Generate our own single-line DKIM entry

I installed pyenv and then installed Python 3.9:
$ pyenv install 3.9
$ eval "$(pyenv init -)"
$ pyenv shell 3.9

In a clean repository I ran
$ scripts/cmdeploy init
$ scripts/cmdeploy run
$ scripts/cmdeploy dns
$ scripts/cmdeploy fmt

With the changes made all these commands work.

scripts/cmdeploy test fails some tests
using maildata fixture at
  importlib.resources.files(__package__).joinpath("mail-data")
line but this is not critical.

cmdeploy/src/cmdeploy/dns.py

@@ -56,22 +56,25 @@ def show_dns(args, out) -> int:
     ssh = f"ssh root@{mail_domain}"
     dns = DNS(out, mail_domain)
 
-    def read_dkim_entries(entry):
-        lines = []
-        for line in entry.split("\n"):
-            if line.startswith(";") or not line.strip():
-                continue
-            line = line.replace("\t", " ")
-            lines.append(line)
-        return "\n".join(lines)
-
     print("Checking your DKIM keys and DNS entries...")
     try:
         acme_account_url = out.shell_output(f"{ssh} -- acmetool account-url")
     except subprocess.CalledProcessError:
         print("Please run `cmdeploy run` first.")
         return 1
-    dkim_entry = read_dkim_entries(out.shell_output(f"{ssh} -- opendkim-genzone -F"))
+
+    dkim_selector = "opendkim"
+    dkim_pubkey = out.shell_output(
+        ssh + f" -- openssl rsa -in /etc/dkimkeys/{dkim_selector}.private"
+        " -pubout 2>/dev/null | awk '/-/{next}{printf(\"%s\",$0)}'"
+    )
+    dkim_entry_value = f"v=DKIM1;k=rsa;p={dkim_pubkey};s=email;t=s"
+    dkim_entry_str = ""
+    while len(dkim_entry_value) >= 255:
+        dkim_entry_str += '"' + dkim_entry_value[:255] + '" '
+        dkim_entry_value = dkim_entry_value[255:]
+    dkim_entry_str += '"' + dkim_entry_value + '"'
+    dkim_entry = f"{dkim_selector}._domainkey.{mail_domain}. TXT {dkim_entry_str}"
 
     ipv6 = dns.get_ipv6()
     reverse_ipv6 = dns.check_ptr_record(ipv6, mail_domain)
@@ -99,7 +102,6 @@ def show_dns(args, out) -> int:
             return 0
         except TypeError:
             pass
-        started_dkim_parsing = False
         for line in zonefile.splitlines():
             line = line.format(
                 acme_account_url=acme_account_url,
@@ -126,28 +128,23 @@ def show_dns(args, out) -> int:
                 current = dns.get("SRV", domain[:-1])
                 if current != f"{prio} {weight} {port} {value}":
                     to_print.append(line)
-            if "  TXT " in line:
+            if " TXT " in line:
                 domain, value = line.split(" TXT ")
                 current = dns.get("TXT", domain.strip()[:-1])
                 if domain.startswith("_mta-sts."):
                     if current:
                         if current.split("id=")[0] == value.split("id=")[0]:
                             continue
-                if current != value:
+
+                # TXT records longer than 255 bytes
+                # are split into multiple <character-string>s.
+                # This typically happens with DKIM record
+                # which contains long RSA key.
+                #
+                # Removing `" "` before comparison
+                # to get back a single string.
+                if current.replace('" "', "") != value.replace('" "', ""):
                     to_print.append(line)
-            if "IN TXT ( " in line:
-                started_dkim_parsing = True
-                dkim_lines = [line]
-            if started_dkim_parsing and line.startswith('"'):
-                dkim_lines.append(" " + line)
-        domain, data = "\n".join(dkim_lines).split(" IN TXT ")
-        current = dns.get("TXT", domain.strip()[:-1])
-        if current:
-            current = "( %s )" % (current.replace('" "', '"\n "'))
-            if current.replace(";", "\\;") != data:
-                to_print.append(dkim_entry)
-        else:
-            to_print.append(dkim_entry)
 
     exit_code = 0
     if to_print: