Allow tarball URLs to redirect to a lockable immutable URL

Previously, for tarball flakes, we recorded the original URL of the
tarball flake, rather than the URL to which it ultimately
redirects. Thus, a flake URL like
http://example.org/patchelf-latest.tar that redirects to
http://example.org/patchelf-<revision>.tar was not really usable. We
couldn't record the redirected URL, because sites like GitHub redirect
to CDN URLs that we can't rely on to be stable.

So now we use the redirected URL only if the server returns the
`x-nix-is-immutable` or `x-amz-meta-nix-is-immutable` headers in its
response.

2 files changed, 22 insertions, 4 deletions

diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index 2346accbe..38b691279 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -186,9 +186,9 @@ struct curlFileTransfer : public FileTransfer
             size_t realSize = size * nmemb;
             std::string line((char *) contents, realSize);
             printMsg(lvlVomit, "got header for '%s': %s", request.uri, trim(line));
+
             static std::regex statusLine("HTTP/[^ ]+ +[0-9]+(.*)", std::regex::extended | std::regex::icase);
-            std::smatch match;
-            if (std::regex_match(line, match, statusLine)) {
+            if (std::smatch match; std::regex_match(line, match, statusLine)) {
                 result.etag = "";
                 result.data.clear();
                 result.bodySize = 0;
@@ -196,9 +196,11 @@ struct curlFileTransfer : public FileTransfer
                 acceptRanges = false;
                 encoding = "";
             } else {
+
                 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));
                         /* Hack to work around a GitHub bug: it sends
@@ -212,10 +214,22 @@ struct curlFileTransfer : public FileTransfer
                             debug("shutting down on 200 HTTP response with expected ETag");
                             return 0;
                         }
-                    } else if (name == "content-encoding")
+                    }
+
+                    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;
@@ -345,7 +359,7 @@ struct curlFileTransfer : public FileTransfer
         {
             auto httpStatus = getHTTPStatus();
 
-            char * effectiveUriCStr;
+            char * effectiveUriCStr = nullptr;
             curl_easy_getinfo(req, CURLINFO_EFFECTIVE_URL, &effectiveUriCStr);
             if (effectiveUriCStr)
                 result.effectiveUri = effectiveUriCStr;
diff --git a/src/libstore/filetransfer.hh b/src/libstore/filetransfer.hh
index 378c6ff78..a3b0dde1f 100644
--- a/src/libstore/filetransfer.hh
+++ b/src/libstore/filetransfer.hh
@@ -80,6 +80,10 @@ struct FileTransferResult
     std::string effectiveUri;
     std::string data;
     uint64_t bodySize = 0;
+    /* An "immutable" URL for this resource (i.e. one whose contents
+       will never change), as returned by the `Link: <url>;
+       rel="immutable"` header. */
+    std::optional<std::string> immutableUrl;
 };
 
 class Store;