Build.Step.Run: fix missing stdin buffer and flush

Writer.sendFileAll() asserts non-zero buffer capacity in the case that
the fallback is hit. It also requires the caller to flush. The buffer
may be bypassed as an optimization but this is not a guarantee.

Also improve the Writer documentation and add an earlier assert on
buffer capacity in sendFileAll().

1 files changed, 9 insertions, 3 deletions

diff --git a/lib/std/Build/Step/Run.zig b/lib/std/Build/Step/Run.zig
index 823e9a8b41..78cf08dd43 100644
--- a/lib/std/Build/Step/Run.zig
+++ b/lib/std/Build/Step/Run.zig
@@ -1780,9 +1780,10 @@ fn evalGeneric(run: *Run, child: *std.process.Child) !StdIoResult {
             };
             defer file.close();
             // TODO https://github.com/ziglang/zig/issues/23955
-            var buffer: [1024]u8 = undefined;
-            var file_reader = file.reader(&buffer);
-            var stdin_writer = child.stdin.?.writer(&.{});
+            var read_buffer: [1024]u8 = undefined;
+            var file_reader = file.reader(&read_buffer);
+            var write_buffer: [1024]u8 = undefined;
+            var stdin_writer = child.stdin.?.writer(&write_buffer);
             _ = stdin_writer.interface.sendFileAll(&file_reader, .unlimited) catch |err| switch (err) {
                 error.ReadFailed => return run.step.fail("failed to read from {f}: {t}", .{
                     path, file_reader.err.?,
@@ -1791,6 +1792,11 @@ fn evalGeneric(run: *Run, child: *std.process.Child) !StdIoResult {
                     stdin_writer.err.?,
                 }),
             };
+            stdin_writer.interface.flush() catch |err| switch (err) {
+                error.WriteFailed => return run.step.fail("failed to write to stdin: {t}", .{
+                    stdin_writer.err.?,
+                }),
+            };
             child.stdin.?.close();
             child.stdin = null;
         },