gateway/lambda_function: redirect to documentation for crawlers

just surfing to https://rpmrepo.osbuild.org/ what also
crawlers do, should result in something other than a 4xx error.

Makefile

@@ -87,6 +87,7 @@ help:
 	@echo
 	@echo "    help:               Print this usage information."
 	@echo "    snapshot-configs:   Regenerate all snapshot configs from definitions."
+	@echo "    test:               Run unit-tests."
 
 $(BUILDDIR)/:
 	$(BIN_MKDIR) -p "$@"
@@ -124,3 +125,7 @@ gateway-zip: $(BUILDDIR)/gateway/rpmrepo-gateway-main.zip
 snapshot-configs:
 	rm -f $(SRCDIR)/repo/*.json
 	./gen-all-repos.py --definitions $(SRCDIR)/repo-definitions.yaml --output $(SRCDIR)/repo/
+
+.PHONY: test
+test:
+	pytest src/gateway/lambda_function.py

src/gateway/lambda_function.py

@@ -35,6 +35,8 @@
     "rhvpn": "https://rpmrepo-storage.bucket.vpce-08d3201af28373567-o10c2v4q.s3.us-east-1.vpce.amazonaws.com/data/rhvpn",
 }
 
+_documentation_url = "https://osbuild.org/docs/developer-guide/projects/rpmrepo/"
+
 
 def _error(code=500):
     """Synthesize API Error Reply"""
@@ -72,6 +74,9 @@ def _parse_proxy(stage, proxy):
     # arguments (this also means you cannot have trailing slashes for now,
     # but API-Gateway drops those silently, anyway).
 
+    if proxy is None:
+        return None
+
     elements = proxy.split("/")
     if len(elements) < 1:
         return None
@@ -336,6 +341,8 @@ def lambda_handler(event, _context):
     if request is None:
         if proxy == "robots.txt":
             return _success(_robots_txt)
+        if proxy is None or proxy in ["", "/"]:
+            return _redirect(_documentation_url)
         return _error(400)
 
     if "enumerate" in request:
@@ -691,3 +698,18 @@ def test_psi():
     )
     assert r["statusCode"] == 301
     assert r["headers"]["Location"] == "https://rhos-d.infra.prod.upshift.rdu2.redhat.com:13808/v1/AUTH_95e858620fb34bcc9162d9f52367a560/manifestdb/rpmrepo/rpm/a/Packages/c/d"
+
+
+def test_basics():
+    """Tests for the basic redirects"""
+    documentation_events = [{"pathParameters": {"proxy": "/"}},
+                            {"pathParameters": {"proxy": ""}},
+                            {}]
+
+    for event in documentation_events:
+        r = lambda_handler(event
+                           ,
+                           None,
+                           )
+        assert r["statusCode"] == 301
+        assert r["headers"]["Location"] == _documentation_url