Fix memory leak in IIS StartupHook by properly disposing PhysicalFileProvider

[Copilot]

The SVACE static analyzer identified a potential handle leak in the IIS StartupHook class where a PhysicalFileProvider was created but not properly disposed.

Problem

In src/Servers/IIS/IIS/src/StartupHook.cs, the unhandled exception handler creates a PhysicalFileProvider to generate error page content but doesn't dispose it:

var model = ErrorPageModelBuilder.CreateErrorPageModel(
    new PhysicalFileProvider(contentRoot), // ❌ Not disposed
    logger: null,
    showDetailedErrors: true,
    exception);

Since PhysicalFileProvider implements IDisposable and manages file system resources, this could lead to handle leaks.

Solution

Wrapped the PhysicalFileProvider creation in a using statement to ensure proper disposal:

using var fileProvider = new PhysicalFileProvider(contentRoot);

var model = ErrorPageModelBuilder.CreateErrorPageModel(
    fileProvider, // ✅ Properly disposed at end of scope
    logger: null,
    showDetailedErrors: true,
    exception);

This approach follows existing patterns in the codebase where PhysicalFileProvider is used with using statements in similar scenarios (e.g., in diagnostic and static file middleware tests).

The fix is safe because ErrorPageModelBuilder.CreateErrorPageModel() only uses the file provider during its execution and doesn't retain any references to it.

Fixes #59524.

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[danmoseley]

@BrennanConroy any opinion on whether ErrorPageModelBuilder.CreateErrorPageModel() ought to do the dispose here? Indeed it is not used after that call so the fix as is is reasonable.

[BrennanConroy]

The CreateErrorPageModel method shouldn't dispose the file provider. It's used by other code that might continue using the file provider later.

[danmoseley]

OK. Are you comfortable with the dispose in this particular context? ie this change.

[Copilot]

Pull Request Overview

This PR fixes a potential memory leak in the IIS StartupHook class by ensuring proper disposal of a PhysicalFileProvider instance used in unhandled exception handling.

• Wrapped PhysicalFileProvider creation in a using statement to ensure automatic disposal
• Replaced inline instantiation with a properly scoped variable reference
• Applied existing codebase patterns for resource management with file providers

[Copilot]

Consider adding a blank line after the using statement declaration to improve readability and follow the formatting guidelines that specify inserting newlines before code blocks.