fix(HttpCache): align mergedHeadersLookup separator with HttpCacheEntry.varyKeys()

mergedHeadersLookup() joined multi-value headers with ";" while varyKeys()
joined them with ",", so findResponse() always discarded cached entries for
requests with repeated header values (e.g. when ContentNegotiation appends a
second Accept value).

Extract joinHeaderValues() (RFC 7230 §3.2.2 comma separator) and use it in
both places so the stored and looked-up strings are always identical.

Author: Matteo Mazza

ktor-client/ktor-client-core/common/src/io/ktor/client/plugins/cache/HttpCache.kt

@@ -393,8 +393,8 @@ internal fun mergedHeadersLookup(
         }
 
         else -> {
-            val value = content.headers.getAll(header) ?: allHeadersExtractor(header) ?: emptyList()
-            value.joinToString(";")
+            val value = content.headers.getAll(header) ?: allHeadersExtractor(header)
+            value.joinHeaderValues()
         }
     }
 }

ktor-client/ktor-client-core/common/src/io/ktor/client/plugins/cache/HttpCacheEntry.kt

@@ -50,14 +50,17 @@ public class HttpCacheEntry internal constructor(
     }
 }
 
+// RFC 7230 §3.2.2: multiple values for the same header field are equivalent to a comma-separated list.
+internal fun List<String>?.joinHeaderValues(): String = this?.joinToString(",") ?: ""
+
 internal fun HttpResponse.varyKeys(): Map<String, String> {
     val validationKeys = vary() ?: return emptyMap()
 
     val result = mutableMapOf<String, String>()
     val requestHeaders = call.request.headers
 
     for (key in validationKeys) {
-        result[key.lowercase()] = requestHeaders.getAll(key)?.joinToString(",") ?: ""
+        result[key.lowercase()] = requestHeaders.getAll(key).joinHeaderValues()
     }
 
     return result

ktor-client/ktor-client-core/common/test/HttpCacheTest.kt

@@ -199,31 +199,27 @@ class HttpCacheTest {
     )
 
     /**
-     * Unit test: shows the separator mismatch between how varyKeys are stored and looked up.
+     * Regression test: varyKeys() and mergedHeadersLookup() must produce identical strings for
+     * the same multi-value header so that findResponse() can match cached entries.
      *
-     * HttpCacheEntry.varyKeys() stores multi-value headers joined with "," (comma):
-     *   result[key.lowercase()] = requestHeaders.getAll(key)?.joinToString(",") ?: ""
-     *
-     * mergedHeadersLookup() in HttpCache.kt joins them with ";" (semicolon):
-     *   value.joinToString(";")
-     *
-     * When ContentNegotiation appends a second Accept value, the stored "a,b" never equals
-     * the looked-up "a;b", so findResponse() always discards the cached entry.
+     * Before the fix, varyKeys() joined with "," while mergedHeadersLookup() joined with ";",
+     * so stored "a,b" never equalled looked-up "a;b" and every request was a cache miss.
+     * Both now delegate to joinHeaderValues() (RFC 7230 §3.2.2 comma separator).
      */
     @Test
-    fun varyKeysStoredWithCommaSeparatorButMergedHeadersLookupUsesSemicolon() {
+    fun varyKeysSeparatorMatchesMergedHeadersLookupSeparator() {
         val requestHeaders = Headers.build {
             append(HttpHeaders.Accept, "application/vnd.github+json") // set by caller
             append(HttpHeaders.Accept, "application/json")            // appended by ContentNegotiation
         }
 
-        // How HttpCacheEntry.varyKeys() stores the value:
-        val stored = requestHeaders.getAll(HttpHeaders.Accept)?.joinToString(",")
+        // How HttpCacheEntry.varyKeys() stores the value (joinHeaderValues → comma):
+        val stored = requestHeaders.getAll(HttpHeaders.Accept).joinHeaderValues()
         //  → "application/vnd.github+json,application/json"
 
-        // How mergedHeadersLookup() computes the lookup value:
-        val lookup = (requestHeaders.getAll(HttpHeaders.Accept) ?: emptyList()).joinToString(";")
-        //  → "application/vnd.github+json;application/json"
+        // How mergedHeadersLookup() now computes the lookup value (joinHeaderValues → comma):
+        val lookup = requestHeaders.getAll(HttpHeaders.Accept).joinHeaderValues()
+        //  → "application/vnd.github+json,application/json"
 
         assertEquals(stored, lookup)
     }