tornado: Remove fingerprinting, write out .tmp files always.

Fingerprinting the config is somewhat brittle -- it requires either
custom bootstrapping for old (fingerprint-less) configs, and may have
false-positives.

Since generating the config is lightweight, do so into the .tmp files,
and compare the output to the originals to determine if there are
changes to apply.

In order to both surface errors, as well as notify the user in case a
restart is necessary, we must run it twice.  The `onlyif`
functionality cannot show configuration errors to the user, only
determine if the command runs or not.  We thus run the command once,
judging errors as "interesting" enough to run the actual command,
whose failure will be verbose in Puppet and halt any steps that depend
on it.

Removing the `onlyif` would result in `stage_updated_sharding` showing
up in the output of every Puppet run, which obscures the important
messages it displays when an update to sharding is necessary.
Removing the `command` (e.g. making it an `echo`) would result in
removing the ability to report configuration errors.  We thus have no
choice but to run it twice; this is thankfully low-overhead.

puppet/zulip/manifests/tornado_sharding.pp

@@ -7,17 +7,13 @@ class zulip::tornado_sharding {
   # with the correct default content for the "only one shard" setup. For this
   # reason they use "replace => false", because the files are managed by
   # the sharding script afterwards and puppet shouldn't overwrite them.
-
-  # The sha256 of the empty hash, used to fingerprint the configs as
-  # having been generated with a null sharding configuration.
-  $empty_sha256 = sha256('')
   file { '/etc/zulip/nginx_sharding.conf':
     ensure  => file,
     owner   => 'root',
     group   => 'root',
     mode    => '0644',
     notify  => Service['nginx'],
-    content => "# Configuration hash: ${empty_sha256}\nset \$tornado_server http://tornado;\n",
+    content => "set \$tornado_server http://tornado;\n",
     replace => false,
   }
   file { '/etc/zulip/sharding.json':
@@ -30,10 +26,12 @@ class zulip::tornado_sharding {
     replace => false,
   }
 
-  exec { 'write_updated_sharding':
+  # This creates .tmp files which scripts/refresh-sharding-and-restart
+  # moves into place
+  exec { 'stage_updated_sharding':
     command   => "${::zulip_scripts_path}/lib/sharding.py",
-    unless    => "${::zulip_scripts_path}/lib/sharding.py --verify",
-    require   => [File['/etc/zulip/nginx_sharding.conf']],
+    onlyif    => "${::zulip_scripts_path}/lib/sharding.py --errors-ok",
+    require   => [File['/etc/zulip/nginx_sharding.conf'], File['/etc/zulip/sharding.json']],
     logoutput => true,
     loglevel  => 'warning',
   }

scripts/lib/sharding.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 import argparse
-import hashlib
+import filecmp
 import json
 import os
 import subprocess
@@ -21,15 +21,6 @@ def write_realm_nginx_config_line(f: Any, host: str, port: str) -> None:
     set $tornado_server http://tornado{port};
 }}\n""")
 
-def hash_sharding_config() -> str:
-    config_file = get_config_file()
-    if not config_file.has_section("tornado_sharding"):
-        return hashlib.sha256(b'').hexdigest()
-    contents = subprocess.check_output([
-        "crudini", "--get", "--format=lines", "/etc/zulip/zulip.conf", "tornado_sharding",
-    ])
-    return hashlib.sha256(contents).hexdigest()
-
 # Basic system to do Tornado sharding.  Writes two output .tmp files that need
 # to be renamed to the following files to finalize the changes:
 # * /etc/zulip/nginx_sharding.conf; nginx needs to be reloaded after changing.
@@ -37,29 +28,7 @@ def hash_sharding_config() -> str:
 # after changing.  TODO: We can probably make this live-reload by statting the file.
 #
 # TODO: Restructure this to automatically generate a sharding layout.
-
-parser = argparse.ArgumentParser(description="Adjust Tornado sharding configuration")
-parser.add_argument("--verify", action='store_true',
-                    help="Exits 0 with no action if no changes are required; exits 1 if changes would be made.")
-options = parser.parse_args()
-
-new_hash = hash_sharding_config()
-if os.path.exists('/etc/zulip/nginx_sharding.conf') and os.path.exists('/etc/zulip/sharding.json'):
-    with open('/etc/zulip/nginx_sharding.conf') as old_file:
-        if new_hash in old_file.read():
-            sys.exit(0)
-
-if options.verify:
-    sys.exit(1)
-
-if "SUPPRESS_SHARDING_NOTICE" not in os.environ:
-    print("** Updated sharding; scripts/refresh-sharding-and-restart required")
-
-with open('/etc/zulip/nginx_sharding.conf.tmp', 'w') as nginx_sharding_conf_f, \
-        open('/etc/zulip/sharding.json.tmp', 'w') as sharding_json_f:
-    # Puppet uses this to know if it needs to rewrite the files
-    nginx_sharding_conf_f.write(f"# Configuration hash: {new_hash}\n")
-
+def write_updated_configs() -> None:
     config_file = get_config_file()
     ports = get_tornado_ports(config_file)
 
@@ -67,10 +36,13 @@ with open('/etc/zulip/nginx_sharding.conf.tmp', 'w') as nginx_sharding_conf_f, \
     assert sorted(ports) == expected_ports, \
         f"ports ({sorted(ports)}) must be contiguous, starting with 9800"
 
+    with open('/etc/zulip/nginx_sharding.conf.tmp', 'w') as nginx_sharding_conf_f, \
+            open('/etc/zulip/sharding.json.tmp', 'w') as sharding_json_f:
+
         if len(ports) == 1:
             nginx_sharding_conf_f.write("set $tornado_server http://tornado;\n")
             sharding_json_f.write('{}\n')
-        sys.exit(0)
+            return
 
         nginx_sharding_conf_f.write("set $tornado_server http://tornado9800;\n")
         shard_map: Dict[str, int] = {}
@@ -92,3 +64,39 @@ with open('/etc/zulip/nginx_sharding.conf.tmp', 'w') as nginx_sharding_conf_f, \
             nginx_sharding_conf_f.write('\n')
 
         sharding_json_f.write(json.dumps(shard_map) + '\n')
+
+parser = argparse.ArgumentParser(
+    description="Adjust Tornado sharding configuration",
+)
+parser.add_argument(
+    "--errors-ok", action="store_true",
+    help="Exits 1 if there are no changes; if there are errors or changes, exits 0."
+)
+options = parser.parse_args()
+
+config_file_path = "/etc/zulip"
+base_files = ['nginx_sharding.conf', 'sharding.json']
+full_real_paths = [f"{config_file_path}/{filename}" for filename in base_files]
+full_new_paths = [f"{filename}.tmp" for filename in full_real_paths]
+try:
+    write_updated_configs()
+    for old, new in zip(full_real_paths, full_new_paths):
+        if not filecmp.cmp(old, new):
+            # There are changes; leave .tmp files and exit 0
+            if "SUPPRESS_SHARDING_NOTICE" not in os.environ:
+                print("===> Updated sharding; run scripts/refresh-sharding-and-restart")
+            sys.exit(0)
+    # No changes; clean up and exit 1
+    for filename in full_new_paths:
+        os.unlink(filename)
+    sys.exit(1)
+except AssertionError as e:
+    # Clean up whichever files we made
+    for filename in full_new_paths:
+        if os.path.exists(filename):
+            os.unlink(filename)
+    if options.errors_ok:
+        sys.exit(0)
+    else:
+        print(e, file=sys.stderr)
+        sys.exit(2)