libstore: Introduce ParsedS3URL type

This systematizes the way our s3:// URLs are parsed in filetransfer.cc.
Yoinked out and refactored out of [1].

[1]: https://github.com/NixOS/nix/pull/13752

Co-authored-by: Bernardo Meurer Costa <beme@anthropic.com>

src/libstore/s3.cc

@@ -0,0 +1,71 @@
+#include "nix/store/s3.hh"
+#include "nix/util/split.hh"
+#include "nix/util/url.hh"
+
+namespace nix {
+
+using namespace std::string_view_literals;
+
+#if NIX_WITH_S3_SUPPORT
+
+ParsedS3URL ParsedS3URL::parse(std::string_view uri)
+try {
+    auto parsed = parseURL(uri);
+
+    if (parsed.scheme != "s3"sv)
+        throw BadURL("URI scheme '%s' is not 's3'", parsed.scheme);
+
+    /* Yeah, S3 URLs in Nix have the bucket name as authority. Luckily registered name type
+       authority has the same restrictions (mostly) as S3 bucket names.
+       TODO: Validate against:
+       https://docs.aws.amazon.com/AmazonS3/latest/userguide/bucketnamingrules.html#general-purpose-bucket-names
+       */
+    if (!parsed.authority || parsed.authority->host.empty()
+        || parsed.authority->hostType != ParsedURL::Authority::HostType::Name)
+        throw BadURL("URI has a missing or invalid bucket name");
+
+    std::string_view key = parsed.path;
+    /* Make the key a relative path. */
+    splitPrefix(key, "/");
+
+    /* TODO: Validate the key against:
+     * https://docs.aws.amazon.com/AmazonS3/latest/userguide/object-keys.html#object-key-guidelines
+     */
+
+    auto getOptionalParam = [&](std::string_view key) -> std::optional<std::string> {
+        const auto & query = parsed.query;
+        auto it = query.find(key);
+        if (it == query.end())
+            return std::nullopt;
+        return it->second;
+    };
+
+    auto endpoint = getOptionalParam("endpoint");
+
+    return ParsedS3URL{
+        .bucket = std::move(parsed.authority->host),
+        .key = std::string{key},
+        .profile = getOptionalParam("profile"),
+        .region = getOptionalParam("region"),
+        .scheme = getOptionalParam("scheme"),
+        .endpoint = [&]() -> decltype(ParsedS3URL::endpoint) {
+            if (!endpoint)
+                return std::monostate();
+
+            /* Try to parse the endpoint as a full-fledged URL with a scheme. */
+            try {
+                return parseURL(*endpoint);
+            } catch (BadURL &) {
+            }
+
+            return ParsedURL::Authority::parse(*endpoint);
+        }(),
+    };
+} catch (BadURL & e) {
+    e.addTrace({}, "while parsing S3 URI: '%s'", uri);
+    throw;
+}
+
+#endif
+
+} // namespace nix