Vault agent memory leak when template can't write to file

[sgorbaty]

Describe the bug

Unbounded memory growth when DAC unix permissions prevent file to be written to disk.

Noticeable CPU usage growth:

To Reproduce
Steps to reproduce the behavior:

1. Configure vault agent with

 "template": [
    {
      "source": "/etc/vault_agent/vault.ctmpl",
      "destination": "/etc/vault_agent/certificate.pem",
      "perms": "0640",
      "error_on_missing_key": true
    }
  ],

2. vault.ctmpl is this:

{{- with pkiCert "pki/internal_certs/issue/site-ec" "common_name=***-***.internal.net" -}}
{{ .Cert }}{{ .CA }}
{{ if .Key }}
  {{ .Key | writeToFile "/etc/vault_agent/private_key.pem" "" "" "0600" }}
{{ end }}
{{- end -}}

3. /etc/vault_agent/private_key.pem is actually configured as chmod 0400.

Expected behavior
Failure to write to disk and no memory leak or CPU spike.

Environment:

• Vault Server Version (retrieve with vault status):
  Vault v1.17.5
• Vault CLI Version (retrieve with vault version):
  Vault v1.17.5
• Server Operating System/Architecture:
  unix

Vault server configuration file(s):

# renew/rotate the cert for associated ica
path "pki/${function}/issue/${pkirole}" {
  capabilities = ["create", "update"]
}

Additional context
Error:

execute: template: :4:12: executing \"\" at <writeToFile \"/etc/vault_agent/private_key.pem\" \"\" \"\" \"0600\">: error calling writeToFile: open /etc/vault_agent/private_key.pem: permission denied"