From 180020e1e53b96a3080f3ac731cb32bbbdee31d5 Mon Sep 17 00:00:00 2001
From: Samuel Williams <samuel.williams@shopify.com>
Date: Sat, 6 Dec 2025 20:35:08 +1300
Subject: [PATCH] Fix `io_pwrite` fiber scheduler hook. (#15428)

Fix io_pwrite fiber scheduler hook.
---
 io.c        | 13 ++++++++++---
 scheduler.c |  8 ++++++--
 vm_dump.c   |  2 +-
 3 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/io.c b/io.c
index 549e7e1317..91537895d2 100644
--- a/io.c
+++ b/io.c
@@ -6254,6 +6254,14 @@ internal_pwrite_func(void *_arg)
 {
     struct prdwr_internal_arg *arg = _arg;
 
+    return (VALUE)pwrite(arg->fd, arg->buf, arg->count, arg->offset);
+}
+
+static VALUE
+pwrite_internal_call(VALUE _arg)
+{
+    struct prdwr_internal_arg *arg = (struct prdwr_internal_arg *)_arg;
+
     VALUE scheduler = rb_fiber_scheduler_current();
     if (scheduler != Qnil) {
         VALUE result = rb_fiber_scheduler_io_pwrite_memory(scheduler, arg->io->self, arg->offset, arg->buf, arg->count, 0);
@@ -6263,8 +6271,7 @@ internal_pwrite_func(void *_arg)
         }
     }
 
-
-    return (VALUE)pwrite(arg->fd, arg->buf, arg->count, arg->offset);
+    return rb_io_blocking_region_wait(arg->io, internal_pwrite_func, arg, RUBY_IO_WRITABLE);
 }
 
 /*
@@ -6316,7 +6323,7 @@ rb_io_pwrite(VALUE io, VALUE str, VALUE offset)
     arg.buf = RSTRING_PTR(tmp);
     arg.count = (size_t)RSTRING_LEN(tmp);
 
-    n = (ssize_t)rb_io_blocking_region_wait(fptr, internal_pwrite_func, &arg, RUBY_IO_WRITABLE);
+    n = (ssize_t)pwrite_internal_call((VALUE)&arg);
     if (n < 0) rb_sys_fail_path(fptr->pathv);
     rb_str_tmp_frozen_release(str, tmp);
 
diff --git a/scheduler.c b/scheduler.c
index 8351fc9945..3e45cbe83e 100644
--- a/scheduler.c
+++ b/scheduler.c
@@ -460,12 +460,14 @@ rb_fiber_scheduler_current_for_threadptr(rb_thread_t *thread)
     }
 }
 
-VALUE
-rb_fiber_scheduler_current(void)
+VALUE rb_fiber_scheduler_current(void)
 {
+    RUBY_ASSERT(ruby_thread_has_gvl_p());
+
     return rb_fiber_scheduler_current_for_threadptr(GET_THREAD());
 }
 
+// This function is allowed to be called without holding the GVL.
 VALUE rb_fiber_scheduler_current_for_thread(VALUE thread)
 {
     return rb_fiber_scheduler_current_for_threadptr(rb_thread_ptr(thread));
@@ -929,6 +931,8 @@ fiber_scheduler_io_pwrite(VALUE _argument) {
 VALUE
 rb_fiber_scheduler_io_pwrite(VALUE scheduler, VALUE io, rb_off_t from, VALUE buffer, size_t length, size_t offset)
 {
+
+
     if (!rb_respond_to(scheduler, id_io_pwrite)) {
         return RUBY_Qundef;
     }
diff --git a/vm_dump.c b/vm_dump.c
index c1a8d70735..e2b4804ab0 100644
--- a/vm_dump.c
+++ b/vm_dump.c
@@ -1436,7 +1436,7 @@ rb_vm_bugreport(const void *ctx, FILE *errout)
                 "---------------------------------------------------\n");
         kprintf("Total ractor count: %u\n", vm->ractor.cnt);
         kprintf("Ruby thread count for this ractor: %u\n", rb_ec_ractor_ptr(ec)->threads.cnt);
-        if (rb_fiber_scheduler_get() != Qnil) {
+        if (ec->thread_ptr->scheduler != Qnil) {
             kprintf("Note that the Fiber scheduler is enabled\n");
         }
         kputs("\n");