Add presigned_url method.

For GCS, return an actual presigned URL for the file. For disk, return a
standard `file://` url. For inmemory, return the original URL, for the
sake of completeness.

lib/bucket_store/disk.rb

@@ -55,6 +55,12 @@ def delete!(bucket:, key:)
       true
     end
 
+    # rubocop: disable Lint/UnusedMethodArgument
+    def presigned_url(bucket:, key:, expiry:)
+      "file://#{key_path(bucket, key)}"
+    end
+    # rubocop: enable Lint/UnusedMethodArgument
+
     private
 
     attr_reader :base_dir

lib/bucket_store/gcs.rb

@@ -66,6 +66,10 @@ def delete!(bucket:, key:)
       true
     end
 
+    def presigned_url(bucket:, key:, expiry:)
+      get_bucket(bucket).file(key).signed_url(expires: expiry)
+    end
+
     private
 
     attr_reader :storage

lib/bucket_store/in_memory.rb

@@ -58,5 +58,11 @@ def delete!(bucket:, key:)
 
       true
     end
+
+    # rubocop: disable Lint/UnusedMethodArgument
+    def presigned_url(bucket:, key:, expiry:)
+      "inmemory://#{bucket}/#{key}"
+    end
+    # rubocop: enable Lint/UnusedMethodArgument
   end
 end

lib/bucket_store/key_storage.rb

@@ -156,6 +156,25 @@ def exists?
       list.first == "#{adapter_type}://#{bucket}/#{key}"
     end
 
+    # Generates a pre-signed URL for the referenced key.
+    #
+    # @param [Integer] expiry The time to expiry for the URL; defaults to 24 hours.
+    # @return [String] The pre-signed URL
+    #
+    # @example Generate a presigned URL for a file
+    #   BucketStore.for("gcs://bucket/file.txt").presigned_url
+    def presigned_url(expiry: 86400)
+      BucketStore.logger.info(event: "key_storage.presigned_url_started")
+
+      start = BucketStore::Timing.monotonic_now
+      url = adapter.presigned_url(bucket: bucket, key: key, expiry: expiry)
+
+      BucketStore.logger.info(event: "key_storage.presigned_url_finished",
+                              duration: BucketStore::Timing.monotonic_now - start)
+
+      url
+    end
+
     private
 
     attr_reader :adapter

spec/bucket_store/key_storage_spec.rb

@@ -154,4 +154,12 @@ def build_for(key)
       expect(build_for("inmemory://bucket/prefix/a").exists?).to be false
     end
   end
+
+  describe "#presigned_url" do
+    let(:url) { "inmemory://bucket/file1" }
+
+    it "returns a URL" do
+      expect(build_for(url).presigned_url).to eq(url)
+    end
+  end
 end