Implement facility based logger, debugging via REST API

This implements a new debug/trace infrastructure based on a slightly
hacked up logger. Instead of the traditional "if debug { ... }" I've
rewritten the logger to have no-op Debugln and Debugf, unless debugging
has been enabled for a given "facility". The "facility" is just a
string, typically a package name.

This will be slightly slower than before; but not that much as it's
mostly a function call that returns immediately. For the cases where it
matters (the Debugln takes a hex.Dump() of something for example, and
it's not in a very occasional "if err != nil" branch) there is an
l.ShouldDebug(facility) that is fast enough to be used like the old "if
debug".

The point of all this is that we can now toggle debugging for the
various packages on and off at runtime. There's a new method
/rest/system/debug that can be POSTed a set of facilities to enable and
disable debug for, or GET from to get a list of facilities with
descriptions and their current debug status.

Similarly a /rest/system/log?since=... can grab the latest log entries,
up to 250 of them (hardcoded constant in main.go) plus the initial few.

Not implemented in this commit (but planned) is a simple debug GUI
available on /debug that shows the current log in an easily pasteable
format and has checkboxes to enable the various debug facilities.

The debug instructions to a user then becomes "visit this URL, check
these boxes, reproduce your problem, copy and paste the log". The actual
log viewer on the hypothetical /debug URL can poll regularly for new log
entries and this bypass the 250 line limit.

The existing STTRACE=foo variable is still obeyed and just sets the
start state of the system.

lib/versioner/staggered.go

@@ -47,14 +47,10 @@ func NewStaggered(folderID, folderPath string, params map[string]string) Version
 	// Use custom path if set, otherwise .stversions in folderPath
 	var versionsDir string
 	if params["versionsPath"] == "" {
-		if debug {
-			l.Debugln("using default dir .stversions")
-		}
+		l.Debugln("using default dir .stversions")
 		versionsDir = filepath.Join(folderPath, ".stversions")
 	} else {
-		if debug {
-			l.Debugln("using dir", params["versionsPath"])
-		}
+		l.Debugln("using dir", params["versionsPath"])
 		versionsDir = params["versionsPath"]
 	}
 
@@ -71,9 +67,7 @@ func NewStaggered(folderID, folderPath string, params map[string]string) Version
 		mutex: sync.NewMutex(),
 	}
 
-	if debug {
-		l.Debugf("instantiated %#v", s)
-	}
+	l.Debugf("instantiated %#v", s)
 
 	go func() {
 		s.clean()
@@ -86,14 +80,10 @@ func NewStaggered(folderID, folderPath string, params map[string]string) Version
 }
 
 func (v Staggered) clean() {
-	if debug {
-		l.Debugln("Versioner clean: Waiting for lock on", v.versionsPath)
-	}
+	l.Debugln("Versioner clean: Waiting for lock on", v.versionsPath)
 	v.mutex.Lock()
 	defer v.mutex.Unlock()
-	if debug {
-		l.Debugln("Versioner clean: Cleaning", v.versionsPath)
-	}
+	l.Debugln("Versioner clean: Cleaning", v.versionsPath)
 
 	if _, err := os.Stat(v.versionsPath); os.IsNotExist(err) {
 		// There is no need to clean a nonexistent dir.
@@ -144,30 +134,22 @@ func (v Staggered) clean() {
 		}
 
 		if path == v.versionsPath {
-			if debug {
-				l.Debugln("Cleaner: versions dir is empty, don't delete", path)
-			}
+			l.Debugln("Cleaner: versions dir is empty, don't delete", path)
 			continue
 		}
 
-		if debug {
-			l.Debugln("Cleaner: deleting empty directory", path)
-		}
+		l.Debugln("Cleaner: deleting empty directory", path)
 		err = os.Remove(path)
 		if err != nil {
 			l.Warnln("Versioner: can't remove directory", path, err)
 		}
 	}
 
-	if debug {
-		l.Debugln("Cleaner: Finished cleaning", v.versionsPath)
-	}
+	l.Debugln("Cleaner: Finished cleaning", v.versionsPath)
 }
 
 func (v Staggered) expire(versions []string) {
-	if debug {
-		l.Debugln("Versioner: Expiring versions", versions)
-	}
+	l.Debugln("Versioner: Expiring versions", versions)
 	var prevAge int64
 	firstFile := true
 	for _, file := range versions {
@@ -185,18 +167,14 @@ func (v Staggered) expire(versions []string) {
 		loc, _ := time.LoadLocation("Local")
 		versionTime, err := time.ParseInLocation(TimeFormat, filenameTag(file), loc)
 		if err != nil {
-			if debug {
-				l.Debugf("Versioner: file name %q is invalid: %v", file, err)
-			}
+			l.Debugf("Versioner: file name %q is invalid: %v", file, err)
 			continue
 		}
 		age := int64(time.Since(versionTime).Seconds())
 
 		// If the file is older than the max age of the last interval, remove it
 		if lastIntv := v.interval[len(v.interval)-1]; lastIntv.end > 0 && age > lastIntv.end {
-			if debug {
-				l.Debugln("Versioner: File over maximum age -> delete ", file)
-			}
+			l.Debugln("Versioner: File over maximum age -> delete ", file)
 			err = os.Remove(file)
 			if err != nil {
 				l.Warnf("Versioner: can't remove %q: %v", file, err)
@@ -220,9 +198,7 @@ func (v Staggered) expire(versions []string) {
 		}
 
 		if prevAge-age < usedInterval.step {
-			if debug {
-				l.Debugln("too many files in step -> delete", file)
-			}
+			l.Debugln("too many files in step -> delete", file)
 			err = os.Remove(file)
 			if err != nil {
 				l.Warnf("Versioner: can't remove %q: %v", file, err)
@@ -237,17 +213,13 @@ func (v Staggered) expire(versions []string) {
 // Archive moves the named file away to a version archive. If this function
 // returns nil, the named file does not exist any more (has been archived).
 func (v Staggered) Archive(filePath string) error {
-	if debug {
-		l.Debugln("Waiting for lock on ", v.versionsPath)
-	}
+	l.Debugln("Waiting for lock on ", v.versionsPath)
 	v.mutex.Lock()
 	defer v.mutex.Unlock()
 
 	_, err := osutil.Lstat(filePath)
 	if os.IsNotExist(err) {
-		if debug {
-			l.Debugln("not archiving nonexistent file", filePath)
-		}
+		l.Debugln("not archiving nonexistent file", filePath)
 		return nil
 	} else if err != nil {
 		return err
@@ -255,9 +227,7 @@ func (v Staggered) Archive(filePath string) error {
 
 	if _, err := os.Stat(v.versionsPath); err != nil {
 		if os.IsNotExist(err) {
-			if debug {
-				l.Debugln("creating versions dir", v.versionsPath)
-			}
+			l.Debugln("creating versions dir", v.versionsPath)
 			osutil.MkdirAll(v.versionsPath, 0755)
 			osutil.HideFile(v.versionsPath)
 		} else {
@@ -265,9 +235,7 @@ func (v Staggered) Archive(filePath string) error {
 		}
 	}
 
-	if debug {
-		l.Debugln("archiving", filePath)
-	}
+	l.Debugln("archiving", filePath)
 
 	file := filepath.Base(filePath)
 	inFolderPath, err := filepath.Rel(v.folderPath, filepath.Dir(filePath))
@@ -283,9 +251,7 @@ func (v Staggered) Archive(filePath string) error {
 
 	ver := taggedFilename(file, time.Now().Format(TimeFormat))
 	dst := filepath.Join(dir, ver)
-	if debug {
-		l.Debugln("moving to", dst)
-	}
+	l.Debugln("moving to", dst)
 	err = osutil.Rename(filePath, dst)
 	if err != nil {
 		return err