improve error handling around disk partitioning

integration/windows/environment_test.go

@@ -35,6 +35,7 @@ func (e *WindowsEnvironment) ShrinkRootPartition() {
 	sizeMinBuffer := 1 * GB
 	cmdFmtString := "Get-Partition -DriveLetter C | Resize-Partition -Size $((Get-PartitionSupportedSize -DriveLetter C).SizeMin + %d)"
 
+	shrunkSuccessfully := false
 	for i := 0; i < 5; i++ {
 		cmd := fmt.Sprintf(cmdFmtString, sizeMinBuffer)
 		stdout, stderr, exitCode, err := e.RunPowershellCommandWithOffsetAndResponses(cmd)
@@ -72,8 +73,14 @@ func (e *WindowsEnvironment) ShrinkRootPartition() {
 			}
 		}
 
+		shrunkSuccessfully = true
 		break
 	}
+
+	Expect(shrunkSuccessfully).WithOffset(1).To(
+		BeTrue(),
+		fmt.Sprintf("Failed to shrink root partition after 5 attempts; last command: %s", fmt.Sprintf(cmdFmtString, sizeMinBuffer)),
+	)
 }
 
 func (e *WindowsEnvironment) EnsureRootPartitionAtMaxSize() {

integration/windows/ephemeral_disk_test.go

@@ -29,6 +29,10 @@ var _ = Describe("EphemeralDisk", func() {
 		agent.EnsureDataDirDoesntExist()
 
 		if diskLetter != "" {
+			Expect(diskLetter).NotTo(Equal("C"),
+				"diskLetter must never be the system partition (C:) — "+
+					"this indicates the agent created the data-dir junction pointing to the root drive, "+
+					"or AssignDriveLetter returned an unexpected value")
 			agent.RunPowershellCommand(fmt.Sprintf("Remove-Partition -DriveLetter %s -Confirm:$false", diskLetter))
 		}
 		if diskNumber != "0" {

platform/windows/disk/partitioner.go

@@ -5,6 +5,7 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"time"
 
 	boshsys "github.com/cloudfoundry/bosh-utils/system"
 )
@@ -13,6 +14,11 @@ var diskNumberPattern = regexp.MustCompile(`^[0-9]+$`)
 
 type Partitioner struct {
 	Runner boshsys.CmdRunner
+
+	// DriveLetterPollInterval controls how long AssignDriveLetter waits between
+	// queries when Get-Partition reports no letter yet (DriveLetter == char(0)).
+	// Zero (the default) uses 1 second. Override in tests to avoid delays.
+	DriveLetterPollInterval time.Duration
 }
 
 // ParseDiskNumberString validates a non-negative decimal disk or partition index for Windows PowerShell -Number parameters.
@@ -151,15 +157,45 @@ func (p *Partitioner) AssignDriveLetter(diskNumber, partitionNumber string) (str
 		dn, pn,
 	)
 
-	stdout, _, _, err := p.ps(driveScript)
-	if err != nil {
-		return "", fmt.Errorf(
-			"failed to find drive letter for partition %s on disk %s: %s",
-			partitionNumber,
-			diskNumber,
-			err,
-		)
+	// PowerShell's Partition.DriveLetter is a .NET char. When no letter has been
+	// assigned the property holds char(0) — the null character — which PowerShell
+	// prints as "\x00\r\n". If we returned that empty string the caller would call
+	// linker.Link with a bare ":" target; on Windows that resolves to the current
+	// directory on the default drive (typically C:\), silently creating a junction
+	// that points back to the system partition.
+	//
+	// Retry up to 10 times to tolerate transient WMI staleness after the access
+	// path is added.
+	pollInterval := p.DriveLetterPollInterval
+	if pollInterval == 0 {
+		pollInterval = time.Second
 	}
 
-	return strings.TrimSpace(stdout), nil
+	for attempt := 0; attempt < 10; attempt++ {
+		stdout, _, _, err := p.ps(driveScript)
+		if err != nil {
+			return "", fmt.Errorf(
+				"failed to find drive letter for partition %s on disk %s: %s",
+				partitionNumber,
+				diskNumber,
+				err,
+			)
+		}
+
+		// The DriveLetter property is a char; an unassigned partition returns the null
+		// character (\x00). Strip it along with whitespace before checking.
+		letter := strings.Trim(strings.TrimSpace(stdout), "\x00")
+		if letter != "" {
+			return letter, nil
+		}
+
+		time.Sleep(pollInterval)
+	}
+
+	return "", fmt.Errorf(
+		"drive letter was not assigned to partition %s on disk %s after %d attempts",
+		partitionNumber,
+		diskNumber,
+		10,
+	)
 }

platform/windows/disk/partitioner_test.go

@@ -4,6 +4,7 @@
 	"errors"
 	"fmt"
 	"strings"
+	"time"
 
 	"github.com/cloudfoundry/bosh-utils/system/fakes"
 	. "github.com/onsi/ginkgo/v2"
@@ -264,5 +265,45 @@
 			Expect(err).To(HaveOccurred())
 			Expect(err.Error()).To(ContainSubstring(`AssignDriveLetter: invalid partition number "2;Invoke-Expression"`))
 		})
+
+		It("retries when Get-Partition reports char(0) before returning the valid letter", func() {
+			// Simulate WMI returning the null character on the first poll, then the
+			// real letter on the second. Without the retry the function would return ""
+			// and the caller would create a junction pointing at bare ":", which Windows
+			// resolves to the current directory on the default drive (often C:\).
+			partitioner.DriveLetterPollInterval = time.Millisecond
+
+			addKey := strings.Join([]string{"-NoProfile", "-NonInteractive", "-Command",
+				`Add-PartitionAccessPath -DiskNumber 1 -PartitionNumber 2 -AssignDriveLetter`}, " ")
+			getKey := strings.Join([]string{"-NoProfile", "-NonInteractive", "-Command",
+				`Get-Partition -DiskNumber 1 -PartitionNumber 2 | Select-Object -ExpandProperty DriveLetter`}, " ")
+
+			cmdRunner.AddCmdResult(addKey, fakes.FakeCmdResult{})
+			cmdRunner.AddCmdResult(getKey, fakes.FakeCmdResult{Stdout: "\x00\n"}) // null char — unassigned
+			cmdRunner.AddCmdResult(getKey, fakes.FakeCmdResult{Stdout: "E\n"})   // assigned on second poll
+
+			driveLetter, err := partitioner.AssignDriveLetter(diskNumber, partitionNumber)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(driveLetter).To(Equal("E"))
+		})
+
+		It("returns an error when DriveLetter stays null across all retry attempts", func() {
+			partitioner.DriveLetterPollInterval = time.Millisecond
+
+			addKey := strings.Join([]string{"-NoProfile", "-NonInteractive", "-Command",
+				`Add-PartitionAccessPath -DiskNumber 1 -PartitionNumber 2 -AssignDriveLetter`}, " ")
+			getKey := strings.Join([]string{"-NoProfile", "-NonInteractive", "-Command",
+				`Get-Partition -DiskNumber 1 -PartitionNumber 2 | Select-Object -ExpandProperty DriveLetter`}, " ")
+
+			cmdRunner.AddCmdResult(addKey, fakes.FakeCmdResult{})
+			// Sticky so every Get-Partition call returns the null char.
+			cmdRunner.AddCmdResult(getKey, fakes.FakeCmdResult{Stdout: "\x00\n", Sticky: true})
+
+			_, err := partitioner.AssignDriveLetter(diskNumber, partitionNumber)
+			Expect(err).To(MatchError(ContainSubstring(
+				fmt.Sprintf("drive letter was not assigned to partition %s on disk %s after 10 attempts",
+					partitionNumber, diskNumber),
+			)))
+		})
 	})
 })