runtime: fix spurious "[string too long]" error

Maxstring is not updated in the new string routines,
this makes runtime think that long strings are bogus.
Fixes #8339.

LGTM=crawshaw, iant
R=golang-codereviews, crawshaw, iant
CC=golang-codereviews, khr, rsc
https://golang.org/cl/110930043

src/pkg/runtime/string_test.go

@@ -6,6 +6,7 @@ package runtime_test
 
 import (
 	"runtime"
+	"strings"
 	"testing"
 )
 
@@ -122,3 +123,25 @@ func TestStringW(t *testing.T) {
 		}
 	}
 }
+
+func TestLargeStringConcat(t *testing.T) {
+	output := executeTest(t, largeStringConcatSource, nil)
+	want := "panic: " + strings.Repeat("0", 1<<10) + strings.Repeat("1", 1<<10) +
+		strings.Repeat("2", 1<<10) + strings.Repeat("3", 1<<10)
+	if !strings.HasPrefix(output, want) {
+		t.Fatalf("output does not start with %q:\n%s", want, output)
+	}
+}
+
+var largeStringConcatSource = `
+package main
+import "strings"
+func main() {
+	s0 := strings.Repeat("0", 1<<10)
+	s1 := strings.Repeat("1", 1<<10)
+	s2 := strings.Repeat("2", 1<<10)
+	s3 := strings.Repeat("3", 1<<10)
+	s := s0 + s1 + s2 + s3
+	panic(s)
+}
+`

src/pkg/runtime/stubs.goc

@@ -24,6 +24,7 @@ package runtime
 
 #pragma textflag NOSPLIT
 func rawstring(size intgo) (s String, b Slice) {
+	uintptr ms;
 	byte *p;
 
 	p = runtime·mallocgc(size, 0, FlagNoScan|FlagNoZero);
@@ -32,6 +33,11 @@ func rawstring(size intgo) (s String, b Slice) {
 	b.array = p;
 	b.len = size;
 	b.cap = size;
+	for(;;) {
+		ms = runtime·maxstring;
+		if((uintptr)size <= ms || runtime·casp((void**)&runtime·maxstring, (void*)ms, (void*)size))
+			break;
+	}
 }
 
 #pragma textflag NOSPLIT