feat: add pre-http-request-hooks config entry and handle it

src/libstore/filetransfer.cc

@@ -116,6 +116,26 @@ struct curlFileTransfer : public FileTransfer
             for (auto it = request.headers.begin(); it != request.headers.end(); ++it){
                 requestHeaders = curl_slist_append(requestHeaders, fmt("%s: %s", it->first, it->second).c_str());
             }
+
+            for (auto &[host, prog] : fileTransferSettings.preHTTPRequestHooks.get())
+            {
+                if (hasPrefix(request.uri, fmt("https://%s/", host)) || hasPrefix(request.uri, fmt("http://%s/", host)))
+                {
+                    auto verb = request.data ? "upload" : "download";
+                    debug("pre-http-request-hooks: '%s' for '%s' (%d)", prog, request.uri, verb);
+                    auto lines = runProgram(prog, false, Strings({ request.uri, verb }));
+                    auto lastPos = std::string::size_type{0};
+                    for (auto nlPos = lines.find('\n'); nlPos != std::string::npos; nlPos = lines.find('\n', lastPos))
+                    {
+                        auto line = lines.substr(lastPos, nlPos - lastPos);
+                        lastPos = nlPos + 1;
+                        auto hn = line.substr(0, line.find(':'));
+                        debug("add header: '%s: *****' to '%s' (%d)", hn, request.uri, verb);
+                        requestHeaders = curl_slist_append(requestHeaders, line.c_str());
+                    }
+                    break;
+                }
+            }
         }
 
         ~TransferItem()

src/libstore/filetransfer.hh

@@ -26,6 +26,35 @@ struct FileTransferSettings : Config
         )",
         {"binary-caches-parallel-connections"}};
 
+    Setting<StringMap> preHTTPRequestHooks{this, {}, "pre-http-request-hooks",
+        R"(
+          Optional. A whitespace-separated list of tuples (`host=<path to a program>`) where 
+          the program can set extra URL-specific HTTP headers. This is used for URLs
+          that can't be accessed publically.
+
+          The hook is passed wth the resource URL and the verb (`upload` or `download`).
+          It can then add https headers to the HTTP request by send them to stdout. 
+          
+          When using the nix-daemon, the daemon executes the hook as `root`.
+          If the nix-daemon is not involved, the hook runs as the user
+          executing the nix-build.
+
+          Example:
+              `#!/usr/bin/env bash
+
+              source /my/lib.sh
+
+              url=$1
+              verb=$2
+
+              host=$(parse_host ${url})
+              token=$(get_cached_CF_token_for ${url} ${verb})
+
+              echo Cookie: CF_Authorization=${token}; Domain=${host}; Secure; HttpOnly
+              echo Origin: ${host}
+              `.
+          )"};
+
     Setting<unsigned long> connectTimeout{
         this, 0, "connect-timeout",
         R"(