Commit 111165513156d5732d85e5ccb52b9d8bded41ffa | zig

author	Matthew Lugg <mlugg@mlugg.co.uk> 2026-01-06 10:50:45+00:00
committer	Matthew Lugg <mlugg@mlugg.co.uk> 2026-01-06 10:50:45+00:00
log	111165513156d5732d85e5ccb52b9d8bded41ffa
tree	d92c2b23c5a611a3383cc4fe3fd441d1724fe831
parent	9d08eba2e111e96133b51c14aca7dc071920b6d2
signature	Commit is signed but in an unrecognized format.

std: block cancelation in default panic and segfault handlers

It doesn't make any sense for a task to be canceled while it's
panicking.

As a happy accident, this also solves some cases where safety panics in
`Io.Threaded` would cause stack traces not to print due to invalid
thread-local state: when cancelation is blocked, `Io.Threaded` doesn't
consult said thread-local state at all. For instance, try inserting a
panic just after a call to `Syscall.start()` in `Io.Threaded`, and then
call the `Io` function in question from a `concurrent` task. Before this
PR, the stack trace fails to print, because the panic handler sees the
thread-local cancelation state in an unexpected state, leading to a
recursive panic. After this PR, the stack trace prints fine.

1 files changed, 8 insertions(+), 0 deletions(-)

lib
- std
  - debug.zig

lib/std/debug.zig+8

 @@ -532,6 +532,10 @@ pub fn defaultPanic(msg: []const u8, first_trace_addr: ?usize) noreturn {
         else => {},
     }
+    // Don't try to cancel during a panic. No need to re-enable cancelation,
+    // because the panic handler doesn't return.
+    _ = std.Options.debug_io.swapCancelProtection(.blocked);
     if (enable_segfault_handler) {
         // If a segfault happens while panicking, we want it to actually segfault, not trigger
         // the handler.
 @@ -1533,6 +1537,10 @@ fn handleSegfault(addr: ?usize, name: []const u8, opt_ctx: ?CpuContextPtr) noret
 }
 pub fn defaultHandleSegfault(addr: ?usize, name: []const u8, opt_ctx: ?CpuContextPtr) noreturn {
+    // Don't try to cancel during a segfault. No need to re-enable cancelation,
+    // because the segfault handler doesn't return.
+    _ = std.Options.debug_io.swapCancelProtection(.blocked);
     // There is very similar logic to the following in `defaultPanic`.
     switch (panic_stage) {
         0 => {