Support registries in gzipped tarballs

Start with Julia 1.7, a registry can also be stored in a depot as a
gzipped tarball. This commit unpacks the tarball into a temporary
directory. This was the simplest way to support this new feature.

This implementation will unpack a fresh copy of the registry on every
call to `reachable_registries`. We could instead cache the temporary
directory for future calls.

There's an open PR for the Tar.jl package that will be of future
interest:

JuliaIO/Tar.jl#95

After that is merged, we could instead stream the individual files
straight out of the tarball, rather than having to unpack it into a
temporary location.

Project.toml

@@ -4,10 +4,12 @@ license = "MIT"
 version = "0.6.2"
 
 [deps]
+CodecZlib = "944b1d66-785c-5afd-91f1-9de20f533193"
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"
 Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"
 REPL = "3fa0cd96-eef1-5676-8a61-b3b8758bbffb"
 TOML = "fa267f1f-6049-4f14-aa54-33bafae1ed76"
+Tar = "a4e569a6-e804-4fa4-b0f3-eef7a1d5b13e"
 UUIDs = "cf7118a7-6976-5b1a-9a39-7adc72f591a4"
 
 [compat]

src/PkgDeps.jl

@@ -6,6 +6,8 @@ using REPL
 using TOML: parsefile
 using UUIDs
 using Compat
+import Tar
+using CodecZlib: GzipDecompressorStream
 
 export PkgEntry, RegistryInstance
 export NoUUIDMatch, PackageNotInRegistry
@@ -53,14 +55,27 @@ function reachable_registries(
 
     for d in depots
         isdir(d) || continue
-        reg_dir = joinpath(d, "registries")
-        isdir(reg_dir) || continue
-
-        for name in readdir(reg_dir)
-            if isempty(registry_names) || name in registry_names
-                file = joinpath(reg_dir, name, "Registry.toml")
-                isfile(file) || continue
-                push!(registries, RegistryInstance(joinpath(reg_dir, name)))
+        registries_dir = joinpath(d, "registries")
+        isdir(registries_dir) || continue
+
+        for file_name in readdir(registries_dir)
+            if isdir(registries_dir, file_name)
+                registry_name = file_name
+                registry_path = joinpath(registries_dir, registry_name)
+            elseif endswith(file_name, ".tar.gz")
+                registry_name = chop(file_name, tail = length(".tar.gz"))
+
+                # We need to unpack the registry into a temporary directory.
+                registry_path = open(joinpath(registries_dir, file_name)) do io
+                    Tar.extract(GzipDecompressorStream(io))
+                end
+            else
+                continue
+            end
+
+            if isempty(registry_names) || registry_name in registry_names
+                isfile(registry_path, "Registry.toml") || continue
+                push!(registries, RegistryInstance(registry_path))
             end
         end
     end