fix: delay-write trace.dat for forgejo diagnosis (#7597)

- Delay the writing of `trace.dat` to the forgejo diagnosis zip file. It's not possible to write `cpu-profile.dat` and `trace.dat` at the same time. This caused the implemention to prematurely close `cpu-profile.dat` and leave it as an empty file.
- Regression of forgejo/forgejo#6470

## Testing
1. Go to `/admin/monitor`
2. Click on "Download diagnosis report"
3. Open the zip file and check that all files are non-empty.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7597
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

routers/web/admin/diagnosis.go

@@ -5,7 +5,9 @@ package admin
 
 import (
 	"archive/zip"
+	"bytes"
 	"fmt"
+	"io"
 	"runtime"
 	"runtime/pprof"
 	"runtime/trace"
@@ -45,14 +47,9 @@ func MonitorDiagnosis(ctx *context.Context) {
 		_, _ = f.Write([]byte(err.Error()))
 	}
 
-	f, err = zipWriter.CreateHeader(&zip.FileHeader{Name: "trace.dat", Method: zip.Deflate, Modified: time.Now()})
+	traceBuf := &bytes.Buffer{}
-	if err != nil {
+	if err := trace.Start(traceBuf); err != nil {
-		ctx.ServerError("Failed to create zip file", err)
+		_, _ = traceBuf.Write([]byte(err.Error()))
-		return
-	}
-
-	if err := trace.Start(f); err != nil {
-		_, _ = f.Write([]byte(err.Error()))
 	}
 
 	select {
@@ -62,6 +59,17 @@ func MonitorDiagnosis(ctx *context.Context) {
 	pprof.StopCPUProfile()
 	trace.Stop()
 
+	f, err = zipWriter.CreateHeader(&zip.FileHeader{Name: "trace.dat", Method: zip.Deflate, Modified: time.Now()})
+	if err != nil {
+		ctx.ServerError("Failed to create zip file", err)
+		return
+	}
+
+	if _, err := io.Copy(f, traceBuf); err != nil {
+		ctx.ServerError("Failed to create zip file", err)
+		return
+	}
+
 	f, err = zipWriter.CreateHeader(&zip.FileHeader{Name: "goroutine-after.txt", Method: zip.Deflate, Modified: time.Now()})
 	if err != nil {
 		ctx.ServerError("Failed to create zip file", err)