record all plugin panics, and print on main exit

Create a logger that will record any apparent crash output for later
processing.

If the cli command returns with a non-zero exit status, check for any
recorded crashes and add those to the output.

internal/logging/logging.go

@@ -24,14 +24,22 @@ const (
 	envLogProvider = "TF_LOG_PROVIDER"
 )
 
-// ValidLevels are the log level names that Terraform recognizes.
-var ValidLevels = []string{"TRACE", "DEBUG", "INFO", "WARN", "ERROR", "OFF"}
+var (
+	// ValidLevels are the log level names that Terraform recognizes.
+	ValidLevels = []string{"TRACE", "DEBUG", "INFO", "WARN", "ERROR", "OFF"}
 
-// logger is the global hclog logger
-var logger hclog.Logger
+	// logger is the global hclog logger
+	logger hclog.Logger
 
-// logWriter is a global writer for logs, to be used with the std log package
-var logWriter io.Writer
+	// logWriter is a global writer for logs, to be used with the std log package
+	logWriter io.Writer
+
+	// initialize our cache of panic output from providers
+	panics = &panicRecorder{
+		panics:   make(map[string][]string),
+		maxLines: 100,
+	}
+)
 
 func init() {
 	logger = newHCLogger("")
@@ -98,13 +106,17 @@ func NewLogger(name string) hclog.Logger {
 	if name == "" {
 		panic("logger name required")
 	}
-	return logger.Named(name)
+	return &logPanicWrapper{
+		Logger: logger.Named(name),
+	}
 }
 
 // NewProviderLogger returns a logger for the provider plugin, possibly with a
 // different log level from the global logger.
 func NewProviderLogger(prefix string) hclog.Logger {
-	l := logger.Named(prefix + "provider")
+	l := &logPanicWrapper{
+		Logger: logger.Named(prefix + "provider"),
+	}
 
 	level := providerLogLevel()
 	logger.Debug("created provider logger", "level", level)

internal/logging/panic.go

@@ -5,7 +5,10 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"strings"
+	"sync"
 
+	"github.com/hashicorp/go-hclog"
 	"github.com/mitchellh/panicwrap"
 )
 
@@ -69,3 +72,111 @@ func PanicHandler(tmpLogPath string) panicwrap.HandlerFunc {
 		fmt.Printf(panicOutput, f.Name())
 	}
 }
+
+const pluginPanicOutput = `
+Stack trace from the %[1]s plugin:
+
+%s
+
+Error: The %[1]s plugin crashed!
+
+This is always indicative of a bug within the plugin. It would be immensely
+helpful if you could report the crash with the plugin's maintainers so that it
+can be fixed. The output above should help diagnose the issue.
+`
+
+// PluginPanics returns a series of provider panics that were collected during
+// execution, and formatted for output.
+func PluginPanics() []string {
+	return panics.allPanics()
+}
+
+// panicRecorder provides a registry to check for plugin panics that may have
+// happened when a plugin suddenly terminates.
+type panicRecorder struct {
+	sync.Mutex
+
+	// panics maps the plugin name to the panic output lines received from
+	// the logger.
+	panics map[string][]string
+
+	// maxLines is the max number of lines we'll record after seeing a
+	// panic header. Since this is going to be printed in the UI output, we
+	// don't want to destroy the scrollback. In most cases, the first few lines
+	// of the stack trace is all that are required.
+	maxLines int
+}
+
+// registerPlugin returns an accumulator function which will accept lines of
+// a panic stack trace to collect into an error when requested.
+func (p *panicRecorder) registerPlugin(name string) func(string) {
+	p.Lock()
+	defer p.Unlock()
+
+	// In most cases we shouldn't be starting a plugin if it already
+	// panicked, but clear out previous entries just in case.
+	delete(p.panics, name)
+
+	count := 0
+
+	// this callback is used by the logger to store panic output
+	return func(line string) {
+		p.Lock()
+		defer p.Unlock()
+
+		// stop recording if there are too many lines.
+		if count > p.maxLines {
+			return
+		}
+		count++
+
+		p.panics[name] = append(p.panics[name], line)
+	}
+}
+
+func (p *panicRecorder) allPanics() []string {
+	p.Lock()
+	defer p.Unlock()
+
+	var res []string
+	for name, lines := range p.panics {
+		if len(lines) == 0 {
+			continue
+		}
+
+		res = append(res, fmt.Sprintf(pluginPanicOutput, name, strings.Join(lines, "\n")))
+	}
+	return res
+}
+
+// logPanicWrapper wraps an hclog.Logger and intercepts and records any output
+// that appears to be a panic.
+type logPanicWrapper struct {
+	hclog.Logger
+	panicRecorder func(string)
+	inPanic       bool
+}
+
+// go-plugin will create a new named logger for each plugin binary.
+func (l *logPanicWrapper) Named(name string) hclog.Logger {
+	return &logPanicWrapper{
+		Logger:        l.Logger.Named(name),
+		panicRecorder: panics.registerPlugin(name),
+	}
+}
+
+// we only need to implement Debug, since that is the default output level used
+// by go-plugin when encountering unstructured output on stderr.
+func (l *logPanicWrapper) Debug(msg string, args ...interface{}) {
+	// We don't have access to the binary itself, so guess based on the stderr
+	// output if this is the start of the traceback. An occasional false
+	// positive shouldn't be a big deal, since this is only retrieved after an
+	// error of some sort.
+	l.inPanic = l.inPanic || strings.HasPrefix(msg, "panic: ") || strings.HasPrefix(msg, "fatal error: ")
+
+	if l.inPanic && l.panicRecorder != nil {
+		l.panicRecorder(msg)
+	}
+
+	l.Logger.Debug(msg, args...)
+}

internal/logging/panic_test.go

@@ -0,0 +1,51 @@
+package logging
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+)
+
+func TestPanicRecorder(t *testing.T) {
+	rec := panics.registerPlugin("test")
+
+	output := []string{
+		"panic: test",
+		"  stack info",
+	}
+
+	for _, line := range output {
+		rec(line)
+	}
+
+	expected := fmt.Sprintf(pluginPanicOutput, "test", strings.Join(output, "\n"))
+
+	res := PluginPanics()
+	if len(res) == 0 {
+		t.Fatal("no output")
+	}
+
+	if res[0] != expected {
+		t.Fatalf("expected: %q\ngot: %q", expected, res[0])
+	}
+}
+
+func TestPanicLimit(t *testing.T) {
+	rec := panics.registerPlugin("test")
+
+	rec("panic: test")
+
+	for i := 0; i < 200; i++ {
+		rec(fmt.Sprintf("LINE: %d", i))
+	}
+
+	res := PluginPanics()
+	// take the extra content into account
+	max := strings.Count(pluginPanicOutput, "\n") + panics.maxLines
+	for _, out := range res {
+		found := strings.Count(out, "\n")
+		if found > max {
+			t.Fatalf("expected no more than %d lines, got: %d", max, found)
+		}
+	}
+}

main.go

@@ -288,6 +288,16 @@ func wrappedMain() int {
 		return 1
 	}
 
+	// if we are exiting with a non-zero code, check if it was caused by any
+	// plugins crashing
+	if exitCode != 0 {
+		for _, panicLog := range logging.PluginPanics() {
+			// we don't write this to Error, or else panicwrap will think this
+			// process panicked
+			Ui.Info(panicLog)
+		}
+	}
+
 	return exitCode
 }