coder · deansheather · Jan 11, 2023 · Jan 11, 2023 · Jan 11, 2023 · Jan 11, 2023
diff --git a/cli/agent.go b/cli/agent.go
@@ -39,6 +39,8 @@ func workspaceAgent() *cobra.Command {
 			ctx, cancel := context.WithCancel(cmd.Context())
 			defer cancel()
 
+			go dumpHandler(ctx)
+
 			rawURL, err := cmd.Flags().GetString(varAgentURL)
 			if err != nil {
 				return xerrors.Errorf("CODER_AGENT_URL must be set: %w", err)

diff --git a/cli/root.go b/cli/root.go
@@ -8,8 +8,11 @@ import (
 	"net/http"
 	"net/url"
 	"os"
+	"os/signal"
+	"path/filepath"
 	"runtime"
 	"strings"
+	"syscall"
 	"text/template"
 	"time"
 
@@ -631,3 +634,93 @@ func (h *headerTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	}
 	return h.transport.RoundTrip(req)
 }
+
+// dumpHandler provides a custom SIGQUIT and SIGTRAP handler that dumps the
+// stacktrace of all goroutines to stderr and a well-known file in the home
+// directory. This is useful for debugging deadlock issues that may occur in
+// production in workspaces, since the default Go runtime will only dump to
+// stderr (which is often difficult/impossible to read in a workspace).
+//
+// SIGQUITs will still cause the program to exit (similarly to the default Go
+// runtime behavior).
+//
+// A SIGQUIT handler will not be registered if GOTRACEBACK=crash.
+//
+// On Windows this immediately returns.
+func dumpHandler(ctx context.Context) {
+	if runtime.GOOS == "windows" {
+		// free up the goroutine since it'll be permanently blocked anyways
+		return
+	}
+
+	listenSignals := []os.Signal{syscall.SIGTRAP}
+	if os.Getenv("GOTRACEBACK") != "crash" {
+		listenSignals = append(listenSignals, syscall.SIGQUIT)
+	}
+
+	sigs := make(chan os.Signal, 1)
+	signal.Notify(sigs, listenSignals...)
+	defer signal.Stop(sigs)
+
+	for {
+		sigStr := ""
+		select {
+		case <-ctx.Done():
+			return
+		case sig := <-sigs:
+			switch sig {
+			case syscall.SIGQUIT:
+				sigStr = "SIGQUIT"
+			case syscall.SIGTRAP:
+				sigStr = "SIGTRAP"
+			}
+		}
+
+		// Start with a 1MB buffer and keep doubling it until we can fit the
+		// entire stacktrace, stopping early once we reach 64MB.
+		buf := make([]byte, 1_000_000)
+		stacklen := 0
+		for {
+			stacklen = runtime.Stack(buf, true)
+			if stacklen < len(buf) {
+				break
+			}
+			if 2*len(buf) > 64_000_000 {
+				// Write a message to the end of the buffer saying that it was
+				// truncated.
+				const truncatedMsg = "\n\n\nstack trace truncated due to size\n"
+				copy(buf[len(buf)-len(truncatedMsg):], truncatedMsg)
+				break
+			}
+			buf = make([]byte, 2*len(buf))
+		}
+
+		_, _ = fmt.Fprintf(os.Stderr, "%s:\n%s\n", sigStr, buf[:stacklen])
+
+		// Write to a well-known file.
+		dir, err := os.UserHomeDir()
+		if err != nil {
+			dir = os.TempDir()
+		}
+		fpath := filepath.Join(dir, fmt.Sprintf("coder-agent-%s.dump", time.Now().Format("2006-01-02T15:04:05.000Z")))
+		_, _ = fmt.Fprintf(os.Stderr, "writing dump to %q\n", fpath)
+
+		f, err := os.Create(fpath)
+		if err != nil {
+			_, _ = fmt.Fprintf(os.Stderr, "failed to open dump file: %v\n", err.Error())
+			goto done
+		}
+		_, err = f.Write(buf[:stacklen])
+		_ = f.Close()
+		if err != nil {
+			_, _ = fmt.Fprintf(os.Stderr, "failed to write dump file: %v\n", err.Error())
+			goto done
+		}
+
+	done:
+		if sigStr == "SIGQUIT" {
+			//nolint:revive
+			os.Exit(1)
+		}
+	}
+}
diff --git a/cli/server.go b/cli/server.go
@@ -81,6 +81,13 @@ func Server(vip *viper.Viper, newAPI func(context.Context, *coderd.Options) (*co
 		Use:   "server",
 		Short: "Start a Coder server",
 		RunE: func(cmd *cobra.Command, args []string) error {
+			// Main command context for managing cancellation of running
+			// services.
+			ctx, cancel := context.WithCancel(cmd.Context())
+			defer cancel()
+
+			go dumpHandler(ctx)
+
 			cfg, err := deployment.Config(cmd.Flags(), vip)
 			if err != nil {
 				return xerrors.Errorf("getting deployment config: %w", err)
@@ -123,11 +130,6 @@ func Server(vip *viper.Viper, newAPI func(context.Context, *coderd.Options) (*co
 				logger = logger.AppendSinks(tracing.SlogSink{})
 			}
 
-			// Main command context for managing cancellation
-			// of running services.
-			ctx, cancel := context.WithCancel(cmd.Context())
-			defer cancel()
-
 			// Register signals early on so that graceful shutdown can't
 			// be interrupted by additional signals. Note that we avoid
 			// shadowing cancel() (from above) here because notifyStop()