fix(libutil/tarfile): normalize legacy HTTP Content-Encoding names

Nix failed to download files served with `Content-Encoding: x-gzip`
because libarchive doesn't recognize the legacy `x-*` compression
format names. Per RFC 9110 §8.4.1.3, HTTP recipients should treat
these as equivalent to their standard counterparts.

Adds `normalizeCompressionMethod()` to map legacy encoding names
before passing to libarchive:
- `x-gzip` → `gzip`
- `x-compress` → `compress`
- `x-bzip2` → `bzip2`

src/libutil-tests/compression.cc

@@ -74,6 +74,30 @@ TEST(decompress, decompressInvalidInputThrowsCompressionError)
     ASSERT_THROW(decompress(method, str), CompressionError);
 }
 
+/* ----------------------------------------------------------------------------
+ * legacy HTTP Content-Encoding names (RFC 9110)
+ * --------------------------------------------------------------------------*/
+
+TEST(decompress, decompressXGzipCompressed)
+{
+    // Test that x-gzip (legacy HTTP Content-Encoding) works like gzip
+    auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+    auto compressedData = compress("gzip", str);
+    auto o = decompress("x-gzip", compressedData);
+
+    ASSERT_EQ(o, str);
+}
+
+TEST(decompress, decompressXBzip2Compressed)
+{
+    // Test that x-bzip2 (legacy HTTP Content-Encoding) works like bzip2
+    auto str = "slfja;sljfklsa;jfklsjfkl;sdjfkl;sadjfkl;sdjf;lsdfjsadlf";
+    auto compressedData = compress("bzip2", str);
+    auto o = decompress("x-bzip2", compressedData);
+
+    ASSERT_EQ(o, str);
+}
+
 /* ----------------------------------------------------------------------------
  * compression sinks
  * --------------------------------------------------------------------------*/

src/libutil/tarfile.cc

@@ -51,6 +51,26 @@ void TarArchive::check(int err, const std::string & reason)
     checkLibArchive(archive, err, reason);
 }
 
+/// @brief Normalize compression method names from legacy HTTP Content-Encoding values.
+///
+/// Per RFC 9110 Section 8.4.1.3, HTTP recipients should treat legacy "x-*" compression
+/// names as equivalent to their standard counterparts:
+/// - "x-gzip" is equivalent to "gzip"
+/// - "x-compress" is equivalent to "compress"
+///
+/// This function maps these legacy names to their libarchive-compatible equivalents.
+static std::string normalizeCompressionMethod(const std::string & method)
+{
+    if (method == "x-gzip")
+        return "gzip";
+    else if (method == "x-compress")
+        return "compress";
+    else if (method == "x-bzip2")
+        return "bzip2";
+    else
+        return method;
+}
+
 /// @brief Get filter_code from its name.
 ///
 /// libarchive does not provide a convenience function like archive_write_add_filter_by_name but for reading.
@@ -59,9 +79,10 @@ void TarArchive::check(int err, const std::string & reason)
 /// hand-rolling the equivalent function that is better implemented in libarchive.
 int getArchiveFilterCodeByName(const std::string & method)
 {
+    auto normalizedMethod = normalizeCompressionMethod(method);
     auto * ar = archive_write_new();
     auto cleanup = Finally{[&ar]() { checkLibArchive(ar, archive_write_close(ar), "failed to close archive: %s"); }};
-    auto err = archive_write_add_filter_by_name(ar, method.c_str());
+    auto err = archive_write_add_filter_by_name(ar, normalizedMethod.c_str());
     checkLibArchive(ar, err, "failed to get libarchive filter by name: %s");
     auto code = archive_filter_code(ar, 0);
     return code;