fix(cody): use client-provided timeout for completions requests (#63875)

Closes
[CODY-2775](https://linear.app/sourcegraph/issue/CODY-2775/%5Bautocomplete-latency%5D-apply-the-same-timeout-on-the-cody-gateway-side)



Enables client control over the request processing timeout on the server
(both Sourcegraph backend and Cody Gateway). The context timeout is set
to the value provided in the `X-Timeout-Ms` header of the client
request. If the header is not provided, the default context timeout is
used (1 minute on both Sourcegraph backend and Cody Gateway).

Previously, we only had a default timeout on the Sourcegraph backend
side (8 minutes).

Corresponding client change:
- https://github.com/sourcegraph/cody/pull/4921

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## Test plan
- Manually tested and confirmed that if the request contains the
`X-Timeout-Ms` header, its value is used. If not, the default maximum
request duration is applied.
- CI
- 
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https://docs-legacy.sourcegraph.com/dev/background-information/testing_principles
-->


## Changelog
- Use the provided timeout from request parameters if available;
otherwise use the default maximum request duration (8 minutes)

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cmd/cody-gateway/internal/httpapi/completions/upstream.go

@@ -116,6 +116,10 @@ type UpstreamRequest interface {
 	BuildPrompt() string
 }
 
+// maxRequestDuration is the maximum amount of time a request can take before
+// being cancelled as DeadlineExceeded.
+const maxRequestDuration = 1 * time.Minute
+
 type UpstreamHandlerConfig struct {
 	// defaultRetryAfterSeconds sets the retry-after policy on upstream rate
 	// limit events in case a retry-after is not provided by the upstream
@@ -161,7 +165,20 @@ func makeUpstreamHandler[ReqT UpstreamRequest](
 	// in order to call the upstream API. e.g. calling the upstreamHandlerMethods in
 	// the correct order, enforcing rate limits and anti-abuse mechanisms, etc.
 	upstreamHandler := func(w http.ResponseWriter, downstreamRequest *http.Request) {
-		ctx := downstreamRequest.Context()
+
+		// Set the context timeout: use the timeout from the request header if provided,
+		// otherwise use the default maximum request duration.
+		ctxTimeout := maxRequestDuration
+		if v := downstreamRequest.Header.Get("X-Timeout-Ms"); v != "" {
+			if t, err := strconv.Atoi(v); err != nil {
+				baseLogger.Warn("error parsing X-Timeout-Ms header", log.Error(err))
+			} else {
+				ctxTimeout = time.Duration(t) * time.Millisecond
+			}
+		}
+		ctx, cancel := context.WithTimeout(downstreamRequest.Context(), ctxTimeout)
+		defer cancel()
+
 		act := actor.FromContext(ctx)
 
 		// TODO: Investigate using actor propagation handler for extracting
@@ -395,7 +412,9 @@ func makeUpstreamHandler[ReqT UpstreamRequest](
 				logger.Error("failed to log event", log.Error(err))
 			}
 		}()
+
 		resp, err := httpClient.Do(upstreamRequest)
+
 		if err != nil {
 			// Ignore reporting errors where client disconnected
 			if upstreamRequest.Context().Err() == context.Canceled && errors.Is(err, context.Canceled) {

cmd/frontend/internal/httpapi/completions/handler.go

@@ -40,7 +40,7 @@ import (
 
 // maxRequestDuration is the maximum amount of time a request can take before
 // being cancelled as DeadlineExceeded.
-const maxRequestDuration = 8 * time.Minute
+const maxRequestDuration = 1 * time.Minute
 
 var timeToFirstEventMetrics = metrics.NewREDMetrics(
 	prometheus.DefaultRegisterer,
@@ -67,7 +67,17 @@ func newCompletionsHandler(
 			return
 		}
 
-		ctx, cancel := context.WithTimeout(r.Context(), maxRequestDuration)
+		// Set the context timeout: use the timeout from the request header if provided,
+		// otherwise use the default maximum request duration.
+		ctxTimeout := maxRequestDuration
+		if v := r.Header.Get("X-Timeout-Ms"); v != "" {
+			if t, err := strconv.Atoi(v); err != nil {
+				logger.Warn("error parsing X-Timeout-Ms header", log.Error(err))
+			} else {
+				ctxTimeout = time.Duration(t) * time.Millisecond
+			}
+		}
+		ctx, cancel := context.WithTimeout(r.Context(), ctxTimeout)
 		defer cancel()
 
 		// First check that Cody is enabled for this Sourcegraph instance.