Introduce debug tags and debug breaks

The environment variable DEBUG_TAGS can be used to control what is
printed on stderr. Example:

    DEBUG_TAGS="+all,-Archiver.*" ./restic backup foo

The variable DEBUG_BREAK can be used to set a breakpoint (implemented
using SIGSTOP):

    DEBUG_BREAK=Archiver.Snapshot ./restic backup foo

debug.go

@@ -4,33 +4,131 @@ package restic
 
 import (
 	"fmt"
-	"io"
 	"log"
 	"os"
+	"path"
 	"path/filepath"
+	"runtime"
+	"strings"
+	"syscall"
 	"time"
 )
 
 var debugLogger = initDebugLogger()
+var debugTags = make(map[string]bool)
+var debugBreak = make(map[string]bool)
 
 func initDebugLogger() *log.Logger {
 	// create new log file
 	filename := fmt.Sprintf("restic-lib-debug-%d-%s",
 		os.Getpid(), time.Now().Format("20060201-150405"))
-	path := filepath.Join(os.TempDir(), filename)
-	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE, 0600)
+	debugfile := filepath.Join(os.TempDir(), filename)
+	f, err := os.OpenFile(debugfile, os.O_WRONLY|os.O_CREATE, 0600)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "unable to create debug log file: %v", err)
 		os.Exit(2)
 	}
 
 	// open logger
-	l := log.New(io.MultiWriter(os.Stderr, f), "DEBUG: ", log.LstdFlags)
-	fmt.Fprintf(os.Stderr, "debug log for restic library activated, writing log file %s\n", path)
+	l := log.New(f, "DEBUG: ", log.LstdFlags)
+	fmt.Fprintf(os.Stderr, "debug log for restic library activated, writing log file %s\n", debugfile)
+
+	// defaults
+	debugTags["break"] = true
+
+	// initialize tags
+	env := os.Getenv("DEBUG_TAGS")
+	if len(env) > 0 {
+		tags := []string{}
+
+		for _, tag := range strings.Split(env, ",") {
+			t := strings.TrimSpace(tag)
+			val := true
+			if t[0] == '-' {
+				val = false
+				t = t[1:]
+			} else if t[0] == '+' {
+				val = true
+				t = t[1:]
+			}
+
+			// test pattern
+			_, err := path.Match(t, "")
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "error: invalid pattern %q: %v\n", t, err)
+				os.Exit(5)
+			}
+
+			debugTags[t] = val
+			tags = append(tags, tag)
+		}
+
+		fmt.Fprintf(os.Stderr, "debug log enabled for: %v\n", tags)
+	}
+
+	// initialize break tags
+	env = os.Getenv("DEBUG_BREAK")
+	if len(env) > 0 {
+		breaks := []string{}
+
+		for _, tag := range strings.Split(env, ",") {
+			t := strings.TrimSpace(tag)
+			debugBreak[t] = true
+			breaks = append(breaks, t)
+		}
+
+		fmt.Fprintf(os.Stderr, "debug breaks enabled for: %v\n", breaks)
+	}
 
 	return l
 }
 
-func debug(fmt string, args ...interface{}) {
-	debugLogger.Printf(fmt, args...)
+func debug(tag string, f string, args ...interface{}) {
+	dbgprint := func() {
+		fmt.Fprintf(os.Stderr, tag+": "+f, args...)
+	}
+
+	debugLogger.Printf(f, args...)
+
+	// check if tag is enabled directly
+	if v, ok := debugTags[tag]; ok {
+		if v {
+			dbgprint()
+		}
+		return
+	}
+
+	// check for globbing
+	for k, v := range debugTags {
+		if m, _ := path.Match(k, tag); m {
+			if v {
+				dbgprint()
+			}
+			return
+		}
+	}
+
+	// check if tag "all" is enabled
+	if v, ok := debugTags["all"]; ok && v {
+		dbgprint()
+	}
+}
+
+func debug_break(tag string) {
+	// check if breaking is enabled
+	if v, ok := debugBreak[tag]; !ok || !v {
+		return
+	}
+
+	_, file, line, _ := runtime.Caller(1)
+	debug("break", "stopping process %d at %s (%v:%v)\n", os.Getpid(), tag, file, line)
+	p, err := os.FindProcess(os.Getpid())
+	if err != nil {
+		panic(err)
+	}
+
+	err = p.Signal(syscall.SIGSTOP)
+	if err != nil {
+		panic(err)
+	}
 }