Remove TTL header from cache key (#1048)

See [discussion on Discord](https://discord.com/channels/928484660785336380/928485908842426389/1231791730513412136). Multiple times, app developers have wanted to cache a piece of content until a particular time, rather than for a particular period. That requires setting ttl_seconds to a variable amount, calculated as the time remaining.

Currently, the cache client won't support that. It generates the cache key from the entire request, which includes the TTL seconds in a header. This change removes that header before generating the key. That seems sensible because the key should be about the content and how it would be fetched, but the TTL is about what to do with the response.

As a transitionary step, this writes to new TTL-less keys, but continues to look up keys with TTLs. Once all the old cache entries have expired, then we can stop looking up the old keys.

runtime/httpcache.go

@@ -23,6 +23,7 @@ const (
 	HTTPTimeout      = 5 * time.Second
 	MaxResponseBytes = 20 * 1024 * 1024 // 20MB
 	HTTPCachePrefix  = "httpcache"
+	TTLHeader        = "X-Tidbyt-Cache-Seconds"
 )
 
 // Status codes that are cacheable as defined here:
@@ -106,10 +107,15 @@ func (c *cacheClient) RoundTrip(req *http.Request) (*http.Response, error) {
 }
 
 func cacheKey(req *http.Request) (string, error) {
+	ttl := req.Header.Get(TTLHeader)
+	req.Header.Del(TTLHeader)
 	r, err := httputil.DumpRequest(req, true)
 	if err != nil {
 		return "", fmt.Errorf("%s: %w", "failed to serialize request", err)
 	}
+	if ttl != "" {
+		req.Header.Set(TTLHeader, ttl)
+	}
 
 	h := sha256.Sum256(r)
 	key := hex.EncodeToString(h[:])

runtime/testdata/httpcache.star

@@ -18,7 +18,7 @@ def main(config):
 
     resp = http.get(
         url = "https://example.com",
-        ttl_seconds = 60,
+        ttl_seconds = 3,
     )
     assert.eq(resp.headers.get("Tidbyt-Cache-Status"), "HIT")
 
@@ -32,7 +32,7 @@ def main(config):
         url = "https://example.com",
         ttl_seconds = 60,
     )
-    assert.eq(resp.headers.get("Tidbyt-Cache-Status"), "MISS")
+    assert.eq(resp.headers.get("Tidbyt-Cache-Status"), "HIT")
 
     resp = http.post(
         url = "https://example.com",