go/printer: comments were unexpected inserted in import decl

[oraluben]

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

$ go version
go version go1.15beta1 darwin/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="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/yyc/Library/Caches/go-build"
GOENV="/Users/yyc/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/yyc/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/yyc/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn"
GOROOT="/Users/yyc/go/go1.15beta1"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/yyc/go/go1.15beta1/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/yyc/GolandProjects/fuzz-test/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/7y/t_wypjhs6hq78tzsj85w2xgw0000gn/T/go-build457538297=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

https://play.golang.org/p/65fHR5rtBXR

What I'm trying to do here is inserting an import decl in the front of a simple go source file, and as shown, an unexpected comment appeared in the middle of the inserted import, which breaks the source file.

What did you expect to see?

What did you see instead?

Buggy version and expected version (fixed with some ad-hoc logic) are both in the above link.

<del>
I believe what happened here is after remembering the offset of one comment at

go/src/go/printer/printer.go

Line 137 in 6f26480

p.commentOffset = p.posFor(list[0].Pos()).Offset

, the offset will not be updated even if new content printed, so commentBefore

go/src/go/printer/printer.go

Line 153 in 6f26480

return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)

will unexpected returns true for the next but not reached comment.
</del>

I can draft a patch if I'm not misunderstanding about the root cause.

update: the guess above is incorrect, still investigating.

[ianlancetaylor]

CC @griesemer

[dgryski]

Given that this breaks go-fuzz, does that increase the priority for 1.15 ?

[griesemer]

Just seeing this (I was on vacation last week). It's too late for 1.15.

[oraluben]

I think I can hardly say this is a bug in go/printer. For instance we have one comment at position n, and we inserted some statements without any position information just before n, it's hard to tell where should the comment be, in the middle of those inserted statements or after.

One workaround useful to go-fuzz would be giving all inserted statements a position not accurate but less than the position of the subsequent comment. How this would work could be found at https://play.golang.org/p/KNEE2J4FrWt.