go/analysis/passes/printf: suppress diagnostic for Println("...%XX...")

A common form of literal string is a URL containing URL-escaped
characters. This CL causes the printf checker not to report a
"Println call has possible Printf formatting directive"
diagnostic for it.

+ test

Fixes golang/go#29854

Change-Id: Ib1dcc44dd8185da17f61296632ad030cb1e58420
Reviewed-on: https://go-review.googlesource.com/c/tools/+/650175
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: adonovan

go/analysis/passes/printf/printf.go

@@ -924,9 +924,14 @@ func checkPrint(pass *analysis.Pass, call *ast.CallExpr, name string) {
 		// The % in "abc 0.0%" couldn't be a formatting directive.
 		s = strings.TrimSuffix(s, "%")
 		if strings.Contains(s, "%") {
-			m := printFormatRE.FindStringSubmatch(s)
-			if m != nil {
-				pass.ReportRangef(call, "%s call has possible Printf formatting directive %s", name, m[0])
+			for _, m := range printFormatRE.FindAllString(s, -1) {
+				// Allow %XX where XX are hex digits,
+				// as this is common in URLs.
+				if len(m) >= 3 && isHex(m[1]) && isHex(m[2]) {
+					continue
+				}
+				pass.ReportRangef(call, "%s call has possible Printf formatting directive %s", name, m)
+				break // report only the first one
 			}
 		}
 	}
@@ -992,3 +997,10 @@ func (ss stringSet) Set(flag string) error {
 //
 // Remove this after the 1.24 release.
 var suppressNonconstants bool
+
+// isHex reports whether b is a hex digit.
+func isHex(b byte) bool {
+	return '0' <= b && b <= '9' ||
+		'A' <= b && b <= 'F' ||
+		'a' <= b && b <= 'f'
+}

go/analysis/passes/printf/testdata/src/a/a.go

@@ -154,6 +154,8 @@ func PrintfTests() {
 	fmt.Println("%v", "hi")                     // want "fmt.Println call has possible Printf formatting directive %v"
 	fmt.Println("%T", "hi")                     // want "fmt.Println call has possible Printf formatting directive %T"
 	fmt.Println("%s"+" there", "hi")            // want "fmt.Println call has possible Printf formatting directive %s"
+	fmt.Println("http://foo.com?q%2Fabc")       // no diagnostic: %XX is excepted
+	fmt.Println("http://foo.com?q%2Fabc-%s")    // want"fmt.Println call has possible Printf formatting directive %s"
 	fmt.Println("0.0%")                         // correct (trailing % couldn't be a formatting directive)
 	fmt.Printf("%s", "hi", 3)                   // want "fmt.Printf call needs 1 arg but has 2 args"
 	_ = fmt.Sprintf("%"+("s"), "hi", 3)         // want "fmt.Sprintf call needs 1 arg but has 2 args"