Preserve partial-response byte ranges

docs/release-control/v6/internal/subsystems/performance-and-scalability.md

@@ -204,9 +204,11 @@ change may globally weaken the Task 03 lifecycle-state idempotency invariant.
    exclusions, and bypass token-valid WebSocket upgrades before wrapping the
    response writer. Informational responses must leave negotiation open for
    the final response, body-forbidden statuses must not gain compression
-   framing, and once a body-permitted successful response advertises gzip, an
-   explicit empty body must still emit a valid empty gzip member. Regression
-   proof lives in `pkg/server/gzip_middleware_test.go`.
+   framing, and partial responses must preserve the identity representation
+   whose byte positions `Content-Range` describes. Once a body-permitted full
+   response advertises gzip, an explicit empty body must still emit a valid
+   empty gzip member. Regression proof lives in
+   `pkg/server/gzip_middleware_test.go`.
    WebSocket state refreshes on monitor, agent-report, and alert mutation hot
    paths must enqueue coalesced current-state invalidations instead of building
    full frontend-state payloads at every signal. The hub owns delayed
@@ -1016,9 +1018,10 @@ supports wildcard acceptance, applies strict three-digit qvalue parsing, and
 lets explicit gzip exclusions override wildcards. WebSocket token lists and
 range requests bypass the wrapper. Explicit empty 200 responses carry a valid
 empty gzip stream whenever `Content-Encoding: gzip` is committed, while
-informational responses defer negotiation to the final response and 204/205/304,
-SSE, already encoded, small declared, and binary responses stay on their
-identity paths.
+informational responses defer negotiation to the final response and
+204/205/206/304, SSE, already encoded, small declared, and binary responses
+stay on their identity paths. Partial content retains the identity bytes that
+its `Content-Range` describes.
 
 ### Libvirt workload projection stays bounded and allocation-local
 

pkg/server/gzip_middleware.go

@@ -180,7 +180,8 @@ func (w *gzipResponseWriter) decide(status int) {
 	header := w.Header()
 	header.Add("Vary", "Accept-Encoding")
 
-	if status == http.StatusNoContent || status == http.StatusResetContent || status == http.StatusNotModified {
+	switch status {
+	case http.StatusNoContent, http.StatusResetContent, http.StatusPartialContent, http.StatusNotModified:
 		return
 	}
 	if header.Get("Content-Encoding") != "" {

pkg/server/gzip_middleware_test.go

@@ -358,6 +358,34 @@ func TestWithGzipPreservesStatusAndEmptyBodies(t *testing.T) {
 	}
 }
 
+func TestWithGzipPreservesPartialContentRepresentation(t *testing.T) {
+	payload := strings.Repeat("x", 1024)
+	handler := withGzip(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Header().Set("Content-Range", "bytes 0-1023/2048")
+		w.WriteHeader(http.StatusPartialContent)
+		_, _ = w.Write([]byte(payload))
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/export?segment=first", nil)
+	req.Header.Set("Accept-Encoding", "gzip")
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusPartialContent {
+		t.Fatalf("status = %d, want 206", rec.Code)
+	}
+	if got := rec.Header().Get("Content-Encoding"); got != "" {
+		t.Fatalf("Content-Encoding = %q, want identity for Content-Range semantics", got)
+	}
+	if got := rec.Header().Get("Content-Range"); got != "bytes 0-1023/2048" {
+		t.Fatalf("Content-Range = %q, want original byte range", got)
+	}
+	if rec.Body.String() != payload {
+		t.Fatalf("partial response body was transformed")
+	}
+}
+
 func TestWithGzipDefersDecisionPastInformationalResponse(t *testing.T) {
 	payload := strings.Repeat(`{"status":"ready"}`, 256)
 	handler := withGzip(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {