From af8a0c46c540ac5bd28ed6e7ff3ae09df1e86a0b Mon Sep 17 00:00:00 2001
From: Vladislav Rassokhin <vladislav.rassokhin@jetbrains.com>
Date: Sun, 26 Nov 2017 00:02:54 +0300
Subject: [PATCH] Do not re-download iso multiple times from different urls

In case of two or more iso_urls checks for downloaded files prior to downloading them.
Speedups case when some iso already downloaded and another url prepended to iso_urls list.
---
 common/step_download.go | 45 ++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 14 deletions(-)

diff --git a/common/step_download.go b/common/step_download.go
index f63070a04..765a07d23 100644
--- a/common/step_download.go
+++ b/common/step_download.go
@@ -61,10 +61,12 @@ func (s *StepDownload) Run(state multistep.StateBag) multistep.StepAction {
 
 	ui.Say(fmt.Sprintf("Downloading or copying %s", s.Description))
 
-	var finalPath string
-	for _, url := range s.Url {
-		ui.Message(fmt.Sprintf("Downloading or copying: %s", url))
+	// First try to use any already downloaded file
+	// If it fails, proceed to regualar download logic
 
+	var downloadConfigs = make([]*DownloadConfig, len(s.Url))
+	var finalPath string
+	for i, url := range s.Url {
 		targetPath := s.TargetPath
 		if targetPath == "" {
 			// Determine a cache key. This is normally just the URL but
@@ -90,22 +92,37 @@ func (s *StepDownload) Run(state multistep.StateBag) multistep.StepAction {
 			Checksum:   checksum,
 			UserAgent:  "Packer",
 		}
+		downloadConfigs[i] = config
 
-		path, err, retry := s.download(config, state)
-		if err != nil {
-			ui.Message(fmt.Sprintf("Error downloading: %s", err))
-		}
-
-		if !retry {
-			return multistep.ActionHalt
-		}
-
-		if err == nil {
-			finalPath = path
+		if match, _ := NewDownloadClient(config).VerifyChecksum(config.TargetPath); match {
+			ui.Message(fmt.Sprintf("Found already downloaded, initial checksum matched, no download needed: %s", url))
+			finalPath = config.TargetPath
 			break
 		}
 	}
 
+	if finalPath == "" {
+		for i, url := range s.Url {
+			ui.Message(fmt.Sprintf("Downloading or copying: %s", url))
+
+			config := downloadConfigs[i]
+
+			path, err, retry := s.download(config, state)
+			if err != nil {
+				ui.Message(fmt.Sprintf("Error downloading: %s", err))
+			}
+
+			if !retry {
+				return multistep.ActionHalt
+			}
+
+			if err == nil {
+				finalPath = path
+				break
+			}
+		}
+	}
+
 	if finalPath == "" {
 		err := fmt.Errorf("%s download failed.", s.Description)
 		state.Put("error", err)