binaryfetcher: atomically rename binary file when fetch succeeds (#57)

* binaryfetcher: atomically rename binary file when fetch succeeds

Otherwise an empty file will be created on errors like HTTP 403
and cached, which will prevent future VMs from running.

* Fix linter errors

internal/binaryfetcher/binaryfetcher.go

@@ -24,23 +24,36 @@ func GetOrFetch(ctx context.Context, fetchFunc FetchFunc, binaryName string, exe
 
 	// Run the user-provided function to fetch the binary file
 	// if not available in the cache
-	binaryFile, err := os.Create(binaryPath)
+	binaryFile, err := os.CreateTemp("", "vetu-binary-file-*")
 	if err != nil {
 		return "", err
 	}
-	defer binaryFile.Close()
 
 	if err := fetchFunc(ctx, binaryFile); err != nil {
+		_ = binaryFile.Close()
+		_ = os.Remove(binaryFile.Name())
+
 		return "", err
 	}
 
 	// Make the binary executable if requested
 	if executable {
 		if err := binaryFile.Chmod(0755); err != nil {
+			_ = binaryFile.Close()
+			_ = os.Remove(binaryFile.Name())
+
 			return "", err
 		}
 	}
 
+	if err := binaryFile.Close(); err != nil {
+		return "", err
+	}
+
+	if err := os.Rename(binaryFile.Name(), binaryPath); err != nil {
+		return "", err
+	}
+
 	return binaryPath, nil
 }
 

internal/filelock/filelock_test.go

@@ -24,7 +24,7 @@ func TestMain(m *testing.M) {
 	} else if lockPath, ok := os.LookupEnv(envTestHelperTrylockShared); ok {
 		testHelperTrylockShared(lockPath)
 	} else {
-		m.Run()
+		os.Exit(m.Run())
 	}
 }
 

internal/pidlock/pidlock_test.go

@@ -25,7 +25,7 @@ func TestMain(m *testing.M) {
 	} else if lockPath, ok := os.LookupEnv(envTestHelperPid); ok {
 		testHelperPid(lockPath)
 	} else {
-		m.Run()
+		os.Exit(m.Run())
 	}
 }
 

internal/sparseio/sparseio_test.go

@@ -40,7 +40,6 @@ func TestCopyRandomized(t *testing.T) {
 
 		// Randomize the contents of some chunks
 		if rand.Intn(2) == 1 {
-			//nolint:staticcheck // what's the alternative to the deprecated rand.Read() anyways?
 			_, err = rand.Read(chunk)
 			require.NoError(t, err)
 		}