Add checks/error reporting to compaction process

* Report compaction results
* Failure to find any disks under the supplied path is treated as a
  'soft' error and a warning message will be printed in place of the
  compaction result. Any other failure will cause the build to fail.

builder/hyperv/common/driver.go

@@ -96,7 +96,7 @@ type Driver interface {
 
 	PreserveLegacyExportBehaviour(string, string) error
 
-	CompactDisks(string) error
+	CompactDisks(string) (string, error)
 
 	RestartVirtualMachine(string) error
 

builder/hyperv/common/driver_mock.go

@@ -193,6 +193,7 @@ type DriverMock struct {
 
 	CompactDisks_Called bool
 	CompactDisks_Path   string
+	CompactDisks_Result string
 	CompactDisks_Err    error
 
 	RestartVirtualMachine_Called bool
@@ -493,10 +494,11 @@ func (d *DriverMock) PreserveLegacyExportBehaviour(srcPath string, dstPath strin
 	return d.PreserveLegacyExportBehaviour_Err
 }
 
-func (d *DriverMock) CompactDisks(path string) error {
+func (d *DriverMock) CompactDisks(path string) (result string, err error) {
 	d.CompactDisks_Called = true
 	d.CompactDisks_Path = path
-	return d.CompactDisks_Err
+	d.CompactDisks_Result = "Mock compact result msg: mockdisk.vhdx. Disk size reduced by 20%"
+	return d.CompactDisks_Result, d.CompactDisks_Err
 }
 
 func (d *DriverMock) RestartVirtualMachine(vmName string) error {

builder/hyperv/common/driver_ps_4.go

@@ -223,7 +223,7 @@ func (d *HypervPS4Driver) PreserveLegacyExportBehaviour(srcPath string, dstPath
 	return hyperv.PreserveLegacyExportBehaviour(srcPath, dstPath)
 }
 
-func (d *HypervPS4Driver) CompactDisks(path string) error {
+func (d *HypervPS4Driver) CompactDisks(path string) (result string, err error) {
 	return hyperv.CompactDisks(path)
 }
 

builder/hyperv/common/step_export_vm.go

@@ -34,13 +34,19 @@ func (s *StepExportVm) Run(_ context.Context, state multistep.StateBag) multiste
 		ui.Say("Skipping disk compaction...")
 	} else {
 		ui.Say("Compacting disks...")
-		err := driver.CompactDisks(tmpPath)
+		// CompactDisks searches for all VHD/VHDX files under the supplied
+		// path and runs the compacting process on each of them. If no disks
+		// are found under the supplied path this is treated as a 'soft' error
+		// and a warning message is printed. All other errors halt the build.
+		result, err := driver.CompactDisks(tmpPath)
 		if err != nil {
 			err := fmt.Errorf("Error compacting disks: %s", err)
 			state.Put("error", err)
 			ui.Error(err.Error())
 			return multistep.ActionHalt
 		}
+		// Report disk compaction results/warn if no disks were found
+		ui.Message(result)
 	}
 
 	if s.SkipExport {

common/powershell/hyperv/hyperv.go

@@ -721,17 +721,40 @@ if ( $((Get-Item $srcPath).GetFileSystemInfos().Count) -eq 0 ) {
 	return err
 }
 
-func CompactDisks(path string) error {
+func CompactDisks(path string) (result string, err error) {
 	var script = `
 param([string]$srcPath)
-Get-ChildItem "$srcPath" -Filter *.vhd* | %{
-    Optimize-VHD -Path $_.FullName -Mode Full
+
+$disks = Get-ChildItem -Path $srcPath -Recurse -Filter *.vhd* -ErrorAction SilentlyContinue | % { $_.FullName }
+# Failure to find any disks is treated as a 'soft' error. Simply print out
+# a warning and exit
+if ($disks.Length -eq 0) {
+    Write-Output "WARNING: No disks found under $srcPath"
+    exit
+}
+
+foreach ($disk in $disks) {
+    Write-Output "Compacting disk: $(Split-Path $disk -leaf)"
+
+    $sizeBefore = $disk.Length
+    Optimize-VHD -Path $disk -Mode Full
+    $sizeAfter = $disk.Length
+
+    # Calculate the percentage change in disk size
+    if ($sizeAfter -gt 0) { # Protect against division by zero
+        $percentChange = ( ( $sizeAfter / $sizeBefore ) * 100 ) - 100
+        switch($percentChange) {
+            {$_ -lt 0} {Write-Output "Disk size reduced by: $(([math]::Abs($_)).ToString("#.#"))%"}
+            {$_ -eq 0} {Write-Output "Disk size is unchanged"}
+            {$_ -gt 0} {Write-Output "WARNING: Disk size increased by: $($_.ToString("#.#"))%"}
+        }
+    }
 }
 `
 
 	var ps powershell.PowerShellCmd
-	err := ps.Run(script, path)
-	return err
+	result, err = ps.Output(script, path)
+	return
 }
 
 func CreateVirtualSwitch(switchName string, switchType string) (bool, error) {