nandi/cosmicnim · fcee0b9

⑂ fcee0b9

Stop Nim's signal handler from crashing on libcosmic's threads

The gallery segfaulted with a stack of signalHandler -> rawNewString ->
rawAlloc on a thread named "notify-rs inoti". Nim installs its signal
handlers process-wide and they allocate to build a traceback, so a signal
landing on a thread libcosmic created — a tokio worker, or the inotify
watcher — faults inside the handler, and the resulting core points at the
allocator rather than at whatever actually happened.

Reproduced it with tgkill against that exact thread: default build dies
with SIGSEGV, -d:noSignalHandler takes the default action and exits
cleanly. examples/nim.cfg sets the flag, and the binding warns when a
host forgets it, since this bites any program that loads the library.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nandithebull committed 2026-09-20T10:52:58-07:00 Browse files

fcee0b9 parent: 6992433

modified README.md +13 -0 Viewed

@@ -72,6 +72,19 @@ v0.2.0 release and no other, so a stale library cannot load against newer
72	72	bindings. `just fetch v0.1.2` (or `COSMICNIM_TAG=v0.1.2 nimble fetchLib`)
73	73	overrides that when you want it.
74	74
	75	+## Signal handling
	76	+
	77	+Compile anything that loads this library with `-d:noSignalHandler`
	78	+(`examples/nim.cfg` does it for the demo, and the binding warns if you
	79	+forget).
	80	+
	81	+libcosmic creates threads Nim knows nothing about — tokio workers and a
	82	+`notify-rs` inotify watcher. Nim installs its signal handlers process-wide,
	83	+and the handler allocates a string to build a traceback, so a signal
	84	+delivered to one of those threads faults inside the handler. The crash it
	85	+produces points at `rawAlloc`, which tells you nothing about the real
	86	+cause; the handler has already destroyed that evidence.
	87	+
75	88	## As a dependency
76	89
77	90	```bash

  bindings. `just fetch v0.1.2` (or `COSMICNIM_TAG=v0.1.2 nimble fetchLib`)
  overrides that when you want it.
++## Signal handling
++
++Compile anything that loads this library with `-d:noSignalHandler`
++(`examples/nim.cfg` does it for the demo, and the binding warns if you
++forget).
++
++libcosmic creates threads Nim knows nothing about — tokio workers and a
++`notify-rs` inotify watcher. Nim installs its signal handlers process-wide,
++and the handler allocates a string to build a traceback, so a signal
++delivered to one of those threads faults inside the handler. The crash it
++produces points at `rawAlloc`, which tells you nothing about the real
++cause; the handler has already destroyed that evidence.
++
  ## As a dependency
  ```bash

added examples/nim.cfg +6 -0 Viewed

new file mode 100644
@@ -0,0 +1,6 @@
	1	+# libcosmic creates threads Nim knows nothing about (tokio workers, the
	2	+# notify-rs inotify watcher). Nim's signal handlers are installed process-wide
	3	+# and allocate a string to build a traceback, so a signal delivered to one of
	4	+# those threads segfaults inside the handler itself. Let the default action
	5	+# stand instead.
	6	+-d:noSignalHandler

++# libcosmic creates threads Nim knows nothing about (tokio workers, the
++# notify-rs inotify watcher). Nim's signal handlers are installed process-wide
++# and allocate a string to build a traceback, so a signal delivered to one of
++# those threads segfaults inside the handler itself. Let the default action
++# stand instead.
++-d:noSignalHandler

modified nim/cosmicnim.nim +5 -0 Viewed

@@ -16,6 +16,11 @@
16	16
17	17	import std/os
18	18
	19	+when not defined(noSignalHandler):
	20	+ {.warning: "compile with -d:noSignalHandler: libcosmic runs threads Nim " &
	21	+ "did not create, and Nim's signal handler allocates, so a signal " &
	22	+ "delivered to one of them crashes inside the handler".}
	23	+
19	24	const libCosmicFfi* {.strdefine.} = currentSourcePath().parentDir /
20	25	"libcosmic_ffi.so"
21	26	## Absolute at compile time, so the library is found wherever nimble put

  import std/os
++when not defined(noSignalHandler):
++  {.warning: "compile with -d:noSignalHandler: libcosmic runs threads Nim " &
++             "did not create, and Nim's signal handler allocates, so a signal " &
++             "delivered to one of them crashes inside the handler".}
++
  const libCosmicFfi* {.strdefine.} = currentSourcePath().parentDir /
                                      "libcosmic_ffi.so"
    ## Absolute at compile time, so the library is found wherever nimble put