Remove convert old states code

Filebeat 1.x had a different registry format from 5.x. So a conversion of the format happened. This code is now removed for 6.0 as the upgrade path to 6.0 is from 5.0.

The code caused some issues in the past so it is better to not have it in anymore.

CHANGELOG.asciidoc

@@ -21,6 +21,7 @@ https://github.com/elastic/beats/compare/v5.1.1...master[Check the HEAD diff]
 
 *Filebeat*
 - Always use absolute path for event and registry. This can lead to issues when relative paths were used before. {pull}3328[3328]
+- Remove code to convert states from 1.x. {pull}3767[3767]
 
 *Heartbeat*
 

filebeat/registrar/registrar.go

@@ -7,8 +7,6 @@ import (
 	"path/filepath"
 	"sync"
 
-	"time"
-
 	cfg "github.com/elastic/beats/filebeat/config"
 	"github.com/elastic/beats/filebeat/input"
 	"github.com/elastic/beats/filebeat/input/file"
@@ -105,12 +103,6 @@ func (r *Registrar) loadStates() error {
 
 	logp.Info("Loading registrar data from %s", r.registryFile)
 
-	// DEPRECATED: This should be removed in 6.0
-	oldStates := r.loadAndConvertOldState(f)
-	if oldStates {
-		return nil
-	}
-
 	decoder := json.NewDecoder(f)
 	states := []file.State{}
 	err = decoder.Decode(&states)
@@ -125,60 +117,6 @@ func (r *Registrar) loadStates() error {
 	return nil
 }
 
-// loadAndConvertOldState loads the old state file and converts it to the new state
-// This is designed so it can be easily removed in later versions
-func (r *Registrar) loadAndConvertOldState(f *os.File) bool {
-	// Make sure file reader is reset afterwards
-	defer f.Seek(0, 0)
-
-	stat, err := f.Stat()
-	if err != nil {
-		logp.Err("Error getting stat for old state: %+v", err)
-		return false
-	}
-
-	// Empty state does not have to be transformed ({} + newline)
-	if stat.Size() <= 4 {
-		return false
-	}
-
-	// Check if already new state format
-	decoder := json.NewDecoder(f)
-	newState := []file.State{}
-	err = decoder.Decode(&newState)
-	// No error means registry is already in new format
-	if err == nil {
-		return false
-	}
-
-	// Reset file offset
-	f.Seek(0, 0)
-	oldStates := map[string]file.State{}
-	err = decoder.Decode(&oldStates)
-	if err != nil {
-		logp.Err("Error decoding old state: %+v", err)
-		return false
-	}
-
-	// No old states found -> probably already new format
-	if oldStates == nil {
-		return false
-	}
-
-	// Convert old states to new states
-	logp.Info("Old registry states found: %v", len(oldStates))
-	states := convertOldStates(oldStates)
-	states = resetStates(states)
-	r.states.SetStates(states)
-
-	// Rewrite registry in new format
-	r.writeRegistry()
-
-	logp.Info("Old states converted to new states and written to registrar: %v", len(oldStates))
-
-	return true
-}
-
 // resetStates sets all states to finished and disable TTL on restart
 // For all states covered by a prospector, TTL will be overwritten with the prospector value
 func resetStates(states []file.State) []file.State {
@@ -192,32 +130,6 @@ func resetStates(states []file.State) []file.State {
 	return states
 }
 
-func convertOldStates(oldStates map[string]file.State) []file.State {
-	// Convert old states to new states
-	states := []file.State{}
-	for _, state := range oldStates {
-		// Makes timestamp time of migration, as this is the best guess
-		state.Timestamp = time.Now()
-
-		// Check for duplicates
-		dupe := false
-		for i, other := range states {
-			if state.FileStateOS.IsSame(other.FileStateOS) {
-				dupe = true
-				if state.Offset > other.Offset {
-					// replace other
-					states[i] = state
-					break
-				}
-			}
-		}
-		if !dupe {
-			states = append(states, state)
-		}
-	}
-	return states
-}
-
 func (r *Registrar) Start() error {
 
 	// Load the previous log file locations now, for use in prospector