fmt: fix size returned on fast path of ReadRune

Fixes #8512.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/130090043

diff --git a/src/pkg/fmt/scan.go b/src/pkg/fmt/scan.go
index 8a337e479d02a22bcc79425372618478d4d967aa..d7befeae43eee88d0543aa89d5b0474e56a95956 100644 (file)
--- a/src/pkg/fmt/scan.go
+++ b/src/pkg/fmt/scan.go
@@ -360,6 +360,7 @@ func (r *readRune) ReadRune() (rr rune, size int, err error) {
        }
        if r.buf[0] < utf8.RuneSelf { // fast check for common ASCII case
                rr = rune(r.buf[0])
+               size = 1 // Known to be 1.
                return
        }
        var n int

diff --git a/src/pkg/fmt/scan_test.go b/src/pkg/fmt/scan_test.go
index d903f0c3ff74cfcf254729439b2293ad1ba3cf1e..541e12df2102fa946ceae6e7f18dbb98a6c4f4d5 100644 (file)
--- a/src/pkg/fmt/scan_test.go
+++ b/src/pkg/fmt/scan_test.go
@@ -842,6 +842,38 @@ func TestLineByLineFscanf(t *testing.T) {
        }
 }
 
+// TestScanStateCount verifies the correct byte count is returned. Issue 8512.
+
+// runeScanner implements the Scanner interface for TestScanStateCount.
+type runeScanner struct {
+       rune rune
+       size int
+}
+
+func (rs *runeScanner) Scan(state ScanState, verb rune) error {
+       r, size, err := state.ReadRune()
+       rs.rune = r
+       rs.size = size
+       return err
+}
+
+func TestScanStateCount(t *testing.T) {
+       var a, b, c runeScanner
+       n, err := Sscanf("12➂", "%c%c%c", &a, &b, &c)
+       if err != nil {
+               t.Fatal(err)
+       }
+       if n != 3 {
+               t.Fatalf("expected 3 items consumed, got %d")
+       }
+       if a.rune != '1' || b.rune != '2' || c.rune != '➂' {
+               t.Errorf("bad scan rune: %q %q %q should be '1' '2' '➂'", a.rune, b.rune, c.rune)
+       }
+       if a.size != 1 || b.size != 1 || c.size != 3 {
+               t.Errorf("bad scan size: %q %q %q should be 1 1 3", a.size, b.size, c.size)
+       }
+}
+
 // RecursiveInt accepts a string matching %d.%d.%d....
 // and parses it into a linked list.
 // It allows us to benchmark recursive descent style scanners.