main: print source location when a panic happens in -monitor

The previous commit started printing the instruction address for runtime
panics. This commit starts using this address to print the source
location.

Here is an example where this feature is very useful. There is a heap
allocation in the Bluetooth package, but we don't know where exactly.
Printing the instruction address of the panic is already useful, but
what is even more useful is looking up this address in the DWARF debug
information that's part of the binary:

    $ tinygo flash -target=circuitplay-bluefruit -monitor ./examples/heartrate
    Connected to /dev/ttyACM0. Press Ctrl-C to exit.
    tick 00:00.810
    tick 00:01.587
    tick 00:02.387
    tick 00:03.244
    panic: runtime error at 0x00027c4d: alloc in interrupt
    [tinygo: panic at /home/ayke/src/tinygo/bluetooth/adapter_sd.go:74:4]

To be clear, this path isn't stored on the microcontroller. It's stored
as part of the build, and `-monitor` just looks up the path from the
panic message.

Possible enhancements:

  - Print such an address for regular panics as well. I'm not sure
    that's so useful, as it's usually a lot easier to look up panics
    just by their message.
  - Use runtimePanicAt (instead of runtimePanic) in other locations, if
    that proves to be beneficial.
  - Print the TinyGo-generated output in some other color, to
    distinguish it from the regular console output.
  - Print more details when panicking (registers, stack values), and
    print an actual backtrace.

main.go

@@ -532,7 +532,7 @@ func Flash(pkgName, port string, options *compileopts.Options) error {
 		return fmt.Errorf("unknown flash method: %s", flashMethod)
 	}
 	if options.Monitor {
-		return Monitor("", options)
+		return Monitor(result.Executable, "", options)
 	}
 	return nil
 }
@@ -1720,7 +1720,7 @@ func main() {
 			os.Exit(1)
 		}
 	case "monitor":
-		err := Monitor(*port, options)
+		err := Monitor("", *port, options)
 		handleCompilerError(err)
 	case "targets":
 		dir := filepath.Join(goenv.Get("TINYGOROOT"), "targets")

monitor.go

@@ -1,9 +1,18 @@
 package main
 
 import (
+	"debug/dwarf"
+	"debug/elf"
+	"debug/macho"
+	"debug/pe"
+	"errors"
 	"fmt"
+	"go/token"
+	"io"
 	"os"
 	"os/signal"
+	"regexp"
+	"strconv"
 	"time"
 
 	"github.com/mattn/go-tty"
@@ -13,7 +22,7 @@ import (
 )
 
 // Monitor connects to the given port and reads/writes the serial port.
-func Monitor(port string, options *compileopts.Options) error {
+func Monitor(executable, port string, options *compileopts.Options) error {
 	config, err := builder.NewConfig(options)
 	if err != nil {
 		return err
@@ -74,17 +83,31 @@ func Monitor(port string, options *compileopts.Options) error {
 
 	go func() {
 		buf := make([]byte, 100*1024)
+		var line []byte
 		for {
 			n, err := p.Read(buf)
 			if err != nil {
 				errCh <- fmt.Errorf("read error: %w", err)
 				return
 			}
-
-			if n == 0 {
-				continue
+			start := 0
+			for i, c := range buf[:n] {
+				if c == '\n' {
+					os.Stdout.Write(buf[start : i+1])
+					start = i + 1
+					address := extractPanicAddress(line)
+					if address != 0 {
+						loc, err := addressToLine(executable, address)
+						if err == nil && loc.IsValid() {
+							fmt.Printf("[tinygo: panic at %s]\n", loc.String())
+						}
+					}
+					line = line[:0]
+				} else {
+					line = append(line, c)
+				}
 			}
-			fmt.Printf("%v", string(buf[:n]))
+			os.Stdout.Write(buf[start:n])
 		}
 	}()
 
@@ -104,3 +127,109 @@ func Monitor(port string, options *compileopts.Options) error {
 
 	return <-errCh
 }
+
+var addressMatch = regexp.MustCompile(`^panic: runtime error at 0x([0-9a-f]+): `)
+
+// Extract the address from the "panic: runtime error at" message.
+func extractPanicAddress(line []byte) uint64 {
+	matches := addressMatch.FindSubmatch(line)
+	if matches != nil {
+		address, err := strconv.ParseUint(string(matches[1]), 16, 64)
+		if err == nil {
+			return address
+		}
+	}
+	return 0
+}
+
+// Convert an address in the binary to a source address location.
+func addressToLine(executable string, address uint64) (token.Position, error) {
+	data, err := readDWARF(executable)
+	if err != nil {
+		return token.Position{}, err
+	}
+	r := data.Reader()
+
+	for {
+		e, err := r.Next()
+		if err != nil {
+			return token.Position{}, err
+		}
+		if e == nil {
+			break
+		}
+		switch e.Tag {
+		case dwarf.TagCompileUnit:
+			r.SkipChildren()
+			lr, err := data.LineReader(e)
+			if err != nil {
+				return token.Position{}, err
+			}
+			var lineEntry = dwarf.LineEntry{
+				EndSequence: true,
+			}
+			for {
+				// Read the next .debug_line entry.
+				prevLineEntry := lineEntry
+				err := lr.Next(&lineEntry)
+				if err != nil {
+					if err == io.EOF {
+						break
+					}
+					return token.Position{}, err
+				}
+
+				if prevLineEntry.EndSequence && lineEntry.Address == 0 {
+					// Tombstone value. This symbol has been removed, for
+					// example by the --gc-sections linker flag. It is still
+					// here in the debug information because the linker can't
+					// just remove this reference.
+					// Read until the next EndSequence so that this sequence is
+					// skipped.
+					// For more details, see (among others):
+					// https://reviews.llvm.org/D84825
+					for {
+						err := lr.Next(&lineEntry)
+						if err != nil {
+							return token.Position{}, err
+						}
+						if lineEntry.EndSequence {
+							break
+						}
+					}
+				}
+
+				if !prevLineEntry.EndSequence {
+					// The chunk describes the code from prevLineEntry to
+					// lineEntry.
+					if prevLineEntry.Address <= address && lineEntry.Address > address {
+						return token.Position{
+							Filename: prevLineEntry.File.Name,
+							Line:     prevLineEntry.Line,
+							Column:   prevLineEntry.Column,
+						}, nil
+					}
+				}
+			}
+		}
+	}
+
+	return token.Position{}, nil // location not found
+}
+
+// Read the DWARF debug information from a given file (in various formats).
+func readDWARF(executable string) (*dwarf.Data, error) {
+	f, err := os.Open(executable)
+	if err != nil {
+		return nil, err
+	}
+	if file, err := elf.NewFile(f); err == nil {
+		return file.DWARF()
+	} else if file, err := macho.NewFile(f); err == nil {
+		return file.DWARF()
+	} else if file, err := pe.NewFile(f); err == nil {
+		return file.DWARF()
+	} else {
+		return nil, errors.New("unknown binary format")
+	}
+}

monitor_test.go

@@ -0,0 +1,66 @@
+package main
+
+import (
+	"bytes"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"testing"
+	"time"
+
+	"github.com/tinygo-org/tinygo/builder"
+	"github.com/tinygo-org/tinygo/compileopts"
+)
+
+func TestTraceback(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		// We care about testing the ELF format, which is only used on Linux
+		// (not on MacOS or Windows).
+		t.Skip("Test only works on Linux")
+	}
+
+	// Build a small binary that only panics.
+	tmpdir := t.TempDir()
+	config, err := builder.NewConfig(&compileopts.Options{
+		GOOS:          runtime.GOOS,
+		GOARCH:        runtime.GOARCH,
+		Opt:           "z",
+		InterpTimeout: time.Minute,
+		Debug:         true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	result, err := builder.Build("testdata/trivialpanic.go", ".elf", tmpdir, config)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Run this binary, and capture the output.
+	buf := &bytes.Buffer{}
+	cmd := exec.Command(result.Binary)
+	cmd.Stdout = buf
+	cmd.Stderr = buf
+	cmd.Run() // this will return an error because of the panic, ignore it
+
+	// Extract the "runtime error at" address.
+	line := bytes.TrimSpace(buf.Bytes())
+	address := extractPanicAddress(line)
+	if address == 0 {
+		t.Fatalf("could not extract panic address from %#v", string(line))
+	}
+
+	// Look up the source location for this address.
+	location, err := addressToLine(result.Executable, address)
+	if err != nil {
+		t.Fatal("could not read source location:", err)
+	}
+
+	// Verify that the source location is as expected.
+	if filepath.Base(location.Filename) != "trivialpanic.go" {
+		t.Errorf("expected path to end with trivialpanic.go, got %#v", location.Filename)
+	}
+	if location.Line != 6 {
+		t.Errorf("expected panic location to be line 6, got line %d", location.Line)
+	}
+}

testdata/trivialpanic.go

@@ -0,0 +1,7 @@
+package main
+
+var n *int
+
+func main() {
+	println(*n) // this will panic
+}