libstore: use curl functions for reading headers

don't reimplement header parsing. this was only really needed due to the
ancient github bug we no longer care about, everything else we have done
in custom code can also be done using curl itself. doing this also fixes
possible sources of header smuggling (because the header function didn't
unfold headers and we'd trim them before parsing, which would've made us
read contents of one header as a fully formed header in itself). this is
a slight behavior change because we now honor only the first instance of
a given header where previous behavior was to honor either the last or a
combination of all of them (accept-ranges was logical-or'd by accident).

Change-Id: I93cb93ddb91ab98c8991f846014926f6ef039fdb

src/libstore/filetransfer.cc

@@ -51,6 +51,7 @@ struct curlFileTransfer : public FileTransfer
         Callback<FileTransferResult> callback;
         CURL * req = 0;
         bool active = false; // whether the handle has been added to the multi object
+        bool headersProcessed = false;
         std::string statusMsg;
 
         unsigned int attempt = 0;
@@ -151,9 +152,33 @@ struct curlFileTransfer : public FileTransfer
 
         std::exception_ptr writeException;
 
+        std::optional<std::string> getHeader(const char * name)
+        {
+            curl_header * result;
+            auto e = curl_easy_header(req, name, 0, CURLH_HEADER, -1, &result);
+            if (e == CURLHE_OK) {
+                return result->value;
+            } else if (e == CURLHE_MISSING || e == CURLHE_NOHEADERS) {
+                return std::nullopt;
+            } else {
+                throw nix::Error("unexpected error from curl_easy_header(): %i", e);
+            }
+        }
+
         size_t writeCallback(void * contents, size_t size, size_t nmemb)
         {
             try {
+                if (!headersProcessed) {
+                    if (auto h = getHeader("content-encoding")) {
+                        encoding = std::move(*h);
+                    }
+                    if (auto h = getHeader("accept-ranges"); h && *h == "bytes") {
+                        acceptRanges = true;
+                    }
+
+                    headersProcessed = true;
+                }
+
                 size_t realSize = size * nmemb;
                 result.bodySize += realSize;
 
@@ -196,31 +221,7 @@ struct curlFileTransfer : public FileTransfer
                 statusMsg = trim(match.str(1));
                 acceptRanges = false;
                 encoding = "";
-            } else {
+                headersProcessed = false;
-
-                auto i = line.find(':');
-                if (i != std::string::npos) {
-                    std::string name = toLower(trim(line.substr(0, i)));
-
-                    if (name == "etag") {
-                        result.etag = trim(line.substr(i + 1));
-                    }
-
-                    else if (name == "content-encoding")
-                        encoding = trim(line.substr(i + 1));
-
-                    else if (name == "accept-ranges" && toLower(trim(line.substr(i + 1))) == "bytes")
-                        acceptRanges = true;
-
-                    else if (name == "link" || name == "x-amz-meta-link") {
-                        auto value = trim(line.substr(i + 1));
-                        static std::regex linkRegex("<([^>]*)>; rel=\"immutable\"", std::regex::extended | std::regex::icase);
-                        if (std::smatch match; std::regex_match(value, match, linkRegex))
-                            result.immutableUrl = match.str(1);
-                        else
-                            debug("got invalid link header '%s'", value);
-                    }
-                }
             }
             return realSize;
         }
@@ -364,6 +365,25 @@ struct curlFileTransfer : public FileTransfer
                 }
             }
 
+            auto link = getHeader("link");
+            if (!link) {
+                link = getHeader("x-amz-meta-link");
+            }
+            if (link) {
+                static std::regex linkRegex(
+                    "<([^>]*)>; rel=\"immutable\"", std::regex::extended | std::regex::icase
+                );
+                if (std::smatch match; std::regex_match(*link, match, linkRegex)) {
+                    result.immutableUrl = match.str(1);
+                } else {
+                    debug("got invalid link header '%s'", *link);
+                }
+            }
+
+            if (auto etag = getHeader("etag")) {
+                result.etag = std::move(*etag);
+            }
+
             if (writeException)
                 failEx(writeException);