windows/bootstrap: repair empty winget source before install

0x8a15000f ("Data required by the source is missing") aborts Git.Git /
GoLang.Go install on a virgin DesktopAppInstaller cache, then iex dies
on missing git. Reset+update the source once, pin --source winget, retry
without the pin, then fail loudly if the index is still empty.

windows/bootstrap.ps1

@@ -11,14 +11,61 @@ $Dest = Join-Path $env:LOCALAPPDATA 'rclone-dropbox-bisync'
 $Dropbox = 'D:\Dropbox'
 $Workdir = 'D:\rclone-bisync'
 
+function Refresh-Path {
+    $env:Path = [Environment]::GetEnvironmentVariable('Path', 'Machine') + ';' +
+        [Environment]::GetEnvironmentVariable('Path', 'User')
+}
+
+# 0x8a15000f: winget source index missing/corrupt (common on a fresh DesktopAppInstaller).
+function Repair-WingetSource {
+    if ($script:WingetSourceRepaired) { return }
+    $winget = Get-Command winget -ErrorAction SilentlyContinue
+    if (-not $winget) { return }
+    $prev = $ErrorActionPreference
+    $ErrorActionPreference = 'Continue'
+    try {
+        & winget source reset --force --disable-interactivity 2>&1 | Out-Null
+        & winget source update --disable-interactivity 2>&1 | Out-Null
+    } finally {
+        $ErrorActionPreference = $prev
+    }
+    $script:WingetSourceRepaired = $true
+}
+
+function Install-WithWinget($WingetId) {
+    Repair-WingetSource
+    $wingetArgs = @(
+        'install', '-e', '--id', $WingetId,
+        '--accept-package-agreements', '--accept-source-agreements',
+        '--disable-interactivity', '--source', 'winget'
+    )
+    $prev = $ErrorActionPreference
+    $ErrorActionPreference = 'Continue'
+    try {
+        & winget @wingetArgs
+        if ($LASTEXITCODE -eq 0) { return }
+        # Index still empty: reset once more, then retry without pinning the source
+        # (some hosts only have msstore until cdn.winget.microsoft.com is reachable).
+        & winget source reset --force --disable-interactivity 2>&1 | Out-Null
+        & winget source update --disable-interactivity 2>&1 | Out-Null
+        & winget install -e --id $WingetId --accept-package-agreements --accept-source-agreements --disable-interactivity
+        if ($LASTEXITCODE -eq 0) { return }
+    } finally {
+        $ErrorActionPreference = $prev
+    }
+    throw "winget failed to install $WingetId (exit $LASTEXITCODE / 0x8a15000f = empty source cache). Run: winget source reset --force; winget source update"
+}
+
 function Ensure-Cmd($Name, $WingetId) {
     if (Get-Command $Name -ErrorAction SilentlyContinue) { return }
     if (-not (Get-Command winget -ErrorAction SilentlyContinue)) {
         throw "Need $Name on PATH (or winget to install $WingetId)"
     }
-    winget install -e --id $WingetId --accept-package-agreements --accept-source-agreements
-    $env:Path = [Environment]::GetEnvironmentVariable('Path', 'Machine') + ';' +
-        [Environment]::GetEnvironmentVariable('Path', 'User')
+    Install-WithWinget $WingetId
+    Refresh-Path
+    if (-not (Get-Command $Name -ErrorAction SilentlyContinue)) {
+        throw "$Name still missing after winget install $WingetId; open a new PowerShell and retry"
+    }
 }
 
 Ensure-Cmd git Git.Git