panic: runtime error: invalid memory address or nil pointer dereference

[deluan]

I'm not totally sure when and why this happens, but one user of my app is reporting this error:

panic: runtime error: invalid memory address or nil pointer dereference

 -> github.com/djherbis/fscache.(*handleCounter).inc
 ->   /go/pkg/mod/github.com/djherbis/fscache@v0.10.0/fscache.go:327

    github.com/djherbis/fscache.(*FSCache).newFile
      /go/pkg/mod/github.com/djherbis/fscache@v0.10.0/fscache.go:227
    github.com/djherbis/fscache.(*FSCache).Get
      /go/pkg/mod/github.com/djherbis/fscache@v0.10.0/fscache.go:154
    github.com/deluan/navidrome/engine.(*cover).Get
      /github/workspace/engine/cover.go:50
    github.com/deluan/navidrome/server/subsonic.(*MediaRetrievalController).GetCoverArt
      /github/workspace/server/subsonic/media_retrieval.go:43
    github.com/deluan/navidrome/server/subsonic.H.func1
      /github/workspace/server/subsonic/api.go:138
    net/http.HandlerFunc.ServeHTTP
      /usr/local/go/src/net/http/server.go:2012
    github.com/go-chi/chi.(*ChainHandler).ServeHTTP
      /go/pkg/mod/github.com/go-chi/chi@v4.1.0+incompatible/chain.go:31
    github.com/go-chi/chi.(*Mux).routeHTTP
      /go/pkg/mod/github.com/go-chi/chi@v4.1.0+incompatible/mux.go:431
    net/http.HandlerFunc.ServeHTTP
      /usr/local/go/src/net/http/server.go:2012
    github.com/deluan/navidrome/server/subsonic.authenticate.func1.1
      /github/workspace/server/subsonic/middlewares.go:93
    net/http.HandlerFunc.ServeHTTP
      /usr/local/go/src/net/http/server.go:2012
    github.com/deluan/navidrome/server/subsonic.checkRequiredParameters.func1
      /github/workspace/server/subsonic/middlewares.go:63
    net/http.HandlerFunc.ServeHTTP
      /usr/local/go/src/net/http/server.go:2012
    github.com/deluan/navidrome/server/subsonic.postFormToQueryParams.func1
      /github/workspace/server/subsonic/middlewares.go:36
    net/http.HandlerFunc.ServeHTTP
      /usr/local/go/src/net/http/server.go:2012
    github.com/go-chi/chi.(*Mux).ServeHTTP
      /go/pkg/mod/github.com/go-chi/chi@v4.1.0+incompatible/mux.go:70
    github.com/deluan/navidrome/server/subsonic.(*Router).ServeHTTP
      /github/workspace/server/subsonic/api.go:48
    github.com/go-chi/chi.(*Mux).Mount.func1
      /go/pkg/mod/github.com/go-chi/chi@v4.1.0+incompatible/mux.go:298
    net/http.HandlerFunc.ServeHTTP
      /usr/local/go/src/net/http/server.go:2012
    github.com/deluan/navidrome/server.RequestLogger.func1
      /github/workspace/server/middlewares.go:21
    net/http.HandlerFunc.ServeHTTP

You can see in the original report that the key files are being created, but the cache files are created with size 0.

Maybe I'm using it wrongly?
https://github.com/deluan/navidrome/blob/master/engine/cover.go#L50

What other information can I give you to help pin this error down?

Original report: navidrome/navidrome#177

[djherbis]

Taking a look, this is very weird though, newFIle creates the cachedFile object (so it's not nil):

fscache/fscache.go

Line 224 in 646777d

cf := &cachedFile{

And then calls inc, where it's failing on nil deref on:

fscache/fscache.go

Line 327 in 646777d

atomic.AddInt64(&h.cnt, 1)

But that should not be nil because we're in newFile and literally just initialized it.

Is this reproducible with a specific setup? I'd like to do some debugging on it to figure out how this is possible.

[djherbis]

I suspect this is the problem:
https://stackoverflow.com/questions/28670232/atomic-addint64-causes-invalid-memory-address-or-nil-pointer-dereference

What's the specific Go build constraints that produces this situation? I'm guessing alignment is messed up. Though it's odd that it would be considering it's the first arg.

I suspect now maybe even though it's the first arg of handleCounter, because that is embedded in cachedFile that might mess up the alignment. This makes me sad because it breaks some of the 'easiness' of Go when a failure can cascade like this.

Related:
golang/go#599
golang/go#36606

[0xERR0R]

This error occurs on arm (raspberry pi 3)

[djherbis]

@0xERR0R Can you dump "go env" so I can match your build setup?
Go version would be helpful too.

[0xERR0R]

I used arm/v7 build from navidrome's release page. I guess, this will be built by goreleaser

[djherbis]

I gotta focus on my day job for awhile, but now that I've got enough details I work on fixing this tonight.

[0xERR0R]

If you want to perform some test -> please contact me, I would be glad to help

[djherbis]

@0xERR0R @deluan I fixed this by adjusting the atomic field alignment as suspected.

To verify, I pulled the old version on my pi, ran go test and got the same panic. I fixed the alignment and re-ran go test a few times, no panic.

Let me know if this fixes things for you too :)

[deluan]

Thanks for the quick turnaround on this issue!