runtime/debug: add SetTraceback

Programs that call panic to crash after detecting a serious problem
may wish to use SetTraceback to force printing of all goroutines first.

Change-Id: Ib23ad9336f405485aabb642ca73f454a14c8baf3
Reviewed-on: https://go-review.googlesource.com/18043
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

3 files changed, 48 insertions, 20 deletions

diff --git a/src/runtime/debug/garbage.go b/src/runtime/debug/garbage.go
index 41202f9e3b..8d6d2782e9 100644
--- a/src/runtime/debug/garbage.go
+++ b/src/runtime/debug/garbage.go
@@ -157,3 +157,14 @@ func SetPanicOnFault(enabled bool) bool {
 // it to the given file descriptor.
 // The heap dump format is defined at https://golang.org/s/go13heapdump.
 func WriteHeapDump(fd uintptr)
+
+// SetTraceback sets the amount of detail printed by the runtime in
+// the traceback it prints before exiting due to an unrecovered panic
+// or an internal runtime error.
+// The level argument takes the same values as the GOTRACEBACK
+// environment variable. For example, SetTraceback("all") ensure
+// that the program prints all goroutines when it crashes.
+// See the package runtime documentation for details.
+// If SetTraceback is called with a level lower than that of the
+// environment variable, the call is ignored.
+func SetTraceback(level string)
diff --git a/src/runtime/extern.go b/src/runtime/extern.go
index a3b23a448b..cab8de0481 100644
--- a/src/runtime/extern.go
+++ b/src/runtime/extern.go
@@ -127,6 +127,10 @@ manner instead of exiting. For example, on Unix systems, the crash raises
 SIGABRT to trigger a core dump.
 For historical reasons, the GOTRACEBACK settings 0, 1, and 2 are synonyms for
 none, all, and system, respectively.
+The runtime/debug package's SetTraceback function allows increasing the
+amount of output at run time, but it cannot reduce the amount below that
+specified by the environment variable.
+See https://golang.org/pkg/runtime/debug/#SetTraceback.
 
 The GOARCH, GOOS, GOPATH, and GOROOT environment variables complete
 the set of Go environment variables. They influence the building of Go programs
diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index 3c4f47dd2e..f63e09cc61 100644
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -22,6 +22,7 @@ const (
 )
 
 var traceback_cache uint32 = 2 << tracebackShift
+var traceback_env uint32
 
 // gotraceback returns the current traceback settings.
 //
@@ -39,9 +40,10 @@ func gotraceback() (level int32, all, crash bool) {
 		level = int32(_g_.m.traceback)
 		return
 	}
-	crash = traceback_cache&tracebackCrash != 0
-	all = all || traceback_cache&tracebackAll != 0
-	level = int32(traceback_cache >> tracebackShift)
+	t := atomic.Load(&traceback_cache)
+	crash = t&tracebackCrash != 0
+	all = all || t&tracebackAll != 0
+	level = int32(t >> tracebackShift)
 	return
 }
 
@@ -382,36 +384,47 @@ func parsedebugvars() {
 		}
 	}
 
-	switch p := gogetenv("GOTRACEBACK"); p {
+	setTraceback(gogetenv("GOTRACEBACK"))
+	traceback_env = traceback_cache
+
+	if debug.gcstackbarrierall > 0 {
+		firstStackBarrierOffset = 0
+	}
+
+	// For cgocheck > 1, we turn on the write barrier at all times
+	// and check all pointer writes.
+	if debug.cgocheck > 1 {
+		writeBarrier.cgo = true
+		writeBarrier.enabled = true
+	}
+}
+
+//go:linkname setTraceback runtime/debug.SetTraceback
+func setTraceback(level string) {
+	var t uint32
+	switch level {
 	case "none":
-		traceback_cache = 0
+		t = 0
 	case "single", "":
-		traceback_cache = 1 << tracebackShift
+		t = 1 << tracebackShift
 	case "all":
-		traceback_cache = 1<<tracebackShift | tracebackAll
+		t = 1<<tracebackShift | tracebackAll
 	case "system":
-		traceback_cache = 2<<tracebackShift | tracebackAll
+		t = 2<<tracebackShift | tracebackAll
 	case "crash":
-		traceback_cache = 2<<tracebackShift | tracebackAll | tracebackCrash
+		t = 2<<tracebackShift | tracebackAll | tracebackCrash
 	default:
-		traceback_cache = uint32(atoi(p))<<tracebackShift | tracebackAll
+		t = uint32(atoi(level))<<tracebackShift | tracebackAll
 	}
 	// when C owns the process, simply exit'ing the process on fatal errors
 	// and panics is surprising. Be louder and abort instead.
 	if islibrary || isarchive {
-		traceback_cache |= tracebackCrash
+		t |= tracebackCrash
 	}
 
-	if debug.gcstackbarrierall > 0 {
-		firstStackBarrierOffset = 0
-	}
+	t |= traceback_env
 
-	// For cgocheck > 1, we turn on the write barrier at all times
-	// and check all pointer writes.
-	if debug.cgocheck > 1 {
-		writeBarrier.cgo = true
-		writeBarrier.enabled = true
-	}
+	atomic.Store(&traceback_cache, t)
 }
 
 // Poor mans 64-bit division.