[cipd] More careful error handling during CIPD bootstrap on Windows.

Errors during the download should be reported as such, not as "failed to grab
the lock file".

R=nodir@chromium.org, iannucci@chromium.org
BUG=853722

Change-Id: If24f4587693852eed0608450bcd311cf3bafd2c3
Reviewed-on: https://chromium-review.googlesource.com/1226106
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>

cipd.ps1

@@ -2,6 +2,15 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# Note: to run this on e.g. OSX for adhoc testing or debugging in case Windows
+# is not around:
+#
+# pwsh cipd.ps1 \
+#     -CipdBinary _cipd.exe \
+#     -BackendURL https://chrome-infra-packages.appspot.com \
+#     -VersionFile ./cipd_client_version
+# file _cipd.exe
+
 Param(
   # Path to download the CIPD binary to.
   [parameter(Mandatory=$true)][string]$CipdBinary,
@@ -82,18 +91,30 @@ $Version = (Get-Content $VersionFile).Trim()
 $URL = "$BackendURL/client?platform=$Platform&version=$Version"
 
 
-# Use a lock file to prevent simultaneous processes from stepping on each other.
+# Grab a lock to prevent simultaneous processes from stepping on each other.
+# This depends on "exclusive write" file sharing mode used by OpenWrite.
 $CipdLockPath = Join-Path $DepotToolsPath -ChildPath ".cipd_client.lock"
-$TmpPath = $CipdBinary + ".tmp"
+$CipdLockFile = $null
-while ($true) {
+while ($CipdLockFile -eq $null) {
-  $CipdLockFile = $null
   try {
     $CipdLockFile = [System.IO.File]::OpenWrite($CipdLockPath)
+  } catch [System.IO.IOException] {
+    echo "CIPD bootstrap lock is held, trying again after delay..."
+    Start-Sleep -s 1
+  }
+}
 
+# Fetch the binary now that the lock is ours.
+$TmpPath = $CipdBinary + ".tmp"
+try {
   echo "Downloading CIPD client for $Platform from $URL..."
   $wc = (New-Object System.Net.WebClient)
   $wc.Headers.Add("User-Agent", $UserAgent)
+  try {
     $wc.DownloadFile($URL, $TmpPath)
+  } catch {
+    throw "Failed to download the file, check your network connection"
+  }
 
   $ActualSHA256 = Get-Actual-SHA256 $TmpPath
   if ($ActualSHA256 -ne $ExpectedSHA256) {
@@ -101,18 +122,8 @@ while ($true) {
   }
 
   Move-Item -LiteralPath $TmpPath -Destination $CipdBinary -Force
-    break
+} finally {
-
-  } catch [System.IO.IOException] {
-    echo "CIPD bootstrap lock is held, trying again after delay..."
-    Start-Sleep -s 1
-  } catch {
-    throw  # for some reason this is needed to exit while(...) loop on errors
-  } finally {
-    Delete-If-Possible $TmpPath
-    if ($CipdLockFile) {
   $CipdLockFile.Close()
   Delete-If-Possible $CipdLockPath
-    }
+  Delete-If-Possible $TmpPath
-  }
 }