Add stub functions for fastly purge

lib/compute-at-edge-abi/compute-at-edge.witx

@@ -486,3 +486,12 @@
         (result $err (expected (error $fastly_status)))
     )
 )
+
+(module $fastly_purge
+    (@interface func (export "purge_surrogate_key")
+        (param $surrogate_key string)
+        (param $options_mask $purge_options_mask)
+        (param $options (@witx pointer $purge_options))
+        (result $err (expected (error $fastly_status)))
+    )
+)

lib/compute-at-edge-abi/typenames.witx

@@ -162,3 +162,19 @@
     (field $sni_hostname (@witx pointer (@witx char8)))
     (field $sni_hostname_len u32)
     ))
+
+(typename $purge_options_mask
+    (flags (@witx repr u32)
+        $soft_purge
+        $ret_buf ;; all ret_buf fields must be populated
+    )
+)
+
+(typename $purge_options
+    (record
+        ;; JSON purge response as in https://developer.fastly.com/reference/api/purging/#purge-tag
+        (field $ret_buf_ptr (@witx pointer u8))
+        (field $ret_buf_len (@witx usize))
+        (field $ret_buf_nwritten_out (@witx pointer (@witx usize)))
+    )
+)

lib/src/linking.rs

@@ -88,6 +88,7 @@ pub fn link_host_functions(linker: &mut Linker<WasmCtx>) -> Result<(), Error> {
     wiggle_abi::fastly_http_resp::add_to_linker(linker, WasmCtx::session)?;
     wiggle_abi::fastly_log::add_to_linker(linker, WasmCtx::session)?;
     wiggle_abi::fastly_object_store::add_to_linker(linker, WasmCtx::session)?;
+    wiggle_abi::fastly_purge::add_to_linker(linker, WasmCtx::session)?;
     wiggle_abi::fastly_uap::add_to_linker(linker, WasmCtx::session)?;
     link_legacy_aliases(linker)?;
     Ok(())

lib/src/wiggle_abi.rs

@@ -48,6 +48,7 @@ macro_rules! multi_value_result {
 mod body_impl;
 mod dictionary_impl;
 mod entity;
+mod fastly_purge_impl;
 mod geo_impl;
 mod headers;
 mod log_impl;

lib/src/wiggle_abi/fastly_purge_impl.rs

@@ -0,0 +1,19 @@
+//! fastly_purge` hostcall implementations.
+
+use {
+    super::types::{PurgeOptions, PurgeOptionsMask},
+    crate::{error::Error, session::Session, wiggle_abi::fastly_purge::FastlyPurge},
+    wiggle::GuestPtr,
+};
+
+impl FastlyPurge for Session {
+    #[allow(unused_variables)] // FIXME FDE 2022-09-26: Remove this directive once implemented.
+    fn purge_surrogate_key<'a>(
+        &mut self,
+        surrogate_key: &GuestPtr<'a, str>,
+        options_mask: PurgeOptionsMask,
+        options: &GuestPtr<'a, PurgeOptions<'a>>,
+    ) -> Result<(), Error> {
+        Err(Error::NotAvailable("FastlyPurge"))
+    }
+}