fmt: should clearflags also set wid and prec to zero

[mitar]

What version of Go are you using (go version)?

$ go version
go version go1.20.7 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/mitar/.cache/go-build"
GOENV="/home/mitar/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/mitar/.go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/mitar/.go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.7"
GCCGO="/usr/bin/gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3086644443=/tmp/go-build -gno-record-gcc-switches"

What did you do?

So I have implemented fmt.Formatter interface for my own struct and wrote bunch of tests. In my case, formatting makes sense only for precision 0 and 1, so I have such check in my code:

		precision, _ := s.Precision()
		if precision < 0 || precision > 1 {
			io.WriteString(s, badPrecString)
			break
		}

What did you expect to see?

I expected to see that all tests which do not provide precision at all in format strings succeed.

What did you see instead?

I see random failures. After debugging I think the reason is that newPrinter() in fmt package uses a sync.Pool and reuses printers. The code calls p.fmt.clearflags() before for simpleFormat formats just calls printArg which calls handleMethods. Because it is a simple format precision is never parsed nor set (just precPresent is reset). Because I do not check for ok value from s.Precision() but just use precision I think I get precision from some other printer instance when it was set in the past.

I find this surprising. In general my experience in Go was that uninitialized values are always 0. So even if precision has not been set I would expect it always to be 0. This happens only because instances are cached (an optimization).

I would suggest that clearflags simply sets wid and prec fields to zero, too. Maybe clearflags should also just be renamed to clear?

I am not sure how to write a test case or reproduction for this.

[mitar]

CC @robpike @martisch via https://dev.golang.org/owners

[robpike]

I suspect you're right and the diagnosis is correct. I'll investigate.

[gopherbot]

Change https://go.dev/cl/556215 mentions this issue: fmt: clear width and precision when recovering formatting object from the pool