pkg/fuzzer: collect executor debug logs in tests

It should hopefully let us debug #5674.

pkg/fuzzer/fuzzer_test.go

@@ -159,25 +159,29 @@ func (f *testFuzzer) run() {
 	f.crashes = make(map[string]int)
 	ctx, done := context.WithCancel(context.Background())
 	f.done = done
+	var output bytes.Buffer
 	cfg := &rpcserver.LocalConfig{
 		Config: rpcserver.Config{
 			Config: vminfo.Config{
+				Debug:    true,
 				Target:   f.target,
 				Features: flatrpc.FeatureSandboxNone,
 				Sandbox:  flatrpc.ExecEnvSandboxNone,
 			},
 			Procs:    4,
 			Slowdown: 1,
 		},
-		Executor: f.executor,
-		Dir:      f.t.TempDir(),
-		Context:  ctx,
+		Executor:     f.executor,
+		Dir:          f.t.TempDir(),
+		Context:      ctx,
+		OutputWriter: &output,
 	}
 	cfg.MachineChecked = func(features flatrpc.Feature, syscalls map[*prog.Syscall]bool) queue.Source {
 		cfg.Cover = true
 		return f
 	}
 	if err := rpcserver.RunLocal(cfg); err != nil {
+		f.t.Logf("executor output:\n%s", output.String())
 		f.t.Fatal(err)
 	}
 	assert.Equal(f.t, len(f.expectedCrashes), len(f.crashes), "not all expected crashes were found")

pkg/rpcserver/local.go

@@ -6,6 +6,7 @@ package rpcserver
 import (
 	"context"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 
@@ -28,9 +29,11 @@ type LocalConfig struct {
 	// Handle ctrl+C and exit.
 	HandleInterrupts bool
 	// Run executor under gdb.
-	GDB         bool
-	MaxSignal   []uint64
-	CoverFilter []uint64
+	GDB bool
+	// Can be used to intercept stdout/stderr output.
+	OutputWriter io.Writer
+	MaxSignal    []uint64
+	CoverFilter  []uint64
 	// RunLocal exits when the context is cancelled.
 	Context        context.Context
 	MachineChecked func(features flatrpc.Feature, syscalls map[*prog.Syscall]bool) queue.Source
@@ -82,7 +85,10 @@ func RunLocal(cfg *LocalConfig) error {
 	}
 	cmd := exec.CommandContext(ctx, bin, args...)
 	cmd.Dir = cfg.Dir
-	if cfg.Debug || cfg.GDB {
+	if cfg.OutputWriter != nil {
+		cmd.Stdout = cfg.OutputWriter
+		cmd.Stderr = cfg.OutputWriter
+	} else if cfg.Debug || cfg.GDB {
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
 	}