Introduce mirrored reads [RHELDST-28332]

This commit modifies exodus-cdn's lookup behavior to enable mirrored
reads. This means, that when exodus-cdn looks up content to be served
for a path with a $releasever alias in effect, exodus-cdn attempts to
look up content on both sides of the alias. (Previously, exodus-cdn
only performed a content lookup on the destination side.)

The mirrored reads are enabled by default, but they can be disabled
by setting the new config option "disable_mirrored_reads" to true
(1, '1', or 'True', where 'True' is not case sensitive.)

configuration/lambda_config.template

@@ -17,6 +17,7 @@
   "headers": {
     "max_age": $EXODUS_HEADERS_MAX_AGE
   },
+  "disable_mirrored_reads": "$EXODUS_LAMBDA_DISABLE_MIRRORED_READS",
   "lambda_version": "$EXODUS_LAMBDA_VERSION",
   "index_filename": "$EXODUS_INDEX_FILENAME",
   "logging": {

exodus_lambda/functions/base.py

@@ -59,6 +59,12 @@ def lambda_version(self):
     def index(self):
         return self.conf["index_filename"]
 
+    @property
+    def disable_mirrored_reads(self):
+        if str(self.conf.get("disable_mirrored_reads", "false")).lower() in ('1', 'true'):
+            return True
+        return False
+
     def set_lambda_version(self, response):
         response.setdefault("headers", {})["x-exodus-version"] = [
             {"key": "X-Exodus-Version", "value": self.lambda_version}

exodus_lambda/functions/origin_request.py

@@ -126,12 +126,14 @@ def uri_alias(self, uri, aliases, ignore_exclusions=False):
 
         return uri
 
-    def resolve_aliases(self, uri, ignore_exclusions=False):
+    def resolve_aliases(
+        self, uri, ignore_exclusions=False, ignore_releasever=False
+    ):
         # aliases relating to origin, e.g. content/origin <=> origin
-
         uri = self.uri_alias(
             uri, self.definitions.get("origin_alias"), ignore_exclusions
         )
+
         # aliases relating to rhui; listing files are a special exemption
         # because they must be allowed to differ for rhui vs non-rhui.
         if not uri.endswith("/listing"):
@@ -140,9 +142,12 @@ def resolve_aliases(self, uri, ignore_exclusions=False):
             )
 
         # aliases relating to releasever; e.g. /content/dist/rhel8/8 <=> /content/dist/rhel8/8.5
-        uri = self.uri_alias(
-            uri, self.definitions.get("releasever_alias"), ignore_exclusions
-        )
+        if not ignore_releasever:
+            uri = self.uri_alias(
+                uri,
+                self.definitions.get("releasever_alias"),
+                ignore_exclusions,
+            )
 
         self.logger.debug("Resolved request URI: %s", uri)
 
@@ -393,6 +398,22 @@ def handler(self, event, context):
         if preferred_uri != fallback_uri:
             uris.append(fallback_uri)
 
+        # When exodus-cdn is looking up content to be served for a path having a
+        # $releasever alias in effect, it should attempt to look up content on
+        # both sides of the alias.
+        if not self.disable_mirrored_reads:
+            # Attempt to look up the original path.
+            # Note: Only the releasever alias is left unresolved. Other alias types
+            # (rhui and origin aliases) are resolved.
+            for ignore_exclusions in (False, True):
+                mirrored_uri = self.resolve_aliases(
+                    request["uri"],
+                    ignore_exclusions=ignore_exclusions,
+                    ignore_releasever=True,
+                )
+                if mirrored_uri not in uris:
+                    uris.append(mirrored_uri)
+
         for uri in uris:
             if listing_response := self.handle_listing_request(uri):
                 self.set_cache_control(uri, listing_response)

scripts/mk-config

@@ -12,6 +12,7 @@ export ENV_TYPE=${ENV_TYPE:-dev}
 export EXODUS_TABLE=${EXODUS_TABLE:-$PROJECT-cdn-$ENV_TYPE}
 export EXODUS_CONFIG_TABLE=${EXODUS_CONFIG_TABLE:-$PROJECT-config-$ENV_TYPE}
 export EXODUS_INDEX_FILENAME=${EXODUS_INDEX_FILENAME:-.__exodus_autoindex}
+export EXODUS_DISABLE_MIRRORED_READS=${EXODUS_DISABLE_MIRRORED_READS:-false}
 
 REVISION="${CODEBUILD_RESOLVED_SOURCE_VERSION:-$(git rev-parse HEAD)}"
 export EXODUS_LAMBDA_VERSION="${EXODUS_LAMBDA_VERSION:-$(date -u --iso=s) ${REVISION}}"

tests/conftest.py

@@ -86,6 +86,7 @@ def mock_conf_file():
     test_env["ORIGIN_REQUEST_LOGGER_LEVEL"] = "DEBUG"
     test_env["EXODUS_LAMBDA_VERSION"] = "fake version"
     test_env["EXODUS_INDEX_FILENAME"] = ".__exodus_autoindex"
+    test_env["EXODUS_DISABLE_MIRRORED_READS"] = "false"
 
     subprocess.run(
         ["envsubst"],

tests/functions/test_base.py

@@ -133,3 +133,30 @@ def test_json_logger_configurable_datefmt(caplog):
             "response": None,
         },
     ]
+
+
+@pytest.mark.parametrize(
+    "config_value,disabled",
+    [
+        ("True", True),
+        ("true", True),
+        (1, True),
+        ("1", True),
+        ("", False),
+        ("false", False),
+    ],
+    ids=[
+        "mixed case true",
+        "lowercase true",
+        "int 1",
+        "string 1",
+        "empty string",
+        "false string",
+    ],
+)
+def test_disable_reads(config_value, disabled):
+    """Verify that the disable_mirrored_reads config value produces the expected
+    disable_mirrored_reads property."""
+    conf = copy.deepcopy(TEST_CONF)
+    conf["disable_mirrored_reads"] = config_value
+    assert LambdaBase(conf_file=conf).disable_mirrored_reads == disabled