Fix relative submodule handling

Tested on

  nix flake prefetch 'git+https://github.com/blender/blender.git?rev=4ed8a360e956daf2591add4d3c9ec0719e2628fe&submodules=1'

src/libfetchers/git-utils.cc

@@ -308,13 +308,21 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 
     std::vector<std::tuple<Submodule, Hash>> getSubmodules(const Hash & rev) override;
 
-    std::string resolveSubmoduleUrl(const std::string & url) override
+    std::string resolveSubmoduleUrl(
+        const std::string & url,
+        const std::string & base) override
     {
         git_buf buf = GIT_BUF_INIT;
         if (git_submodule_resolve_url(&buf, *this, url.c_str()))
             throw Error("resolving Git submodule URL '%s'", url);
         Finally cleanup = [&]() { git_buf_dispose(&buf); };
-        return buf.ptr;
+
+        std::string res(buf.ptr);
+
+        if (!hasPrefix(res, "/") && res.find("://") == res.npos)
+            res = parseURL(base + "/" + res).canonicalise().to_string();
+
+        return res;
     }
 
     bool hasObject(const Hash & oid_) override

src/libfetchers/git-utils.hh

@@ -59,7 +59,9 @@ struct GitRepo
      */
     virtual std::vector<std::tuple<Submodule, Hash>> getSubmodules(const Hash & rev) = 0;
 
-    virtual std::string resolveSubmoduleUrl(const std::string & url) = 0;
+    virtual std::string resolveSubmoduleUrl(
+        const std::string & url,
+        const std::string & base) = 0;
 
     struct TarballInfo
     {

src/libfetchers/git.cc

@@ -594,7 +594,7 @@ struct GitInputScheme : InputScheme
             std::map<CanonPath, nix::ref<InputAccessor>> mounts;
 
             for (auto & [submodule, submoduleRev] : repo->getSubmodules(rev)) {
-                auto resolved = repo->resolveSubmoduleUrl(submodule.url);
+                auto resolved = repo->resolveSubmoduleUrl(submodule.url, repoInfo.url);
                 debug("Git submodule %s: %s %s %s -> %s",
                     submodule.path, submodule.url, submodule.branch, submoduleRev.gitRev(), resolved);
                 fetchers::Attrs attrs;

src/libutil/url.cc

@@ -2,6 +2,7 @@
 #include "url-parts.hh"
 #include "util.hh"
 #include "split.hh"
+#include "canon-path.hh"
 
 namespace nix {
 
@@ -141,6 +142,13 @@ bool ParsedURL::operator ==(const ParsedURL & other) const
         && fragment == other.fragment;
 }
 
+ParsedURL ParsedURL::canonicalise()
+{
+    ParsedURL res(*this);
+    res.path = CanonPath(res.path).abs();
+    return res;
+}
+
 /**
  * Parse a URL scheme of the form '(applicationScheme\+)?transportScheme'
  * into a tuple '(applicationScheme, transportScheme)'

src/libutil/url.hh

@@ -19,6 +19,11 @@ struct ParsedURL
     std::string to_string() const;
 
     bool operator ==(const ParsedURL & other) const;
+
+    /**
+     * Remove `.` and `..` path elements.
+     */
+    ParsedURL canonicalise();
 };
 
 MakeError(BadURL, Error);