Add log to debug delta failures after they run

Delta creation runs in parallel, so use descriptive prefixes that can be
easily queried from the log file later on since they will not all be
grouped.

Signed-off-by: Tudor Marcu <tudor.marcu@intel.com>

swupd/delta.go

@@ -2,6 +2,7 @@ package swupd
 
 import (
 	"fmt"
+	"log"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -32,6 +33,8 @@ type Delta struct {
 	to    *File
 }
 
+var bsdiffLog *log.Logger
+
 // CreateDeltasForManifest creates all delta files between the previous and current version of the
 // supplied manifest. Returns a list of deltas (which contains information about
 // individual delta errors). Returns error (and no deltas) if it can't assemble the delta
@@ -58,6 +61,15 @@ func CreateDeltasForManifest(manifest, statedir string, from, to uint32, numWork
 }
 
 func createDeltasFromManifests(c *config, oldManifest, newManifest *Manifest, numWorkers int) ([]Delta, error) {
+	logFile, err := os.OpenFile(filepath.Join(c.stateDir, "bsdiff_errors.log"), os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
+	if err != nil {
+		return nil, errors.Wrap(err, "Cannot create log file for delta creation")
+	}
+	defer func() {
+		_ = logFile.Close()
+	}()
+	bsdiffLog = log.New(logFile, "DELTA: ", log.Lshortfile)
+
 	deltas, err := findDeltas(c, oldManifest, newManifest)
 	if err != nil {
 		return nil, errors.Wrapf(err, "Failed to create deltas list %s", newManifest.Name)
@@ -141,20 +153,29 @@ func createDelta(c *config, delta *Delta) error {
 				}
 			}
 		}
-		return errors.Wrapf(err, "Failed to create delta for %s (%d) -> %s (%d)", delta.from.Name, delta.from.Version, delta.to.Name, delta.to.Version)
+		errStr := fmt.Sprintf("Failed to create delta for %s (%d) -> %s (%d)", delta.from.Name, delta.from.Version, delta.to.Name, delta.to.Version)
+		bsdiffLog.SetPrefix("BSDIFF: ")
+		bsdiffLog.Println(errStr)
+		return errors.Wrap(err, errStr)
 	}
 
 	// Check that delta is smaller than compressed full file
 	if deltaTooLarge(c, delta, newPath) {
 		_ = os.Remove(delta.Path)
-		return fmt.Errorf("Delta file %s (%d) larger than compressed full file %s",
-			delta.to.Name, delta.to.Version, newPath)
+
+		errStr := fmt.Sprintf("Delta file %s (%d) larger than compressed full file %s", delta.to.Name, delta.to.Version, newPath)
+		bsdiffLog.SetPrefix("LARGER-DELTA: ")
+		bsdiffLog.Println(errStr)
+		return errors.New(errStr)
 	}
 
 	// Check that the delta actually applies correctly.
 	testPath := delta.Path + ".testnewfile"
 	if err := helpers.RunCommandSilent("bspatch", oldPath, testPath, delta.Path); err != nil {
-		return errors.Wrapf(err, "Failed to apply delta %s", delta.Path)
+		errStr := fmt.Sprintf("Failed to apply delta %s", delta.Path)
+		bsdiffLog.SetPrefix("BSPATCH: ")
+		bsdiffLog.Println(errStr)
+		return errors.Wrapf(err, errStr)
 	}
 	defer func() {
 		_ = os.Remove(testPath)