cmd/vet: should check for missing call to "Err() error" methods

[mdlayher]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.7.3 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/matt"
GORACE=""
GOROOT="/home/matt/.gvm/gos/go1.7.3"
GOTOOLDIR="/home/matt/.gvm/gos/go1.7.3/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build698545690=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

https://play.golang.org/p/zvrWRVgw4M

Used a bufio.Scanner without checking bufio.Scanner.Err() for error. The same mistake occurs frequently with sql.Rows.Err().

What did you expect to see?

go vet should warn that s.Err() was not called, and that it should be checked to determine if an error occurred while scanning.

This frequently occurs in our internal codebase at work. Arguably, go vet could check if any type has a method Err() error and report if the error is not checked or returned, instead of just checking the special cases with bufio.Scanner and sql.Rows. There may also be more types that use this pattern in the standard library.

What did you see instead?

No warnings from go vet.

[josharian]

There are legitimate cases in which you might elide the s.Err check, such as when reading lines from a strings.Reader; we know a priori that that cannot fail. False positives from vet are painful.

This frequently occurs in our internal codebase at work.

How frequently is frequently? How bad are the resulting bugs?

cc @robpike

[mdlayher]

How frequently is frequently? How bad are the resulting bugs?

It probably comes up in code review at least once or twice per week. Typically when interacting with a database.

As far as the severity of the resulting bugs, I am not sure. I'm on a different team and often just get asked to do code review sporadically, even though I typically don't maintain the projects where these problems occur.