(stderr);
+    const debug_info = std.debug.getSelfDebugInfo() catch |err| {
+        w.print("Unable to dump stack trace: Unable to open debug info: {s}\n", .{
+            @errorName(err),
+        }) catch {};
+        return;
+    };
+    const ally = debug_info.allocator;
+    w.print("name: '{s}'. creation stack trace:\n", .{step.name}) catch {};
+    std.debug.writeStackTrace(step.getStackTrace(), w, ally, debug_info, tty_config) catch |err| {
+        stderr.writer().print("Unable to dump stack trace: {s}\n", .{@errorName(err)}) catch {};
+        return;
+    };
+}
+
 const Step = @This();
 const std = @import("../std.zig");
 const Build = std.Build;