elastic-agent: don't swallow download errors (elastic#23235)

* elastic-agent: don't swallow download errors

Stop swallowing the error from io.Copy when reading
from response bodies in the http downloader. This
prevents storing a partial download, which leads to
a permanent error state.

(cherry picked from commit 3cb1aa3)

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -29,6 +29,7 @@
 - Fix sysv init files for deb/rpm installation {pull}22543[22543]
 - Fix shell wrapper for deb/rpm packaging {pull}23038[23038]
 - Fixed parsing of npipe URI {pull}22978[22978]
+- Remove artifacts on transient download errors {pull}23235[23235]
 
 ==== New features
 

x-pack/elastic-agent/pkg/artifact/download/http/downloader.go

@@ -151,5 +151,9 @@ func (e *Downloader) downloadFile(ctx context.Context, artifactName, filename, f
 	}
 
 	_, err = io.Copy(destinationFile, resp.Body)
+	if err != nil {
+		return "", errors.New(err, "fetching package failed", errors.TypeNetwork, errors.M(errors.MetaKeyURI, sourceURI))
+	}
+
 	return fullPath, nil
 }

x-pack/elastic-agent/pkg/artifact/download/http/downloader_test.go

@@ -0,0 +1,55 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package http
+
+import (
+	"context"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+
+	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/artifact"
+)
+
+func TestDownloadBodyError(t *testing.T) {
+	// This tests the scenario where the download encounters a network error
+	// part way through the download, while copying the response body.
+
+	type connKey struct{}
+	var srv *httptest.Server
+	srv = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.(http.Flusher).Flush()
+		conn := r.Context().Value(connKey{}).(net.Conn)
+		conn.Close()
+	}))
+	defer srv.Close()
+	client := srv.Client()
+	srv.Config.ConnContext = func(ctx context.Context, c net.Conn) context.Context {
+		return context.WithValue(ctx, connKey{}, c)
+	}
+
+	targetDir, err := ioutil.TempDir(os.TempDir(), "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	config := &artifact.Config{
+		SourceURI:       srv.URL,
+		TargetDirectory: targetDir,
+		OperatingSystem: "linux",
+		Architecture:    "64",
+	}
+
+	testClient := NewDownloaderWithClient(config, *client)
+	artifactPath, err := testClient.Download(context.Background(), beatSpec, version)
+	if err == nil {
+		os.Remove(artifactPath)
+		t.Fatal("expected Download to return an error")
+	}
+}