Print thread ID in panic message #115746

tgross35 · 2023-09-11T05:19:01Z

panic! does not print any identifying information for threads that are
unnamed. However, in many cases, the thread ID can be determined.

This changes the panic message from something like this:

thread '<unnamed>' panicked at src/main.rs:3:5:
explicit panic

To something like this:

thread '<unnamed>' (12345) panicked at src/main.rs:3:5:
explicit panic

Stack overflow messages are updated as well.

This change applies to both named and unnamed threads. The ID printed is
the OS integer thread ID rather than the Rust thread ID, which should
also be what debuggers print.

try-job: x86_64-msvc-1
try-job: x86_64-mingw-*

rustbot · 2023-09-11T05:19:09Z

r? @davidtwco

(rustbot has picked a reviewer for you, use r? to override)

tgross35 · 2023-09-11T05:19:52Z

I think this needs FCP since it's a visible change

@rustbot label -T-libs -T-compiler +T-libs-api
r? libs-api

rustbot · 2023-09-11T05:35:09Z

The Miri subtree was changed

cc @rust-lang/miri

tgross35 · 2023-09-11T06:21:00Z

cc @m-ou-se since you recently did the formatting in #112849

bjorn3 · 2023-09-11T08:16:28Z

Maybe

thread '<unnamed 2>' panicked at src/main.rs:3:5:
explicit panic

or

thread '<unnamed#2>' panicked at src/main.rs:3:5:
explicit panic

? That fits more with the way named threads are shown.

the8472 · 2023-09-11T09:22:55Z

Note that this is a rust-internal thread ID, which isn't the OS thread ID which can be relevant if you're also logging from C code or looking at things with GDB. And as_u64() is unstable and there are questions about stabilizing it (#110738).

tgross35 · 2023-09-11T10:04:18Z

Maybe

thread '<unnamed 2>' panicked at src/main.rs:3:5:
explicit panic

or

thread '<unnamed#2>' panicked at src/main.rs:3:5:
explicit panic

I just proposed the syntax as a demo, figured libs-api would make the final choice - though between those two, I like the first one better

Note that this is a rust-internal thread ID, which isn't the OS thread ID which can be relevant if you're also logging from C code or looking at things with GDB. And as_u64() is unstable and there are questions about stabilizing it (#110738).

It is kind of unfortunate that Rust and C won't use the same thread IDs. But this seems worthwhile still - Rust-only applications printing the thread ID in logs is common (tracing even has a builtin way to do this), probably more common than Rust thread IDs silently mixing with C thread IDs in logs. And even in that case, it still gives you something more concrete to trace than "something failed".

This is all inspired by the below failure I recently had in an highly threaded system. The entire backtrace uselessly displayed one repeated function, with no hints about which preceding log events were relevant:

thread '<unnamed>' panicked at src/scan.rs:101:55:
called `Result::unwrap()` on an `Err` value: ()
stack backtrace:
   0:        0x1009661d0 - __mh_execute_header
   1:        0x10088869b - __mh_execute_header
   2:        0x10093ba0e - __mh_execute_header
   4:        0x10096757c - __mh_execute_header
   5:        0x100968632 - __mh_execute_header
   6:        0x1009680f4 - __mh_execute_header
   7:        0x100968059 - __mh_execute_header
   8:        0x100968042 - __mh_execute_header
   9:        0x100bdd913 - __mh_execute_header

I don't think that stability of the thread ID is much of a concern since we will always have some integer ID for a thread, even if the semantics of that ID might change. The important thing is probably just that this panic message thread ID is the same as what shows up as Debug for ThreadId (or as_u64 as applicable) since that is what other libraries are directed to use. That is the case here

tgross35 · 2023-09-15T07:18:44Z

Another display option - it looks a bit cleaner to not even call out that a thread is unnamed, and I don't think it adds much

thread with ID 2 panicked at src/main.rs:3:5:
explicit panic

Or we could use ThreadId's debug formatting which tracing and I think log do, but that seems unnecessarily clunky

thread with `ThreadId(2)` panicked at src/main.rs:3:5:
explicit panic

I also tried to poke some discussion at #67939

bors · 2023-09-19T19:48:12Z

☔ The latest upstream changes (presumably #115627) made this pull request unmergeable. Please resolve the merge conflicts.

tgross35 · 2023-10-23T23:50:36Z

@joshtriplett do you have any thoughts on this?

timClicks · 2023-11-10T03:02:51Z

Sorry to add bike shedding to the thread. Here is a slight derivative of the last suggestion, which omits a "with":

thread `ThreadId(2)` panicked at src/main.rs:3:5:
explicit panic

Although it's uglier than using the thread identifier in a sentence, using the Debug form makes it look like a Rust value rather than something else. It's also consistent with other output from Rust, although the case can definitely be made that thread IDs deserve a special case.

tgross35 · 2023-12-01T05:35:09Z

I know Josh has a pretty busy queue, so perhaps it is best to reroll

r? libs-api

dtolnay · 2023-12-01T06:37:52Z

I think this needs FCP since it's a visible change

@rustbot label -T-libs -T-compiler +T-libs-api

I think it wouldn't need one. T-libs-api FCP is primarily for any time a permanent API commitment is being made by the standard library, and deprecations. In contrast, anything that we can just change back with minimal effort or disruption would not warrant FCP.

#112849 had a T-libs FCP but I am not familiar with their criteria for that. @rust-lang/libs would you want to handle this PR?

In any case, this looks good to me.

thomcc · 2023-12-01T06:44:15Z

I don't know that this needs FCP. I'm not opposed and there is precedent for it, but OTOH we wouldn't FCP a panic message change somewhere in the stdlib. So I don't feel strongly.

In any case, I think this change is a good idea, and am in favor.

`panic!` does not print any identifying information for threads that are unnamed. However, in many cases, the thread ID can be determined. This changes the panic message from something like this: thread '<unnamed>' panicked at src/main.rs:3:5: explicit panic To something like this: thread '<unnamed>' (0xff9bf) panicked at src/main.rs:3:5: explicit panic Stack overflow messages are updated as well. This change applies to both named and unnamed threads. The ID printed is the OS integer thread ID rather than the Rust thread ID, which should also be what debuggers print.

tgross35 · 2025-06-13T22:03:58Z

After a very long time I finally got around to fixing up my oldest open r-l/r PR. There are likely to be some rough edges still and tests that I need to update still, but @cuviper this should be ready for a look.

This now prints the OS thread ID except on platforms where this isn't easy to access, in which case the Rust thread ID's integral value is printed. I believe this should address the FCP concern. The output format is what Josh T suggested above:

thread '<unnamed>' (1234) panicked at $DIR/test-panic-abort-nocapture.rs:32:5:
thread 'main' (1234) panicked at $DIR/test-panic-abort-nocapture.rs:32:5:

Which also matches the most popular voted option at https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/Printing.20thread.20ID.20on.20panic.20bikeshed/near/407588656.

And since it seems like there really should be a more cross platform way to do this kind of thing, I wrote a short POSIX RFC so maybe one decade we can provide user API to access this identifier https://www.mail-archive.com/austin-group-l@opengroup.org/msg13876.html.

@rustbot review

Various platforms provide a function to return the current OS thread ID, but they all use a slightly different name. Add shims for these functions for Apple, FreeBSD, and Windows, with tests to account for those and a few more platforms that are not yet supported by Miri. These should be useful in general but should also help support printing the OS thread ID in panic messages [1]. [1]: rust-lang/rust#115746

tgross35 · 2025-06-13T22:16:44Z

library/std/src/sys/pal/windows/thread.rs

@@ -115,6 +115,14 @@ impl Thread {
    }
 }

+pub(crate) fn current_os_id() -> Option<u64> {
+    // SAFETY: FFI call with no preconditions.
+    let id: u32 = unsafe { c::GetThreadId(c::GetCurrentThread()) };


I am going to change this to GetCurrentThreadId that does both steps, but not until the Miri shims merge so I can stop manually syncing changes.

Print thread ID in panic message `panic!` does not print any identifying information for threads that are unnamed. However, in many cases, the thread ID can be determined. This changes the panic message from something like this: thread '<unnamed>' panicked at src/main.rs:3:5: explicit panic To something like this: thread '<unnamed>' (12345) panicked at src/main.rs:3:5: explicit panic Stack overflow messages are updated as well. This change applies to both named and unnamed threads. The ID printed is the OS integer thread ID rather than the Rust thread ID, which should also be what debuggers print. try-job: `*various*` try-job: `x86_64-msvc*`

Print thread ID in panic message `panic!` does not print any identifying information for threads that are unnamed. However, in many cases, the thread ID can be determined. This changes the panic message from something like this: thread '<unnamed>' panicked at src/main.rs:3:5: explicit panic To something like this: thread '<unnamed>' (12345) panicked at src/main.rs:3:5: explicit panic Stack overflow messages are updated as well. This change applies to both named and unnamed threads. The ID printed is the OS integer thread ID rather than the Rust thread ID, which should also be what debuggers print. try-job: aarch64-gnu try-job: test-various try-job: x86_64-gnu try-job: x86_64-gnu-aux try-job: x86_64-msvc-1

tgross35 · 2025-06-14T19:07:28Z

@bors2 try

rust-bors · 2025-06-14T19:07:32Z

⌛ Trying commit a08fb17 with merge 2591081…

To cancel the try build, run the command @bors2 try cancel.

Print thread ID in panic message `panic!` does not print any identifying information for threads that are unnamed. However, in many cases, the thread ID can be determined. This changes the panic message from something like this: thread '<unnamed>' panicked at src/main.rs:3:5: explicit panic To something like this: thread '<unnamed>' (12345) panicked at src/main.rs:3:5: explicit panic Stack overflow messages are updated as well. This change applies to both named and unnamed threads. The ID printed is the OS integer thread ID rather than the Rust thread ID, which should also be what debuggers print. try-job: x86_64-msvc-1 try-job: `x86_64-mingw-*`

rust-bors · 2025-06-14T21:41:28Z

☀️ Try build successful (CI)
Build commit: 2591081 (2591081df6a34c230b27998270b2e49294c7aa24, parent: 4a73e3c224465c0c0e71b39b479a0911460dd794)

rustbot assigned davidtwco Sep 11, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Sep 11, 2023

rustbot added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. and removed T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Sep 11, 2023

rustbot assigned joshtriplett and unassigned davidtwco Sep 11, 2023

tgross35 force-pushed the unnamed-threads-panic-message branch from d68fab6 to 371dd28 Compare September 11, 2023 05:35

tgross35 force-pushed the unnamed-threads-panic-message branch 2 times, most recently from 8e72037 to 3da1046 Compare September 11, 2023 06:13

rustbot removed the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Sep 11, 2023

tgross35 force-pushed the unnamed-threads-panic-message branch from 3da1046 to e055544 Compare November 9, 2023 17:20

rustbot assigned dtolnay and unassigned joshtriplett Dec 1, 2023

dtolnay approved these changes Dec 1, 2023

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tgross35 added 2 commits June 13, 2025 21:59

Work around stage1 vs. stage2 differences in miri

3bab080

tgross35 force-pushed the unnamed-threads-panic-message branch from fa35fe7 to 3bab080 Compare June 13, 2025 22:01

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 13, 2025

tgross35 commented Jun 13, 2025

View reviewed changes

tgross35 added 3 commits June 13, 2025 23:15

fixup! Print thread ID in panic message if thread name is unknown

8eeabd5

fixup! Add normalization for thread IDs in panic messages

ee6756b

fixup! Work around stage1 vs. stage2 differences in miri

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fixup! Print thread ID in panic message if thread name is unknown

a08fb17

Print thread ID in panic message #115746

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Print thread ID in panic message #115746

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