From 07c4480e3ef7e3cb850aeffe4bc6c7d374952330 Mon Sep 17 00:00:00 2001
From: Damian Gryski <damian@gryski.com>
Date: Wed, 1 Apr 2026 16:46:25 -0700
Subject: [PATCH] internal,fsthttp: add backend to HTTPCacheTransactionLookup()

---
 fsthttp/request.go                       | 2 ++
 internal/abi/fastly/hostcalls_noguest.go | 4 ++++
 internal/abi/fastly/httpcache_guest.go   | 8 ++++++++
 internal/abi/fastly/types.go             | 3 +++
 4 files changed, 17 insertions(+)

diff --git a/fsthttp/request.go b/fsthttp/request.go
index 2c4908a..c59c13e 100644
--- a/fsthttp/request.go
+++ b/fsthttp/request.go
@@ -624,6 +624,8 @@ func (req *Request) sendWithGuestCache(ctx context.Context, backend string) (*Re
 		req.CacheOptions.OverrideKey = ""
 	}
 
+	options.Backend(backend)
+
 	// force the lookup to await in the host, retrieving any errors synchronously
 	cacheHandle, err := fastly.HTTPCacheTransactionLookup(req.abi.req, &options)
 	if err != nil {
diff --git a/internal/abi/fastly/hostcalls_noguest.go b/internal/abi/fastly/hostcalls_noguest.go
index 988984e..b0a6ba1 100644
--- a/internal/abi/fastly/hostcalls_noguest.go
+++ b/internal/abi/fastly/hostcalls_noguest.go
@@ -720,6 +720,10 @@ type HTTPCacheLookupOptions struct{}
 func (HTTPCacheLookupOptions) OverrideKey(key string) {
 }
 
+func (HTTPCacheLookupOptions) Backend(backend string) {
+}
+
+
 func HTTPCacheIsRequestCacheable(req *HTTPRequest) (bool, error) {
 	return false, fmt.Errorf("not implemented")
 }
diff --git a/internal/abi/fastly/httpcache_guest.go b/internal/abi/fastly/httpcache_guest.go
index 71d7fc4..18162c1 100644
--- a/internal/abi/fastly/httpcache_guest.go
+++ b/internal/abi/fastly/httpcache_guest.go
@@ -21,6 +21,14 @@ func (o *HTTPCacheLookupOptions) OverrideKey(key string) {
 	o.mask |= httpCacheLookupOptionsFlagOverrideKey
 }
 
+func (o *HTTPCacheLookupOptions) Backend(backend string) {
+	b := []byte(backend)
+	buf := prim.NewReadBufferFromBytes(b)
+	o.opts.backendPtr = prim.ToPointer(buf.Char8Pointer())
+	o.opts.backendLen = buf.Len()
+	o.mask |= httpCacheLookupOptionsFlagBackend
+}
+
 type HTTPCacheWriteOptions struct {
 	mask httpCacheWriteOptionsMask
 	opts httpCacheWriteOptions
diff --git a/internal/abi/fastly/types.go b/internal/abi/fastly/types.go
index e30d54b..06825c2 100644
--- a/internal/abi/fastly/types.go
+++ b/internal/abi/fastly/types.go
@@ -1824,6 +1824,8 @@ const (
 type httpCacheLookupOptions struct {
 	overrideKeyPtr prim.Pointer[prim.Char8]
 	overrideKeyLen prim.Usize
+	backendPtr     prim.Pointer[prim.Char8]
+	backendLen     prim.Usize
 }
 
 type httpCacheLookupOptionsMask prim.U32
@@ -1831,6 +1833,7 @@ type httpCacheLookupOptionsMask prim.U32
 const (
 	httpCacheLookupOptionsFlagReserved    httpCacheLookupOptionsMask = 1 << 0
 	httpCacheLookupOptionsFlagOverrideKey httpCacheLookupOptionsMask = 1 << 1
+	httpCacheLookupOptionsFlagBackend     httpCacheLookupOptionsMask = 1 << 2
 )
 
 type (