cmd/compile: panic by building with GOSSAFUNC in Go1.15

[tenntenn]

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

$ go version
go version go1.15 darwin/amd64

Does this issue reproduce with the latest release?

Yes. It does not happen in Go1.14.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/tenntenn/Library/Caches/go-build"
GOENV="/Users/tenntenn/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/tenntenn/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/tenntenn/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
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/dr/gn16sh9935zcz37xt2ghpk480000gn/T/go-build058106775=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

% cat main.go
package main
func main() {}
% GOSSAFUNC=main go build main.go
# runtime
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x4 pc=0x12708a7]
goroutine 121 [running]:
cmd/compile/internal/ssa.(*Func).Fatalf(...)
	/usr/local/go/src/cmd/compile/internal/ssa/func.go:625
cmd/compile/internal/ssa.NewHTMLWriter(0x1843e7d, 0x8, 0xc00555bce0, 0x0, 0x0, 0x1)
	/usr/local/go/src/cmd/compile/internal/ssa/html.go:33 +0xe7
cmd/compile/internal/gc.buildssa(0xc00194c840, 0x2, 0x0)
	/usr/local/go/src/cmd/compile/internal/gc/ssa.go:342 +0x1172
cmd/compile/internal/gc.compileSSA(0xc00194c840, 0x2)
	/usr/local/go/src/cmd/compile/internal/gc/pgen.go:317 +0x5d
cmd/compile/internal/gc.compileFunctions.func2(0xc00448f080, 0xc0008f7e80, 0x2)
	/usr/local/go/src/cmd/compile/internal/gc/pgen.go:382 +0x4d
created by cmd/compile/internal/gc.compileFunctions
	/usr/local/go/src/cmd/compile/internal/gc/pgen.go:380 +0x129

What did you expect to see?

Build successfully and generate ssa.html.

What did you see instead?

panic has been occurred.

[randall77]

It's trying to print you an error because it couldn't open the ssa.html file. Is the directory not writeable? Ah, maybe its trying to print an ssa.html for runtime.main, and you have the Go distribution directories read only. I can reproduce by making the runtime directory unwriteable.

Part of the problem is GOSSAFUNC uses a package-local name, so it can match in multiple packages, including stdlib.
I'm not sure why it is crashing when it tries to print you an error. That's something we should probably fix in the distribution.

[randall77]

I see now. The error writer tries to use the entry block's line number to annotate the error, but the entry block has not been set yet. Oops.

[gopherbot]

Change https://golang.org/cl/249497 mentions this issue: cmd/compile: define starting block before opening html writer

[randall77]

I don't think this is worth backporting, GOSSAFUNC is a best-effort service.
The fix will come out in 1.16. Until then, make the destination directory (or directories) writeable. Use a unique function name to prevent collision with stdlib names, if needed.

[nmiyake]

As noted already, the specific panic here seems to be around error printing. The underlying issue that causes the error in this case seems like it could be similar to #33278, so it may be helpful to consult that issue for reference (it looks like using go tool compile instead of go build may be sufficient to work around the issue).

[dr2chase]

See also https://go-review.googlesource.com/c/go/+/246937

[gopherbot]

Change https://golang.org/cl/250340 mentions this issue: cmd/compile: also check package.function for GOSSAFUNC match