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Go Debugging

packages/shared-skills/skills/debugging/references/runtimes/go.md

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Go Debugging

Covers goroutines, dlv (Delve), pprof, the race detector, and the fact that Go's concurrency model means most bugs are about goroutines doing something quiet and wrong.

Environment detection (Phase 0)

bash
go version
cat go.mod | head -5

# Delve installed?
which dlv
dlv version

# Build constraints
grep -r '// +build\|//go:build' cmd/ internal/ pkg/ 2>/dev/null | head

# pprof wired up?
grep -r 'net/http/pprof\|runtime/pprof' --include='*.go' | head -3

Delve (dlv) — the Go debugger

Go's gc compiler emits DWARF, but plain gdb barely understands goroutines. Use dlv, not gdb. Plain gdb on a Go binary will miss goroutine state and print garbage for interface values.

The five dlv launch modes

bash
# Build and launch under debugger (equivalent to `go run` + debug)
dlv debug ./cmd/server -- --port=8080

# Debug a test binary
dlv test ./internal/handler/           # enters the test package under debug

# Debug an existing binary (must be built with -gcflags="all=-N -l" for best results)
dlv exec ./bin/myserver

# Attach to a running process
dlv attach $(pgrep myserver)

# Headless mode (IDE / remote attach) — default port 2345
dlv debug --headless --listen=:2345 --api-version=2 ./cmd/server

Building a debuggable binary

The compiler inlines and optimizes aggressively in normal builds, which makes stepping confusing. For serious debugging:

bash
go build -gcflags="all=-N -l" -o ./bin/server ./cmd/server
# -N disables optimization
# -l disables inlining

Then dlv exec ./bin/server.

Essential dlv commands

(dlv) b main.main                      # breakpoint at function
(dlv) b handler.go:42                  # breakpoint at file:line
(dlv) b pkg/foo.Bar                    # breakpoint at type method (Go path syntax)
(dlv) c / continue                     # continue until next break
(dlv) n / next                         # step over
(dlv) s / step                         # step into
(dlv) so / stepout                     # step out
(dlv) bt / stack                       # stack trace of current goroutine
(dlv) goroutines                       # list all goroutines
(dlv) goroutine <id>                   # switch to goroutine N
(dlv) goroutine <id> bt                # stack of a specific goroutine
(dlv) locals                           # all locals in frame
(dlv) args                             # function args
(dlv) p <expr>                         # print value (understands interfaces, maps, slices)
(dlv) vars <regex>                     # package vars matching regex
(dlv) regs                             # registers (rare in Go debugging)
(dlv) on <bpid> print <expr>           # auto-print on breakpoint hit (powerful!)
(dlv) trace <location>                 # like breakpoint but just logs, doesn't stop

The trace command is underused — it's like a logpoint, no stepping required.

Goroutine-centric debugging

Goroutine leaks and deadlocks are the most common Go bugs. dlv's goroutines command is the starting point.

(dlv) goroutines -t                    # with truncated stack
(dlv) goroutines -s                    # sorted by stack
(dlv) goroutines -with user            # filter user-spawned goroutines

Common patterns:

You see in goroutinesUsually means
100s of goroutines stuck at chan receiveProducer died; consumers leak
100s stuck at semacquireLock contention; a holder probably deadlocked
One stuck at select with no defaultMissing case or closed channel scenario
Stuck at netpollExternal I/O not responding — not a Go bug, check downstream
Growing count over timeGoroutine leak — need to find who's spawning without cleanup

Panic signals in Go

go
// Without recovery, panics crash the program with a stack trace of ALL goroutines
// With recovery, they're silent unless explicitly logged:
defer func() {
    if r := recover(); r != nil {
        log.Printf("recovered panic: %v\n%s", r, debug.Stack())   // GOOD
        // log.Printf("recovered")                                  // BAD — silent
    }
}()

Always check for silent recovers in Phase 8. Grep:

bash
rg 'recover\(\)' --type go

And inspect each site for whether the panic is actually surfaced.

Race detector — ALWAYS run when the bug is intermittent

bash
go test -race ./...
go run -race ./cmd/server
go build -race ./cmd/server

The race detector wraps memory accesses and catches concurrent read/write without synchronization. Run this before attaching dlv if intermittency is involved — it often finds the bug directly.

Output shape:

WARNING: DATA RACE
Read at 0x00c0001a0080 by goroutine 7:
  main.(*Counter).Value()
      /path/to/counter.go:14 +0x3c
Previous write at 0x00c0001a0080 by goroutine 6:
  main.(*Counter).Inc()
      /path/to/counter.go:10 +0x5f

Both stacks. Both goroutines. The race is obvious from the line pair.

pprof — for perf, memory, goroutine leaks

Wire it up (idempotent; usually already present)

go
import _ "net/http/pprof"

func main() {
    go func() {
        log.Println(http.ListenAndServe("localhost:6060", nil))
    }()
    // ... rest of your server
}

Queries

bash
# CPU profile (30s)
go tool pprof http://localhost:6060/debug/pprof/profile?seconds=30

# Heap snapshot
go tool pprof http://localhost:6060/debug/pprof/heap

# Goroutine snapshot — find leaks
go tool pprof http://localhost:6060/debug/pprof/goroutine

# Block profile — find blocking ops (needs runtime.SetBlockProfileRate)
go tool pprof http://localhost:6060/debug/pprof/block

# Mutex profile — find lock contention (needs runtime.SetMutexProfileFraction)
go tool pprof http://localhost:6060/debug/pprof/mutex

Inside pprof:

(pprof) top                 # top functions by self time
(pprof) list main.handler   # annotated source of a function
(pprof) web                 # SVG callgraph in browser (requires graphviz)
(pprof) traces              # sample traces

For goroutine leaks, take two snapshots 30s apart and diff:

bash
go tool pprof -base prof1.pb.gz prof2.pb.gz

Goroutines that appear in prof2 but not prof1 are new; if they stick around, they're leaking.

GODEBUG — runtime-level observability

bash
GODEBUG=gctrace=1 ./myserver              # print GC stats
GODEBUG=schedtrace=1000 ./myserver        # scheduler trace every 1000ms
GODEBUG=scheddetail=1,schedtrace=1000     # detailed scheduler state
GODEBUG=allocfreetrace=1 ./myserver       # every alloc/free (noisy!)
GODEBUG=memprofilerate=1 ./myserver       # profile every allocation

Useful for diagnosing GC pressure, goroutine starvation, or memory pattern issues.

Silent-failure patterns in Go

PatternWhy it's silent
if err != nil { return err } that returns to a caller that ignoresError bubbles up, then gets discarded at the top
defer func() { recover() }() — bare recover, no logPanic swallowed, program continues with state corruption
_, _ = conn.Write(data)Intentionally discarded error
Buffered channel send that blocks foreverSender hangs; hard to see if no deadlock detection
time.Sleep in a test"Works on my machine"; test passes locally, fails in CI
go func() { ... }() with no error pathGoroutine dies silently on panic unless recover+log
Context canceled but operation continuesIgnored ctx.Err() check
json.Unmarshal of zero-value struct fieldInput missing the key; silently zero
Closed channel read returning zero valueConsumer doesn't check ok; reads forever

Phase 9 cleanup specifics

bash
# Kill dlv sessions
pkill -f 'dlv' || true
lsof -iTCP:2345 -sTCP:LISTEN -nP 2>/dev/null      # dlv default

# Kill pprof HTTP endpoint if you started it just for this session
lsof -iTCP:6060 -sTCP:LISTEN -nP 2>/dev/null

# Revert any `fmt.Println("DEBUG: ...")` or `log.Printf("DEBUG: ...")` additions
git diff | grep -E '(fmt\.Println\("DEBUG|log\.Printf\("DEBUG|println!)'
git checkout <file>

# Unset env vars
unset GODEBUG