std.debug: fix some corner cases #23927

rootbeer · 2025-05-19T04:58:36Z

Add infinite loop detection to the std.debug backtraces. Make the backtrace and stacktrace code more robust on corner-case architectures.

Expand the "unwind.zig" test case to exercise std.debug.dumpCurrentStackTrace(). And trigger a signal handler so the test can exercise std.debug.dumpStackTraceFromBase() and std.debug.StackIterator.initWithContext() using a kernel-constructed context.

This is preparation for moving std.debug away from getContext() (#23801).

lib/std/debug.zig

alexrp · 2025-05-19T06:48:20Z

test/standalone/stack_iterator/unwind.zig

+// Getting the backtrace inside the signal handler (with the ucontext_t)
+// gets stuck in a loop on some systems:
+const expect_signal_frame_overflow =
+    (native_arch == .arm and link_libc); // loops above main()


glibc vs musl? armeb, thumb, thumbeb?

I haven't investigated the failure cases too closely yet. I'm just trying to get the test to compile and not blow up on every architecture. And I'm trying to avoid watering the test down too much on the platforms where it works reliably. That said, the failure I see for this one is when statically linking musl to the test case.

I haven't been building the other ARM variants, so I'll try and mix those in too.

I see this code was unchanged, so do the others work?

alexrp · 2025-05-19T06:52:06Z

test/standalone/stack_iterator/unwind.zig

+    native_arch == .mips or
+    native_arch == .mipsel or
+    native_arch == .mips64 or
+    native_arch == .mips64el or
+    native_arch == .powerpc64 or
+    native_arch == .powerpc64le;


What's wrong with these?

Generally they don't seem to generate traces at all (either through dumpCurrentStackTrace or with StackIterator). Zig doesn't have a ucontext_t on MIPS. I'm not sure what's up with the PowerPC ones.

On a related note, Zig CI failed on aarch64-linux (no libc) because the stack trace gets stuck in a loop above main (see https://github.com/ziglang/zig/actions/runs/15104670128/job/42451228605?pr=23927). So I've added .aarch64 to this list of ignorable failures for now. Its flaky though. The test sometimes builds traces without looping for me locally, and sometimes not. (From what I can tell a specific build of test test is deterministic, but across multiple builds its not.)

This test creates three nested stack frames and then tests stack trace creation. Add some additional tests of stack traces by invoking "dumpCurrentStackTrace()" and by using a signal handler's "context" parameter to feed backtrace construction. Make the test case at least runnable on a wide variety of systems (including Windows, and WASI). Because `ucontext_t` and `getcontext` are not evenly supported everywhere, some systems are expected only get through parts of the test.

rootbeer · 2025-05-20T15:56:32Z

This is ready for a review. I think the actual fixes are all straightforward, but the test is generating a lot of stderr spew (both from the dump-stack-trace functions being tested and my verbose std.debug.print logging. This all shows up in "passing" test output (e.g., https://github.com/ziglang/zig/actions/runs/15124641832/job/42514310709?pr=23927#step:3:2243). Is there something I can do in the build.zig to hide the stderr output when the test passes? Or should I remove my print statements? But, the dump-stack-trace functions are doing most of the logging (and I suspect its generally confusing to see stack traces in passing tests). Should I disable those? (The StackIterator testing covers the meat of the code, but it does seem like the complete routines should get some testing ...)

alexrp · 2025-05-20T17:03:44Z

Is there something I can do in the build.zig to hide the stderr output when the test passes?

You can capture the output from the build script, e.g. by adding an "expected output" check. See for example #23892.

alexrp · 2025-05-20T17:05:02Z

lib/std/debug.zig

@@ -13,6 +13,9 @@ const native_arch = builtin.cpu.arch;
 const native_os = builtin.os.tag;
 const native_endian = native_arch.endian();

+/// Maximum distance to walk when iterating through a stack trace.


Suggested change

/// Maximum distance to walk when iterating through a stack trace.

/// Maximum number of frames to walk when iterating through a stack trace.

alexrp · 2025-05-20T17:06:08Z

test/standalone/stack_iterator/unwind.zig

+    (native_arch != .wasm32) and
+    (native_arch != .wasm64); // wasm has no introspection


nit:

Suggested change

(native_arch != .wasm32) and

(native_arch != .wasm64); // wasm has no introspection

!native_arch.isWasm(); // wasm has no introspection

alexrp · 2025-05-20T17:06:40Z

test/standalone/stack_iterator/unwind.zig

+// gets stuck in a loop on some systems:
+const expect_signal_frame_overflow =
+    (native_arch == .arm and link_libc) or // loops above main()
+    (native_arch == .aarch64); // non-deterministic, sometimes overflows, sometimes not


aarch64_be?

alexrp · 2025-05-20T17:09:06Z

test/standalone/stack_iterator/unwind.zig

+    (native_arch == .x86_64 and link_libc and builtin.abi.isGnu()) or // stuck on pthread_kill?
+    (native_arch == .x86_64 and link_libc and builtin.abi.isMusl() and builtin.omit_frame_pointer) or // immediately confused backtrace
+    (native_arch == .x86_64 and builtin.os.tag.isDarwin()) or // immediately confused backtrace
+    (native_arch == .aarch64 or native_arch == .aarch64_be) or // non-deterministic, sometimes overflows, sometimes confused


nit:

Suggested change

(native_arch == .aarch64 or native_arch == .aarch64_be) or // non-deterministic, sometimes overflows, sometimes confused

native_arch.isAARCH64() or // non-deterministic, sometimes overflows, sometimes confused

alexrp · 2025-05-20T17:09:42Z

test/standalone/stack_iterator/unwind.zig

+    (native_arch == .x86_64 and link_libc and builtin.abi.isMusl() and builtin.omit_frame_pointer) or // immediately confused backtrace
+    (native_arch == .x86_64 and builtin.os.tag.isDarwin()) or // immediately confused backtrace
+    (native_arch == .aarch64 or native_arch == .aarch64_be) or // non-deterministic, sometimes overflows, sometimes confused
+    (native_arch == .riscv64 and link_libc) or // `ucontext_t` not defined yet


riscv32? Also has no ucontext_t IIRC.

alexrp · 2025-05-20T17:10:10Z

test/standalone/stack_iterator/unwind.zig

+    native_arch == .mips or // Missing ucontext_t. Most stack traces are empty ... (with or without libc)
+    native_arch == .mipsel or // same as .mips
+    native_arch == .mips64 or // same as .mips
+    native_arch == .mips64el or // same as .mips


nit:

Suggested change

native_arch == .mips or // Missing ucontext_t. Most stack traces are empty ... (with or without libc)

native_arch == .mipsel or // same as .mips

native_arch == .mips64 or // same as .mips

native_arch == .mips64el or // same as .mips

native_arch.isMIPS() or // Missing ucontext_t. Most stack traces are empty ... (with or without libc)

alexrp · 2025-05-20T17:10:40Z

test/standalone/stack_iterator/unwind.zig

+    native_arch == .powerpc64 or // dumpCurrent* useless, StackIterator empty, ctx-based trace empty (with or without libc)
+    native_arch == .powerpc64le; // same as .powerpc64


nit:

Suggested change

native_arch == .powerpc64 or // dumpCurrent* useless, StackIterator empty, ctx-based trace empty (with or without libc)

native_arch == .powerpc64le; // same as .powerpc64

native_arch.isPowerPC64() or // dumpCurrent* useless, StackIterator empty, ctx-based trace empty (with or without libc)

Also what about powerpc?

rootbeer · 2025-05-20T18:26:59Z

@alexrp Thanks again for the reviews! One more question before I push a new version up: Should I make changes anywhere to get this test to compile/run against targets other than the default on CI?

alexrp · 2025-05-20T18:32:14Z

I guess you could just change the test's build.zig to ignore the standard target option and instead build & run for a predefined set of targets? test/llvm_targets.zig is a good resource for a list of targets that are actually relevant/real (although some of them we obviously don't fully support yet).

rootbeer added 3 commits May 18, 2025 15:17

posix: windows does not have getcontext

37937c1

debug.zig: protect against incomplete configurations

ac2289e

debug.zig: use Windows APIs on .windows codepath

c7b5dc4

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rootbeer added 2 commits May 19, 2025 14:36

debug.zig: prevent infinite loops during backtraces

44f476c

rootbeer force-pushed the debug-mcontext-stage1 branch from b95af2b to 37ebc96 Compare May 19, 2025 22:41

rootbeer mentioned this pull request May 19, 2025

Stack trace on panic loops forever #18286

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std.debug: fix some corner cases #23927

std.debug: fix some corner cases #23927

rootbeer commented May 19, 2025

alexrp May 19, 2025 •

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rootbeer commented May 20, 2025

alexrp commented May 20, 2025

	/// Maximum distance to walk when iterating through a stack trace.
	/// Maximum number of frames to walk when iterating through a stack trace.

		(native_arch != .wasm32) and
		(native_arch != .wasm64); // wasm has no introspection

	(native_arch != .wasm32) and
	(native_arch != .wasm64); // wasm has no introspection
	!native_arch.isWasm(); // wasm has no introspection

	(native_arch == .aarch64 or native_arch == .aarch64_be) or // non-deterministic, sometimes overflows, sometimes confused
	native_arch.isAARCH64() or // non-deterministic, sometimes overflows, sometimes confused

		native_arch == .powerpc64 or // dumpCurrent* useless, StackIterator empty, ctx-based trace empty (with or without libc)
		native_arch == .powerpc64le; // same as .powerpc64

	native_arch == .powerpc64 or // dumpCurrent* useless, StackIterator empty, ctx-based trace empty (with or without libc)
	native_arch == .powerpc64le; // same as .powerpc64
	native_arch.isPowerPC64() or // dumpCurrent* useless, StackIterator empty, ctx-based trace empty (with or without libc)

std.debug: fix some corner cases #23927

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std.debug: fix some corner cases #23927

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