cmd/vet: don't suggest ... if it breaks a program

It is possible to write a function that seems to wrap a print/printf
call, but then doesn't. For example, if the string parameter we thought
was the format is used as another argument.

One option would be to make vet's print analysis smarter, to detect when
format strings are indeed used like we initially suspected.

However, I've opted for a simpler solution - check if the print/printf
call is already using more than one variadic argument, in which case
using an ellipsis in the last one would break the program:

	// too many arguments in call to fmt.Printf
	fmt.Printf(format, arg0, args...)

Fixes #26979.

Change-Id: I39371f1cec8483cfd2770a91670c1e80cbb9efdf
Reviewed-on: https://go-review.googlesource.com/129575
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

diff --git a/src/cmd/vet/print.go b/src/cmd/vet/print.go
index 90fd4ed379760bd4645a320923c25aaf1193599f..a55da1d3c815c78dc7aa8a122374f2bb61f44eee 100644 (file)
--- a/src/cmd/vet/print.go
+++ b/src/cmd/vet/print.go
@@ -259,6 +259,20 @@ func checkPrintfFwd(pkg *Package, w *printfWrapper, call *ast.CallExpr, kind int
        }
 
        if !call.Ellipsis.IsValid() {
+               typ, ok := pkg.types[call.Fun].Type.(*types.Signature)
+               if !ok {
+                       return
+               }
+               if len(call.Args) > typ.Params().Len() {
+                       // If we're passing more arguments than what the
+                       // print/printf function can take, adding an ellipsis
+                       // would break the program. For example:
+                       //
+                       //   func foo(arg1 string, arg2 ...interface{} {
+                       //       fmt.Printf("%s %v", arg1, arg2)
+                       //   }
+                       return
+               }
                if !vcfg.VetxOnly {
                        desc := "printf"
                        if kind == kindPrint {

diff --git a/src/cmd/vet/testdata/print.go b/src/cmd/vet/testdata/print.go
index 7c0cbcf05adac60c1469f48aa0acc16c8110683a..ecafed5fa2f2cc05621668c5ee7caa5b5931204c 100644 (file)
--- a/src/cmd/vet/testdata/print.go
+++ b/src/cmd/vet/testdata/print.go
@@ -446,6 +446,10 @@ func (*ptrStringer) BadWrapf(x int, format string, args ...interface{}) string {
        return fmt.Sprintf(format, args) // ERROR "missing ... in args forwarded to printf-like function"
 }
 
+func (*ptrStringer) WrapfFalsePositive(x int, arg1 string, arg2 ...interface{}) string {
+       return fmt.Sprintf("%s %v", arg1, arg2)
+}
+
 type embeddedStringer struct {
        foo string
        ptrStringer