[cipd] Simplify .cipd_impl.ps1.

This changes cipd_impl.ps1 to work with "Constrained Language Mode",
just using built in powershell utils. This also now has a
substantially simpler implementation.

A lost feature here is that multiple cipd_impl invocations would
serialize... however this required using System functions which
don't work in Constrained Language Mode. The new impl goes for
an eventually consistent approach; Each invocation of the script
will use a tempfile with a unique name, and will move it into
place after downloading if the file hash matches.

In the cold start &&& higly parallel case, this could cause multiple
downloads, but the eventual state will be consistent.

R=actodd, alexschulze, fancl, vadimsh

Bug: 1442051
Change-Id: I4e7a808eb60654d96fe7f20f36b642e8792b4193
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4507643
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>

.cipd_impl.ps1

@@ -5,10 +5,11 @@
 # 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
+# pwsh
+# PS ...> $ExecutionContext.SessionState.LanguageMode = "ConstrainedLanguage"
+# PS ...> ./.cipd_impl.ps1 -CipdBinary _cipd.exe `
+#         -BackendURL https://chrome-infra-packages.appspot.com `
+#         -VersionFile ./cipd_client_version
 # file _cipd.exe
 
 param(
@@ -16,7 +17,7 @@ param(
     [Parameter(Mandatory = $true)]
     [string]
     $CipdBinary,
-    
+
     # CIPD platform to download the client for.
     [string]
     $Platform = "windows-amd64",
@@ -47,18 +48,6 @@ try {
 }
 $Env:CIPD_HTTP_USER_AGENT_PREFIX = $UserAgent
 
-
-# Tries to delete the file, ignoring errors. Used for best-effort cleanups.
-function Delete-If-Possible($path) {
-  try {
-    [System.IO.File]::Delete($path)
-  } catch {
-    $err = $_.Exception.Message
-    echo "Warning: error when deleting $path - $err. Ignoring."
-  }
-}
-
-
 # Returns the expected SHA256 hex digest for the given platform reading it from
 # *.digests file.
 function Get-Expected-SHA256($platform) {
@@ -73,60 +62,6 @@ function Get-Expected-SHA256($platform) {
   throw "No SHA256 digests for $platform in $digestsFile"
 }
 
-
-# Returns SHA256 hex digest of a binary file at the given path.
-function Get-Actual-SHA256($path) {
-  # Note: we don't use Get-FileHash to be compatible with PowerShell v3.0
-  $file = [System.IO.File]::Open($path, [System.IO.FileMode]::Open)
-  try {
-    $algo = New-Object System.Security.Cryptography.SHA256Managed
-    $hash = $algo.ComputeHash($file)
-  } finally {
-    $file.Close()
-  }
-  $hex = ""
-  foreach ($byte in $hash) {
-    $hex += $byte.ToString("x2")
-  }
-  return $hex
-}
-
-
-# Download a file to a particular path.
-function Download-File {
-  [CmdletBinding()]
-  param (
-      [Parameter(Mandatory = $true)]
-      [string]
-      $UserAgent,
-
-      [Parameter(Mandatory = $true)]
-      [string]
-      $Url,
-
-      [Parameter(Mandatory = $true)]
-      [string]
-      $Path
-  )
-
-  $wc = (New-Object System.Net.WebClient)
-  $wc.Headers.Add("User-Agent", $UserAgent)
-  try {
-    # Download failures were reported on Windows 8.1 without this line.
-    [System.Net.ServicePointManager]::SecurityProtocol = `
-            [System.Net.SecurityProtocolType]::Tls12
-    $wc.DownloadFile($Url, $Path)
-  }
-  catch {
-    $err = $_.Exception.Message
-    throw "Failed to download the file, check your network connection, $err"
-  }
-  finally {
-    $wc.Dispose()
-  }
-}
-
-
 # Retry a command with a delay between each.
 function Retry-Command {
   [CmdletBinding()]
@@ -145,15 +80,15 @@ function Retry-Command {
   $attempt = 0
   while ($attempt -lt $MaxAttempts) {
     try {
-      $Command.Invoke()
+      Invoke-Command -ScriptBlock $Command
       return
     }
     catch {
       $attempt += 1
       $exception = $_.Exception.InnerException
       if ($attempt -lt $MaxAttempts) {
-        echo $exception.Message
-        echo "Retrying after a short nap..."
+        Write-Output "FAILURE: " + $_
+        Write-Output "Retrying after a short nap..."
         Start-Sleep -Seconds $Delay.TotalSeconds
       } else {
         throw $exception
@@ -162,41 +97,25 @@ function Retry-Command {
   }
 }
 
-
 $ExpectedSHA256 = Get-Expected-SHA256 $Platform
 $Version = (Get-Content $VersionFile).Trim()
 $URL = "$BackendURL/client?platform=$Platform&version=$Version"
 
-
-# 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 $DepotToolsPath -ChildPath ".cipd_client.lock"
-$CipdLockFile = $null
-while ($CipdLockFile -eq $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 = $CipdBinary + ".tmp"
+$TmpPath = $CipdBinary + ".tmp." + $PID
 try {
-  echo "Downloading CIPD client for $Platform from $URL..."
-  Retry-Command -Command {
-    Download-File -UserAgent $UserAgent -Url $URL -Path $TmpPath
+  Write-Output "Downloading CIPD client for $Platform from $URL..."
+  Retry-Command {
+    Invoke-WebRequest -UserAgent $UserAgent -Uri $URL -OutFile $TmpPath
   }
 
-  $ActualSHA256 = Get-Actual-SHA256 $TmpPath
+  $ActualSHA256 = (Get-FileHash -Path $TmpPath -Algorithm "SHA256").Hash.toLower()
   if ($ActualSHA256 -ne $ExpectedSHA256) {
     throw "Invalid SHA256 digest: $ActualSHA256 != $ExpectedSHA256"
   }
 
   Move-Item -LiteralPath $TmpPath -Destination $CipdBinary -Force
-} finally {
-  $CipdLockFile.Close()
-  Delete-If-Possible $CipdLockPath
-  Delete-If-Possible $TmpPath
+} catch {
+  Remove-Item -Path $TmpPath -ErrorAction Ignore
+  throw  # Re raise any error
 }