Move isUri() and resolveUri() out of filetransfer.cc

These are purely related to NIX_PATH / -I command line parsing, so put
them in libexpr.

src/libexpr/eval.cc

@@ -402,7 +402,7 @@ static Strings parseNixPath(const std::string & s)
         }
 
         if (*p == ':') {
-            if (isUri(std::string(start2, s.end()))) {
+            if (EvalSettings::isPseudoUrl(std::string(start2, s.end()))) {
                 ++p;
                 while (p != s.end() && *p != ':') ++p;
             }
@@ -2583,6 +2583,23 @@ Strings EvalSettings::getDefaultNixPath()
     return res;
 }
 
+bool EvalSettings::isPseudoUrl(std::string_view s)
+{
+    if (s.compare(0, 8, "channel:") == 0) return true;
+    size_t pos = s.find("://");
+    if (pos == std::string::npos) return false;
+    std::string scheme(s, 0, pos);
+    return scheme == "http" || scheme == "https" || scheme == "file" || scheme == "channel" || scheme == "git" || scheme == "s3" || scheme == "ssh";
+}
+
+std::string EvalSettings::resolvePseudoUrl(std::string_view url)
+{
+    if (hasPrefix(url, "channel:"))
+        return "https://nixos.org/channels/" + std::string(url.substr(8)) + "/nixexprs.tar.xz";
+    else
+        return std::string(url);
+}
+
 EvalSettings evalSettings;
 
 static GlobalConfig::Register rEvalSettings(&evalSettings);

src/libexpr/eval.hh

@@ -590,6 +590,10 @@ struct EvalSettings : Config
 
     static Strings getDefaultNixPath();
 
+    static bool isPseudoUrl(std::string_view s);
+
+    static std::string resolvePseudoUrl(std::string_view url);
+
     Setting<bool> enableNativeCode{this, false, "allow-unsafe-native-code-during-evaluation",
         "Whether builtin functions that allow executing native code should be enabled."};
 

src/libexpr/parser.y

@@ -805,10 +805,11 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
 
     std::pair<bool, std::string> res;
 
-    if (isUri(elem.second)) {
+    if (EvalSettings::isPseudoUrl(elem.second)) {
         try {
-            res = { true, store->toRealPath(fetchers::downloadTarball(
-                        store, resolveUri(elem.second), "source", false).first.storePath) };
+            auto storePath = fetchers::downloadTarball(
+                store, EvalSettings::resolvePseudoUrl(elem.second), "source", false).first.storePath;
+            res = { true, store->toRealPath(storePath) };
         } catch (FileTransferError & e) {
             logWarning({
                 .msg = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", elem.second)

src/libexpr/primops/fetchTree.cc

@@ -220,8 +220,6 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
-    url = resolveUri(*url);
-
     state.checkURI(*url);
 
     if (name == "")