From 8bda42e6f56b09d6cad4326b4c3241652f43dbc9 Mon Sep 17 00:00:00 2001
From: Robert Buessow <robert.buessow@relational.ai>
Date: Thu, 28 May 2026 13:24:29 +0000
Subject: [PATCH] Add profiler signal masking to init_runtime

Block SIGUSR2/SIGPROF on the calling thread before starting the Tokio
runtime so worker threads inherit a mask that excludes Julia profiler
signals, preventing UB hangs when Profile.@profile is active.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
---
 src/RustyIceberg.jl | 80 ++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 72 insertions(+), 8 deletions(-)

diff --git a/src/RustyIceberg.jl b/src/RustyIceberg.jl
index b68f935..d2ac905 100644
--- a/src/RustyIceberg.jl
+++ b/src/RustyIceberg.jl
@@ -85,6 +85,63 @@ end
 const _INIT_LOCK::ReentrantLock = ReentrantLock()
 _PANIC_HOOK::Function = default_panic_hook
 
+# ---------------------------------------------------------------------------
+# Profiler signal masking
+# ---------------------------------------------------------------------------
+# Julia's CPU profiler delivers SIGUSR2 to every adopted Julia thread to
+# collect backtraces. Tokio worker threads become adopted Julia threads via
+# the @ccallable preamble in notify_result_iceberg, but while they execute
+# Rust code between Julia callbacks their Julia task context is incomplete.
+# Receiving SIGUSR2 in that state causes UB in Julia's signal handler,
+# manifesting as a hang when Profile.@profile is active.
+#
+# Fix: block SIGUSR2 (and SIGPROF, used by older Julia / macOS profilers)
+# on the calling thread before starting the Tokio runtime. Tokio worker
+# threads inherit the mask and are never targeted by the profiler.
+# The mask is restored on the calling thread after init_runtime returns.
+
+@static if Sys.islinux()
+    const _SIGSET_T_BYTES = 128   # sizeof(sigset_t) on Linux
+    const _SIG_BLOCK      = Cint(0)
+    const _SIG_SETMASK    = Cint(2)
+    const _SIGUSR2_NUM    = 12
+    const _SIGPROF_NUM    = 27
+
+    function _profiler_sigset()
+        buf = zeros(UInt8, _SIGSET_T_BYTES)
+        for sig in (_SIGUSR2_NUM, _SIGPROF_NUM)
+            buf[div(sig - 1, 8) + 1] |= UInt8(1 << ((sig - 1) % 8))
+        end
+        buf
+    end
+    const _PROFILER_SIGSET = _profiler_sigset()
+
+    function _block_profiler_signals()
+        old = zeros(UInt8, _SIGSET_T_BYTES)
+        GC.@preserve old _PROFILER_SIGSET begin
+            @ccall pthread_sigmask(
+                _SIG_BLOCK::Cint,
+                pointer(_PROFILER_SIGSET)::Ptr{Cvoid},
+                pointer(old)::Ptr{Cvoid},
+            )::Cint
+        end
+        return old
+    end
+
+    function _restore_signal_mask(old::Vector{UInt8})
+        GC.@preserve old begin
+            @ccall pthread_sigmask(
+                _SIG_SETMASK::Cint,
+                pointer(old)::Ptr{Cvoid},
+                C_NULL::Ptr{Cvoid},
+            )::Cint
+        end
+    end
+else
+    _block_profiler_signals() = nothing
+    _restore_signal_mask(::Nothing) = nothing
+end
+
 Base.@ccallable function iceberg_panic_hook_wrapper()::Cint
     global _PANIC_HOOK
     _PANIC_HOOK()
@@ -144,15 +201,22 @@ function init_runtime(
         _PANIC_HOOK = on_rust_panic
         panic_fn_ptr = @cfunction(iceberg_panic_hook_wrapper, Cint, ())
         fn_ptr = @cfunction(notify_result_iceberg, Cint, (Ptr{Nothing},))
-        res = @ccall rust_lib.iceberg_init_runtime(config::StaticConfig, panic_fn_ptr::Ptr{Nothing}, fn_ptr::Ptr{Nothing})::Cint
-        if res != 0
-            throw(IcebergException(
-                INTERNAL,
-                "Failed to initialize Iceberg runtime",
-                "iceberg_init_runtime returned $res",
-            ))
+        # Block profiler signals before starting Tokio so its worker threads
+        # inherit a mask that excludes SIGUSR2/SIGPROF (see _block_profiler_signals).
+        old_mask = _block_profiler_signals()
+        try
+            res = @ccall rust_lib.iceberg_init_runtime(config::StaticConfig, panic_fn_ptr::Ptr{Nothing}, fn_ptr::Ptr{Nothing})::Cint
+            if res != 0
+                throw(IcebergException(
+                    INTERNAL,
+                    "Failed to initialize Iceberg runtime",
+                    "iceberg_init_runtime returned $res",
+                ))
+            end
+            _ICEBERG_STARTED[] = true
+        finally
+            _restore_signal_mask(old_mask)
         end
-        _ICEBERG_STARTED[] = true
     end
     return nothing
 end