libstore: remove FiletransferRequest::head

add a method to FileTransfer that provides this functionality instead.

Change-Id: Ic1933a5df76a109c248c9c5efea065356b20a6f9

src/libstore/filetransfer.cc

@@ -48,6 +48,7 @@ struct curlFileTransfer : public FileTransfer
         FileTransferResult result;
         Activity act;
         std::optional<std::string> uploadData;
+        bool noBody = false; // \equiv HTTP HEAD, don't download data
         bool done = false; // whether either the success or failure function has been called
         std::packaged_task<FileTransferResult(std::exception_ptr, FileTransferResult)> callback;
         std::function<void(TransferItem &, std::string_view data)> dataCallback;
@@ -92,7 +93,8 @@ struct curlFileTransfer : public FileTransfer
             ActivityId parentAct,
             std::invocable<std::exception_ptr> auto callback,
             std::function<void(TransferItem &, std::string_view data)> dataCallback,
-            std::optional<std::string> uploadData
+            std::optional<std::string> uploadData,
+            bool noBody
         )
             : fileTransfer(fileTransfer)
             , request(request)
@@ -100,6 +102,7 @@ struct curlFileTransfer : public FileTransfer
                 fmt(uploadData ? "uploading '%s'" : "downloading '%s'", request.uri),
                 {request.uri}, parentAct)
             , uploadData(std::move(uploadData))
+            , noBody(noBody)
             , callback([cb{std::move(callback)}] (std::exception_ptr ex, FileTransferResult r) {
                 cb(ex);
                 return r;
@@ -299,7 +302,7 @@ struct curlFileTransfer : public FileTransfer
             if (settings.downloadSpeed.get() > 0)
                 curl_easy_setopt(req, CURLOPT_MAX_RECV_SPEED_LARGE, (curl_off_t) (settings.downloadSpeed.get() * 1024));
 
-            if (request.head)
+            if (noBody)
                 curl_easy_setopt(req, CURLOPT_NOBODY, 1);
 
             if (uploadData) {
@@ -661,17 +664,18 @@ struct curlFileTransfer : public FileTransfer
 
     std::future<FileTransferResult> enqueueDownload(const FileTransferRequest & request) override
     {
-        return enqueueFileTransfer(request, std::nullopt);
+        return enqueueFileTransfer(request, std::nullopt, false);
     }
 
     std::future<FileTransferResult>
     enqueueUpload(const FileTransferRequest & request, std::string data) override
     {
-        return enqueueFileTransfer(request, std::move(data));
+        return enqueueFileTransfer(request, std::move(data), false);
     }
 
-    std::future<FileTransferResult>
+    std::future<FileTransferResult> enqueueFileTransfer(
-    enqueueFileTransfer(const FileTransferRequest & request, std::optional<std::string> data)
+        const FileTransferRequest & request, std::optional<std::string> data, bool noBody
+    )
     {
         return enqueueFileTransfer(
             request,
@@ -681,14 +685,16 @@ struct curlFileTransfer : public FileTransfer
                 }
             },
             {},
-            std::move(data)
+            std::move(data),
+            noBody
         );
     }
 
     std::future<FileTransferResult> enqueueFileTransfer(const FileTransferRequest & request,
         std::invocable<std::exception_ptr> auto callback,
         std::function<void(TransferItem &, std::string_view data)> dataCallback,
-        std::optional<std::string> data
+        std::optional<std::string> data,
+        bool noBody
     )
     {
         /* Ugly hack to support s3:// URIs. */
@@ -724,11 +730,26 @@ struct curlFileTransfer : public FileTransfer
                                getCurActivity(),
                                std::move(callback),
                                std::move(dataCallback),
-                               std::move(data)
+                               std::move(data),
+                               noBody
                            ))
             ->callback.get_future();
     }
 
+    bool exists(std::string_view uri) override
+    {
+        try {
+            enqueueFileTransfer(FileTransferRequest{uri}, std::nullopt, true).get();
+            return true;
+        } catch (FileTransferError & e) {
+            /* S3 buckets return 403 if a file doesn't exist and the
+                bucket is unlistable, so treat 403 as 404. */
+            if (e.error == FileTransfer::NotFound || e.error == FileTransfer::Forbidden)
+                return false;
+            throw;
+        }
+    }
+
     box_ptr<Source> download(FileTransferRequest && request) override
     {
         struct State {
@@ -776,7 +797,8 @@ struct curlFileTransfer : public FileTransfer
                 state->data.append(data);
                 state->avail.notify_one();
             },
-            std::nullopt
+            std::nullopt,
+            false
         );
 
         struct InnerSource : Source

src/libstore/filetransfer.hh

@@ -58,7 +58,6 @@ struct FileTransferRequest
     Headers headers;
     std::string expectedETag;
     bool verifyTLS = true;
-    bool head = false;
 
     FileTransferRequest(std::string_view uri)
         : uri(uri) { }
@@ -96,6 +95,18 @@ struct FileTransfer
     virtual std::future<FileTransferResult>
     enqueueUpload(const FileTransferRequest & request, std::string data) = 0;
 
+    /**
+     * Checks whether the given URI exists. For historical reasons this function
+     * treats HTTP 403 responses like HTTP 404 responses and returns `false` for
+     * both. This was originally done to handle unlistable S3 buckets, which may
+     * return 403 (not 404) if the reuqested object doesn't exist in the bucket.
+     *
+     * ## Bugs
+     *
+     * S3 objects are downloaded completely to answer this request.
+     */
+    virtual bool exists(std::string_view uri) = 0;
+
     /**
      * Download a file, returning its contents through a source. Will not return
      * before the transfer has fully started, ensuring that any errors thrown by

src/libstore/http-binary-cache-store.cc

@@ -114,15 +114,8 @@ protected:
         checkEnabled();
 
         try {
-            FileTransferRequest request{makeURI(path)};
+            return getFileTransfer()->exists(makeURI(path));
-            request.head = true;
-            getFileTransfer()->enqueueDownload(request).get();
-            return true;
         } catch (FileTransferError & e) {
-            /* S3 buckets return 403 if a file doesn't exist and the
-               bucket is unlistable, so treat 403 as 404. */
-            if (e.error == FileTransfer::NotFound || e.error == FileTransfer::Forbidden)
-                return false;
             maybeDisable();
             throw;
         }