x/debug: setting debug breakpoints by file/line broken

[fabxc]

• go1.10.2
• linux/amd64

I tried to setup the GCP cloud debugger for a Go application but it fails to set breakpoints with: error setting breakpoint at scrape/scrape.go:818: couldn't find file "scrape/scrape.go".

I reproduced this locally by loading a compiled Go program with golang.org/x/debug/local.New. Calling Program.BreakpointAtLine("scrape/scrape.go", 818) fails with the same error. Using more specific paths (e.g. full file path at compile time) does not change anything.
Setting a breakpoint directly through symbol name with Program.BreakpointAtFunction works.

When re-compiling the debugged program with go1.9, BreakpointAtLine works without issues again.
In both cases the binary was compiled with -gcflags='-N -l' and CGO_ENABLED=0.

This blog post reported a similar problem some time ago. I could not find a matching issue though.

[heschi]

I thought this might be the change to DW_AT_comp_dir in CL 74790 but looking at the code for LineToBreakpointPCs that's giving the error, it seems like it should be resilient to that.

https://github.com/golang/debug/blob/cb5e6d581c5b9d2e229187cecb199106564df20e/dwarf/line.go#L47-L94

So I'm not sure offhand. @fabxc, can you put your repro somewhere I can try it?

cc @zombiezen

[fabxc]

Thanks for looking into this. The following gist shows a minimal example: https://gist.github.com/fabxc/99c9f81adafda4969deafad94ed14224

[heschi]

Ouch. x/debug/dwarf (this code doesn't exist in pkg/debug/dwarf) assumes that there's only one CU:
https://github.com/golang/debug/blob/cb5e6d581c5b9d2e229187cecb199106564df20e/dwarf/cache.go#L146
and we switched to CU-per-package in 1.10.

Fixing it probably isn't too hard but requires more than a couple minutes of thought to do right.

[fabxc]

Seems like any related code was purged from x/debug. So this can probably be closed?

The Go cloud debugger agent vendored the former x/debug and took the bug with it. Any recommendations on how to address the issue there?

[jba]

Moving to googleapis/google-cloud-go#1079.