Fix negative revalidating for zero-length objects

src/iocore/cache/CacheRead.cc

@@ -1105,6 +1105,10 @@ CacheVC::openReadStartHead(int event, Event *e)
     }
     // the first fragment might have been gc'ed. Make sure the first
     // fragment is there before returning CACHE_EVENT_OPEN_READ
+    // For zero-length documents, don't attempt to read additional fragments.
+    if (doc_len == 0 && !f.single_fragment) {
+      f.single_fragment = true;
+    }
     if (!f.single_fragment) {
       goto Learliest;
     }

tests/gold_tests/cache/replay/negative-revalidating-enabled.replay.yaml

@@ -81,6 +81,29 @@ sessions:
     proxy-response:
       status: 200
 
+  # Also populate cache with a zero-length response (for issue #6649).
+  - client-request:
+      method: "GET"
+      version: "1.1"
+      scheme: "http"
+      url: /path/zero_length_item
+      headers:
+        fields:
+        - [ Host, example.com ]
+        - [ uuid, 31 ]
+
+    # Populate the cache with a 200 response with zero-length content.
+    server-response:
+      status: 200
+      reason: "OK"
+      headers:
+        fields:
+        - [ Content-Length, 0 ]
+        - [ Cache-Control, max-age=2 ]
+
+    proxy-response:
+      status: 200
+
   # Verify we serve the 200 OK out of the cache if it is not stale.
   - client-request:
       method: "GET"
@@ -107,6 +130,29 @@ sessions:
     proxy-response:
       status: 200
 
+  # Verify zero-length is also served from cache when fresh.
+  - client-request:
+      method: "GET"
+      version: "1.1"
+      scheme: "http"
+      url: /path/zero_length_item
+      headers:
+        fields:
+        - [ Host, example.com ]
+        - [ uuid, 32 ]
+
+    # This should not reach the origin server.
+    server-response:
+      status: 503
+      reason: "Service Unavailable"
+      headers:
+        fields:
+        - [ Content-Length, 0 ]
+
+    # Again, we should serve this out of the cache.
+    proxy-response:
+      status: 200
+
   # Verify that with negative_revalidating enabled, we serve the 200 OK out of
   # the cache even though it is stale (but younger than max-age + max_stale_age).
   - client-request:
@@ -134,6 +180,54 @@ sessions:
     proxy-response:
       status: 200
 
+  # Test zero-length while stale - first request (regression test for issue #6649).
+  - client-request:
+      method: "GET"
+      version: "1.1"
+      scheme: "http"
+      url: /path/zero_length_item
+      headers:
+        fields:
+        - [ Host, example.com ]
+        - [ uuid, 33 ]
+
+    server-response:
+      status: 503
+      reason: "Service Unavailable"
+      headers:
+        fields:
+        - [ Content-Length, 0 ]
+
+    # With negative_revalidating enabled, the cached response should be served
+    # even though it is stale.
+    proxy-response:
+      status: 200
+
+  # Make a second consecutive stale request for zero-length to verify the bug scenario.
+  # Per issue #6649: "only the first get on a 0 length cached object comes back
+  # as stale and the second succeeds" (meaning the second might fail).
+  - client-request:
+      method: "GET"
+      version: "1.1"
+      scheme: "http"
+      url: /path/zero_length_item
+      headers:
+        fields:
+        - [ Host, example.com ]
+        - [ uuid, 34 ]
+
+    server-response:
+      status: 503
+      reason: "Service Unavailable"
+      headers:
+        fields:
+        - [ Content-Length, 0 ]
+
+    # This second consecutive stale request should also be served from cache.
+    # This is where the #6649 bug manifests - without the fix, it returns 503.
+    proxy-response:
+      status: 200
+
   # Verify that max_stale_age is respected.
   - client-request:
       method: "GET"
@@ -161,6 +255,29 @@ sessions:
     proxy-response:
       status: 503
 
+  # Verify max_stale_age is also respected for zero-length responses.
+  - client-request:
+      method: "GET"
+      version: "1.1"
+      scheme: "http"
+      url: /path/zero_length_item
+      headers:
+        fields:
+        - [ Host, example.com ]
+        - [ uuid, 35 ]
+
+    server-response:
+      status: 503
+      reason: "Service Unavailable"
+      headers:
+        fields:
+        - [ Content-Length, 0 ]
+
+    # negative_revalidating is enabled, but now the cached item is older than
+    # max_stale_age.
+    proxy-response:
+      status: 503
+
   #
   # Test 2: Negative revalidating for a cached chunk encoded response.
   #