runtime: print signal name in panic, if name is known

Adds a small function signame that infers a signal name
from the signal table, otherwise will fallback to using
hex(sig) as previously. No signal table is present for
Windows hence it will always print the hex value.

Sample code and new result:
```go
package main

import (
  "fmt"
  "time"
)

func main() {
  defer func() {
    if err := recover(); err != nil {
      fmt.Printf("err=%v\n", err)
    }
  }()

  ticker := time.Tick(1e9)
  for {
    <-ticker
  }
}
```

```shell
$ go run main.go &
$ kill -11 <pid>
fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb01dfacedebac1e
pc=0xc71db]
...
```

Fixes #13969

Change-Id: Ie6be312eb766661f1cea9afec352b73270f27f9d
Reviewed-on: https://go-review.googlesource.com/22753
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

5 files changed, 32 insertions, 1 deletions

diff --git a/src/runtime/os1_nacl.go b/src/runtime/os1_nacl.go
index 622755119d..feea496652 100644
--- a/src/runtime/os1_nacl.go
+++ b/src/runtime/os1_nacl.go
@@ -56,6 +56,13 @@ func osinit() {
 	//nacl_exception_handler(funcPC(sigtramp), nil);
 }
 
+func signame(sig uint32) string {
+	if sig >= uint32(len(sigtable)) {
+		return ""
+	}
+	return sigtable[sig].name
+}
+
 func crash() {
 	*(*int32)(nil) = 0
 }
diff --git a/src/runtime/os1_plan9.go b/src/runtime/os1_plan9.go
index eb7a0c6481..6c7e36d062 100644
--- a/src/runtime/os1_plan9.go
+++ b/src/runtime/os1_plan9.go
@@ -286,3 +286,10 @@ func _atoi(b []byte) int {
 	}
 	return n
 }
+
+func signame(sig uint32) string {
+	if sig >= uint32(len(sigtable)) {
+		return ""
+	}
+	return sigtable[sig].name
+}
diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index 382a20e4e7..60b277d52c 100644
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -641,7 +641,13 @@ var deadlock mutex
 
 func dopanic_m(gp *g, pc, sp uintptr) {
 	if gp.sig != 0 {
-		print("[signal ", hex(gp.sig), " code=", hex(gp.sigcode0), " addr=", hex(gp.sigcode1), " pc=", hex(gp.sigpc), "]\n")
+		signame := signame(gp.sig)
+		if signame != "" {
+			print("[signal ", signame)
+		} else {
+			print("[signal ", hex(gp.sig))
+		}
+		print(" code=", hex(gp.sigcode0), " addr=", hex(gp.sigcode1), " pc=", hex(gp.sigpc), "]\n")
 	}
 
 	level, all, docrash := gotraceback()
diff --git a/src/runtime/signal_unix.go b/src/runtime/signal_unix.go
index 5ce2380daa..f59c9b9549 100644
--- a/src/runtime/signal_unix.go
+++ b/src/runtime/signal_unix.go
@@ -12,3 +12,10 @@ import _ "unsafe" // for go:linkname
 func os_sigpipe() {
 	systemstack(sigpipe)
 }
+
+func signame(sig uint32) string {
+	if sig >= uint32(len(sigtable)) {
+		return ""
+	}
+	return sigtable[sig].name
+}
diff --git a/src/runtime/signal_windows.go b/src/runtime/signal_windows.go
index d54dbf7616..298dcc96a0 100644
--- a/src/runtime/signal_windows.go
+++ b/src/runtime/signal_windows.go
@@ -209,6 +209,10 @@ func raisebadsignal(sig int32) {
 	badsignal2()
 }
 
+func signame(sig uint32) string {
+	return ""
+}
+
 func crash() {
 	// TODO: This routine should do whatever is needed
 	// to make the Windows program abort/crash as it