runtime: don't duplicate reraised panic values in printpanics

Change the output printed when crashing with a reraised panic value
to not duplicate that value.

Changes output of panicking with "PANIC", recovering, and reraising
from:

  panic: PANIC [recovered]
    panic: PANIC

to:

  panic: PANIC [recovered, reraised]

Fixes #71517

Change-Id: Id59938c4ea0df555b851ffc650fe6f94c0845499
Reviewed-on: https://go-review.googlesource.com/c/go/+/645916
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index 3ffb3966d0..2dd3c3c2db 100644
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -649,6 +649,13 @@ func preprintpanics(p *_panic) {
 		}
 	}()
 	for p != nil {
+		if p.link != nil && *efaceOf(&p.link.arg) == *efaceOf(&p.arg) {
+			// This panic contains the same value as the next one in the chain.
+			// Mark it as reraised. We will skip printing it twice in a row.
+			p.link.reraised = true
+			p = p.link
+			continue
+		}
 		switch v := p.arg.(type) {
 		case error:
 			p.arg = v.Error()
@@ -664,6 +671,9 @@ func preprintpanics(p *_panic) {
 func printpanics(p *_panic) {
 	if p.link != nil {
 		printpanics(p.link)
+		if p.link.reraised {
+			return
+		}
 		if !p.link.goexit {
 			print("\t")
 		}
@@ -673,7 +683,9 @@ func printpanics(p *_panic) {
 	}
 	print("panic: ")
 	printpanicval(p.arg)
-	if p.recovered {
+	if p.reraised {
+		print(" [recovered, reraised]")
+	} else if p.recovered {
 		print(" [recovered]")
 	}
 	print("\n")