log chat sessions to database

server/routers/aiGateway/streamAiGatewayResponse.ts

@@ -0,0 +1,86 @@
+import { Response } from "express";
+import { stripInjectedUsageFrame } from "@server/lib/aiUsageExtraction";
+
+/**
+ * Reads an upstream AI provider response, writes it through to the client
+ * (streaming or buffered), and returns the full response text once done, so
+ * the caller can extract usage/cost and log the completed session. Shared by
+ * both the direct-upstream path (pipeline.ts) and the "custom"/target
+ * routing-mode path (targetRouting.ts) so usage/cost tracking and session
+ * logging apply identically to both instead of each maintaining its own copy
+ * of this loop.
+ *
+ * Callers own fetching the upstream response and the AbortController/
+ * `res.on("close", onClientClose)` wiring, since those differ meaningfully
+ * between the two transports (direct upstream fetch with TLS-skip support vs
+ * a plain fetch to gerbil) - only the "read the stream, write to the client,
+ * accumulate the full text" part is actually identical logic between them.
+ */
+export async function streamAiGatewayResponse(args: {
+    res: Response;
+    upstreamRes: globalThis.Response;
+    isStream: boolean;
+    // True when we injected stream_options.include_usage ourselves (the
+    // caller didn't ask for it) and need to strip the extra usage-only frame
+    // back out of what's forwarded to the client.
+    injectedUsageOurselves: boolean;
+    abortController: AbortController;
+    onClientClose: () => void;
+}): Promise<{ fullText: string; aborted: boolean }> {
+    const {
+        res,
+        upstreamRes,
+        isStream,
+        injectedUsageOurselves,
+        abortController,
+        onClientClose
+    } = args;
+
+    const contentType = upstreamRes.headers.get("content-type") || "";
+    res.status(upstreamRes.status);
+    res.setHeader("Content-Type", contentType || "application/json");
+
+    if (isStream && upstreamRes.body) {
+        res.flushHeaders();
+        const reader = upstreamRes.body.getReader();
+        const decoder = new TextDecoder();
+        let fullText = "";
+        // Frame-boundary buffer, only used when we need to filter the
+        // usage-only frame we injected out of what reaches the client.
+        let sseCarry = "";
+        try {
+            while (!abortController.signal.aborted) {
+                const { done, value } = await reader.read();
+                if (done) break;
+                const chunkText = decoder.decode(value, { stream: true });
+                fullText += chunkText;
+                if (injectedUsageOurselves) {
+                    sseCarry += chunkText;
+                    const lastBoundary = sseCarry.lastIndexOf("\n\n");
+                    if (lastBoundary !== -1) {
+                        const toEmit = sseCarry.slice(0, lastBoundary + 2);
+                        sseCarry = sseCarry.slice(lastBoundary + 2);
+                        res.write(stripInjectedUsageFrame(toEmit));
+                    }
+                } else {
+                    res.write(value);
+                }
+            }
+            if (injectedUsageOurselves && sseCarry) {
+                res.write(stripInjectedUsageFrame(sseCarry));
+            }
+        } finally {
+            await reader.cancel().catch(() => {});
+            res.off("close", onClientClose);
+        }
+        if (!res.writableEnded) {
+            res.end();
+        }
+        return { fullText, aborted: abortController.signal.aborted };
+    }
+
+    res.off("close", onClientClose);
+    const text = await upstreamRes.text();
+    res.send(text);
+    return { fullText: text, aborted: abortController.signal.aborted };
+}