Factor out abstract syntax for Store URIs

Need to decouple parsing from actually opening a store for Machine
configs.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore/store-reference.cc

@@ -0,0 +1,92 @@
+#include <regex>
+
+#include "error.hh"
+#include "url.hh"
+#include "store-reference.hh"
+#include "file-system.hh"
+
+namespace nix {
+
+static bool isNonUriPath(const std::string & spec)
+{
+    return
+        // is not a URL
+        spec.find("://") == std::string::npos
+        // Has at least one path separator, and so isn't a single word that
+        // might be special like "auto"
+        && spec.find("/") != std::string::npos;
+}
+
+StoreReference StoreReference::parse(const std::string & uri, const StoreReference::Params & extraParams)
+{
+    auto params = extraParams;
+    try {
+        auto parsedUri = parseURL(uri);
+        params.insert(parsedUri.query.begin(), parsedUri.query.end());
+
+        auto baseURI = parsedUri.authority.value_or("") + parsedUri.path;
+
+        return {
+            .variant =
+                Specified{
+                    .scheme = std::move(parsedUri.scheme),
+                    .authority = std::move(baseURI),
+                },
+            .params = std::move(params),
+        };
+    } catch (BadURL &) {
+        auto [baseURI, uriParams] = splitUriAndParams(uri);
+        params.insert(uriParams.begin(), uriParams.end());
+
+        if (baseURI == "" || baseURI == "auto") {
+            return {
+                .variant = Auto{},
+                .params = std::move(params),
+            };
+        } else if (baseURI == "daemon") {
+            return {
+                .variant =
+                    Specified{
+                        .scheme = "unix",
+                        .authority = "",
+                    },
+                .params = std::move(params),
+            };
+        } else if (baseURI == "local") {
+            return {
+                .variant =
+                    Specified{
+                        .scheme = "local",
+                        .authority = "",
+                    },
+                .params = std::move(params),
+            };
+        } else if (isNonUriPath(baseURI)) {
+            return {
+                .variant =
+                    Specified{
+                        .scheme = "local",
+                        .authority = absPath(baseURI),
+                    },
+                .params = std::move(params),
+            };
+        }
+    }
+
+    throw UsageError("Cannot parse Nix store '%s'", uri);
+}
+
+/* Split URI into protocol+hierarchy part and its parameter set. */
+std::pair<std::string, StoreReference::Params> splitUriAndParams(const std::string & uri_)
+{
+    auto uri(uri_);
+    StoreReference::Params params;
+    auto q = uri.find('?');
+    if (q != std::string::npos) {
+        params = decodeQuery(uri.substr(q + 1));
+        uri = uri_.substr(0, q);
+    }
+    return {uri, params};
+}
+
+}