How do you debug a Go program with Delve (dlv)? How do you attach to a running process, debug a test, and debug goroutines?

Delve is the Go debugger. It understands goroutines, the Go scheduler and Go types better than gdb does. The main entry points are:

# build and debug the main package (Delve turns optimizations off for you)
dlv debug ./cmd/api -- -config=dev.yaml

# debug the tests of one package, running only one test
dlv test ./internal/store -- -test.run '^TestUpsert
  

 -test.v

# attach to a running process (Linux may need ptrace_scope=0 or CAP_SYS_PTRACE)
dlv attach 12345

# headless server for an IDE or a remote container
dlv attach 12345 --headless --listen=:2345 --api-version=2 --accept-multiclient

# debug a binary you built yourself: keep variables visible
go build -gcflags=all="-N -l" -o app ./cmd/api && dlv exec ./app

Useful commands: break store.go:42, cond 1 id == 7, continue, next, step, stepout, print x, locals, goroutines (lists every goroutine with its current location), goroutines -group userloc, goroutine 18 followed by bt to switch to a goroutine and see its stack, and threads.

Gotchas: in an optimized binary, variables show as "optimized away" and lines seem to jump around. Rebuild with -N -l. Stopping at a breakpoint pauses the whole process, so timeouts and deadlines in other goroutines may fire when you resume. Attaching to a production pod stops it from serving traffic. Take it out of load balancing first, or capture a goroutine dump (/debug/pprof/goroutine?debug=2) instead.

What the interviewer is looking for: you know the difference between debug, test, attach, exec and core, and you can inspect goroutines to find deadlocks.