fmt: scanning widths apply after leading spaces

When scanning with a width, as in %5s, C skips leading spaces
brefore counting the 5 characters. We should do the same.

Reword the documentation about widths to make this clear.

Fixes #9444

Change-Id: I443a6441adcf1c834057ef3977f9116a987a79cd
Reviewed-on: https://go-review.googlesource.com/10997Reviewed-by: Andrew Gerrand <adg@golang.org>

src/fmt/doc.go

@@ -271,13 +271,18 @@
 		Flags # and + are not implemented.
 
 	The familiar base-setting prefixes 0 (octal) and 0x
-	(hexadecimal) are accepted when scanning integers without a
-	format or with the %v verb.
-
-	Width is interpreted in the input text (%5s means at most
-	five runes of input will be read to scan a string) but there
-	is no syntax for scanning with a precision (no %5.2f, just
-	%5f).
+	(hexadecimal) are accepted when scanning integers without
+	a format or with the %v verb.
+
+	Width is interpreted in the input text but there is no
+	syntax for scanning with a precision (no %5.2f, just %5f).
+	If width is provided, it applies after leading spaces are
+	trimmed and specifies the maximum number of runes to read
+	to satisfy the verb. For example,
+	   Sscanf(" 1234567 ", "%5s%d", &s, &i)
+	will set s to "12345" and i to 67 while
+	   Sscanf(" 12 34 567 ", "%5s%d", &s, &i)
+	will set s to "12" and i to 34.
 
 	In all the scanning functions, a carriage return followed
 	immediately by a newline is treated as a plain newline

src/fmt/scan.go

@@ -1165,6 +1165,7 @@ func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err erro
 		if !widPresent {
 			s.maxWid = hugeWid
 		}
+		s.SkipSpace()
 		s.argLimit = s.limit
 		if f := s.count + s.maxWid; f < s.argLimit {
 			s.argLimit = f

src/fmt/scan_test.go

@@ -340,6 +340,8 @@ var multiTests = []ScanfMultiTest{
 	{"%6vX=%3fY", "3+2iX=2.5Y", args(&c, &f), args((3 + 2i), 2.5), ""},
 	{"%d%s", "123abc", args(&i, &s), args(123, "abc"), ""},
 	{"%c%c%c", "2\u50c2X", args(&r1, &r2, &r3), args('2', '\u50c2', 'X'), ""},
+	{"%5s%d", " 1234567 ", args(&s, &i), args("12345", 67), ""},
+	{"%5s%d", " 12 34 567 ", args(&s, &i), args("12", 34), ""},
 
 	// Custom scanners.
 	{"%e%f", "eefffff", args(&x, &y), args(Xs("ee"), Xs("fffff")), ""},