Fix: HTTPHdr host cache invalidation when Host header modified

include/proxy/hdrs/HTTP.h

@@ -444,13 +444,22 @@ bool is_http1_hdr_version_supported(const HTTPVersion &http_version);
 
 class IOBufferReader;
 
+/** HTTP Header class.
+ *
+ * @warning Changing the size of this class (adding/removing fields) will change
+ * the on-disk cache format and cause cache incompatibility. The HTTPCacheAlt
+ * structure contains embedded HTTPHdr objects, and the cache marshalling code
+ * uses sizeof(HTTPCacheAlt) to read/write cache entries. Any size change will
+ * cause "vector inconsistency" errors when reading cache entries written by a
+ * different version.
+ */
 class HTTPHdr : public MIMEHdr
 {
 public:
   HTTPHdrImpl       *m_http = nullptr;
   mutable URL        m_url_cached;
   mutable MIMEField *m_host_mime             = nullptr;
-  mutable int        m_host_length           = 0;     ///< Length of hostname.
+  mutable int        m_host_length           = 0;     ///< Length of hostname (parsed, excludes port).
   mutable int        m_port                  = 0;     ///< Target port.
   mutable bool       m_target_cached         = false; ///< Whether host name and port are cached.
   mutable bool       m_target_in_url         = false; ///< Whether host name and port are in the URL.
@@ -732,8 +741,34 @@ HTTPHdr::print(char *buf, int bufsize, int *bufindex, int *dumpoffset) const
 inline void
 HTTPHdr::_test_and_fill_target_cache() const
 {
-  if (!m_target_cached)
+  if (!m_target_cached) {
     this->_fill_target_cache();
+    return;
+  }
+
+  // If host came from the Host header (not URL), check for staleness by verifying
+  // the current Host header value length matches what we expect from cached values.
+  if (!m_target_in_url && m_host_mime != nullptr) {
+    int expected_len = m_host_length;
+    if (m_port_in_header && m_port > 0) {
+      // Account for ":port" suffix in the raw Host header value.
+      expected_len += 1; // colon
+      if (m_port < 10) {
+        expected_len += 1;
+      } else if (m_port < 100) {
+        expected_len += 2;
+      } else if (m_port < 1000) {
+        expected_len += 3;
+      } else if (m_port < 10000) {
+        expected_len += 4;
+      } else {
+        expected_len += 5;
+      }
+    }
+    if (m_host_mime->m_len_value != expected_len) {
+      this->_fill_target_cache();
+    }
+  }
 }
 
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