[supervisor] Better reflect ♻️ incremental prebuilds in prebuild logs

components/supervisor/pkg/supervisor/tasks.go

@@ -9,7 +9,9 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
+	"regexp"
 	"strconv"
 	"strings"
 	"sync"
@@ -377,9 +379,29 @@ func (tm *tasksManager) watch(task *task, terminal *terminal.Term) {
 	go func() {
 		defer stdout.Close()
 
-		fileName := tm.prebuildLogFileName(task)
-		// TODO(janx): If the file already exists (from a parent prebuild), extract its "time saved", and log that below
-		// (instead, or in addition to, the incremental prebuild time).
+		var (
+			fileName                    = tm.prebuildLogFileName(task)
+			parentLog                   = ""
+			parentElapsed time.Duration = 0
+		)
+		if _, err := os.Stat(fileName); err == nil {
+			// If the file already exists (from a parent prebuild), extract its "elapsed time" before overwriting the prebuild logs.
+			content, err := ioutil.ReadFile(fileName)
+			if err == nil {
+				parentLog = string(content)
+				reg, err := regexp.Compile(`\n🤙 This task ran as a workspace prebuild\n(🎉 Well done on saving (\d+) minutes?\n)?\n`)
+				if err == nil {
+					res := reg.FindStringSubmatch(parentLog)
+					if res != nil {
+						parentLog = reg.ReplaceAllString(parentLog, "\n♻️ Re-running this task as an incremental workspace prebuild\n")
+						parentElapsedInMinutes, err := strconv.Atoi(res[2])
+						if err == nil {
+							parentElapsed = time.Duration(parentElapsedInMinutes) * time.Minute
+						}
+					}
+				}
+			}
+		}
 		file, err := os.Create(fileName)
 		var fileWriter *bufio.Writer
 		if err != nil {
@@ -391,12 +413,15 @@ func (tm *tasksManager) watch(task *task, terminal *terminal.Term) {
 			fileWriter = bufio.NewWriter(file)
 			defer fileWriter.Flush()
 		}
+		if parentLog != "" {
+			fileWriter.WriteString(parentLog + "\n")
+		}
 
 		buf := make([]byte, 4096)
 		for {
 			n, err := stdout.Read(buf)
 			if err == io.EOF {
-				elapsed := time.Since(start)
+				elapsed := time.Since(start) + parentElapsed
 				duration := ""
 				if elapsed >= 1*time.Minute {
 					elapsedInMinutes := strconv.Itoa(int(elapsed.Minutes()))