runtime: don't write unique string to trace if it's length zero

While we're here, document that ID 0 is implicitly assigned to an empty
set of data for both stacks and strings.

Change-Id: Ic52ff3a1132abc5a8f6f6c4e4357e31e6e7799fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/723061
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

src/runtime/tracemap.go

@@ -23,6 +23,13 @@ import (
 	"unsafe"
 )
 
+// traceMap is a map of a variable-sized array of bytes to a unique ID.
+//
+// Because traceMap just operates on raw bytes, this type is used as the
+// backing store for both the trace string table and trace stack table,
+// the latter of which is just an array of PCs.
+//
+// ID 0 is reserved for arrays of bytes of size zero.
 type traceMap struct {
 	root atomic.UnsafePointer // *traceMapNode (can't use generics because it's notinheap)
 	_    cpu.CacheLinePad

src/runtime/tracestack.go

@@ -136,8 +136,9 @@ func traceStack(skip int, gp *g, tab *traceStackTable) uint64 {
 	return id
 }
 
-// traceStackTable maps stack traces (arrays of PC's) to unique uint32 ids.
-// It is lock-free for reading.
+// traceStackTable maps stack traces (arrays of PC's) to unique IDs.
+//
+// ID 0 is reserved for a zero-length stack.
 type traceStackTable struct {
 	tab traceMap
 }
@@ -145,8 +146,10 @@ type traceStackTable struct {
 // put returns a unique id for the stack trace pcs and caches it in the table,
 // if it sees the trace for the first time.
 func (t *traceStackTable) put(pcs []uintptr) uint64 {
+	// Even though put will handle this for us, taking the address of pcs forces a bounds check
+	// that will fail if len(pcs) == 0.
 	if len(pcs) == 0 {
-		return 0
+		return 0 // ID 0 is reserved for zero-length stacks.
 	}
 	id, _ := t.tab.put(noescape(unsafe.Pointer(&pcs[0])), uintptr(len(pcs))*unsafe.Sizeof(uintptr(0)))
 	return id

src/runtime/tracestring.go

@@ -12,6 +12,8 @@ import "internal/trace/tracev2"
 
 // traceStringTable is map of string -> unique ID that also manages
 // writing strings out into the trace.
+//
+// ID 0 is reserved for the empty string.
 type traceStringTable struct {
 	// lock protects buf.
 	lock mutex
@@ -37,6 +39,9 @@ func (t *traceStringTable) put(gen uintptr, s string) uint64 {
 
 // emit emits a string and creates an ID for it, but doesn't add it to the table. Returns the ID.
 func (t *traceStringTable) emit(gen uintptr, s string) uint64 {
+	if len(s) == 0 {
+		return 0 // Empty strings are implicitly assigned ID 0 already.
+	}
 	// Grab an ID and write the string to the buffer.
 	id := t.tab.stealID()
 	systemstack(func() {