fix(sidecarassets): publish the extracted jar through a rename

Extract wrote a 96 MB jar with a plain WriteFile into a temp path every
sanderling process on the host shares. On a cold host several concurrent
workers all miss the checksum and all write the same path, and O_TRUNC lets one
spawn a JVM against another's half-written archive. A fresh experiment host is
exactly a cold host.

Claude-Session: https://claude.ai/code/session_01A5KmftdEJ49A9z5mF5ESrX

internal/sidecarassets/embed.go

@@ -39,11 +39,35 @@ func Extract(dir string) (string, error) {
 		}
 	}
 
-	if err := os.WriteFile(jarPath, embeddedJAR, 0o644); err != nil {
+	if err := writeAtomic(jarPath, embeddedJAR); err != nil {
 		return "", fmt.Errorf("write %s: %w", jarPath, err)
 	}
-	if err := os.WriteFile(checksumPath, []byte(checksum), 0o644); err != nil {
+	if err := writeAtomic(checksumPath, []byte(checksum)); err != nil {
 		return "", fmt.Errorf("write checksum: %w", err)
 	}
 	return jarPath, nil
 }
+
+// writeAtomic publishes content at path through a rename, so a reader never
+// observes a partial file. dir is shared between every sanderling process on
+// the host, so a campaign running one worker per device has several of them
+// extracting the same JAR at once on a cold host; a plain write let one
+// process spawn a JVM against another's half-written file.
+func writeAtomic(path string, content []byte) error {
+	temporary, err := os.CreateTemp(filepath.Dir(path), filepath.Base(path)+".*.partial")
+	if err != nil {
+		return err
+	}
+	defer os.Remove(temporary.Name())
+	if _, err := temporary.Write(content); err != nil {
+		temporary.Close()
+		return err
+	}
+	if err := temporary.Close(); err != nil {
+		return err
+	}
+	if err := os.Chmod(temporary.Name(), 0o644); err != nil {
+		return err
+	}
+	return os.Rename(temporary.Name(), path)
+}

internal/sidecarassets/embed_withsidecar_test.go

@@ -6,8 +6,11 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"encoding/hex"
+	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
+	"sync"
 	"testing"
 )
 
@@ -89,3 +92,76 @@ func TestExtract_RewritesIfChecksumMissing(t *testing.T) {
 		t.Errorf("checksum should have been written: %v", err)
 	}
 }
+
+// TestExtract_ReadersNeverSeeAPartialJAR covers the cold-host case a parallel
+// campaign hits: every worker process extracts into the same shared directory
+// under os.TempDir, and one of them spawns a JVM against the JAR while another
+// is still writing it. A plain write truncates the file in place, so the
+// reader gets a short JAR and the JVM dies on a corrupt archive.
+func TestExtract_ReadersNeverSeeAPartialJAR(t *testing.T) {
+	directory := t.TempDir()
+	want := EmbeddedSHA256()
+	jarPath, err := Extract(directory)
+	if err != nil {
+		t.Fatal(err)
+	}
+	checksumPath := jarPath + ".sha256"
+
+	stop := make(chan struct{})
+	torn := make(chan string, 1)
+	var readers sync.WaitGroup
+	for range 4 {
+		readers.Add(1)
+		go func() {
+			defer readers.Done()
+			for {
+				select {
+				case <-stop:
+					return
+				default:
+				}
+				content, err := os.ReadFile(jarPath)
+				if err != nil {
+					continue
+				}
+				sum := sha256.Sum256(content)
+				if got := hex.EncodeToString(sum[:]); got != want {
+					select {
+					case torn <- fmt.Sprintf("read %d bytes, sha256 %s", len(content), got):
+					default:
+					}
+					return
+				}
+			}
+		}()
+	}
+
+	for range 8 {
+		// Dropping the checksum is what a cold host looks like: no extraction
+		// has been recorded, so every caller writes the JAR again.
+		if err := os.Remove(checksumPath); err != nil && !os.IsNotExist(err) {
+			t.Fatal(err)
+		}
+		if _, err := Extract(directory); err != nil {
+			t.Fatal(err)
+		}
+	}
+	close(stop)
+	readers.Wait()
+
+	select {
+	case detail := <-torn:
+		t.Fatalf("a reader observed a partial JAR while another caller extracted: %s", detail)
+	default:
+	}
+
+	entries, err := os.ReadDir(directory)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, entry := range entries {
+		if strings.Contains(entry.Name(), ".partial") {
+			t.Errorf("Extract left a staging file behind: %s", entry.Name())
+		}
+	}
+}