runtime/trace: missing stacks for trace events emitted when getg().mp.curg == nil, like GoCreate for time.AfterFunc #68093
Description
Here is a simple program that reproduces the issue:
package main
import (
"log"
"os"
"runtime/trace"
"time"
)
func main() {
f, err := os.Create("trace.out")
if err != nil {
log.Fatal(err)
}
defer f.Close()
trace.Start(f)
time.AfterFunc(1*time.Second, func() {
println("hello!")
})
time.Sleep(5 * time.Second)
trace.Stop()
}
In this trace, the GoCreate event that creates the goroutine to run println("hello") will have a stack trace for the new goroutine (really, the start PC) but will not have the stack trace in the scheduler/timer code that actually creates the goroutine.
The reason for this is simple: traceStack, the function that obtains a stack trace for the tracer, skips taking a stack if it's called from a system stack. This has been true as far back as I can find. The reason why it does this is because it uses the fact that it runs on a system stack as a signal that it should collect gp.mp.curg's stack instead. This is reasonable -- if a goroutine calls into the scheduler to yield, it'll be running on a system stack when we emit the trace event, but we crucially do not care about that location, but where the user code was when we yielded.
The fix here is, I think, straightforward: collect a trace from a system stack if and only if gp.mp.curg is nil. This should be sufficient to capture the time.AfterFunc case and cases like it.
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