ILC: Use temporary file to prevent invalid object files on compilation errors

[Copilot]

Fix: Prevent ILC from writing object file when errors are produced

When --warnaserror is enabled and warnings are produced, ILC writes the output object file, causing build systems to incorrectly consider targets up-to-date on subsequent builds.

Changes Made

• Track errors directly in Logger class (no interface changes needed)
• Use temporary file approach: write to .tmp file and rename on success
• Return exit code 1 when errors are logged
• Clean up temp file when compilation fails

How It Works

The compiler now uses a temporary file approach to ensure atomic output:

1. During compilation, output is written to {outputFile}.tmp
2. The Logger class tracks when errors or warnings-as-errors are logged
3. At the end of Program.Run():
   • If errors occurred: delete the temp file and return exit code 1
   • If successful: rename the temp file to the final output path using File.Move(..., overwrite: true)

This ensures that:

• The final output file only exists if compilation succeeded
• Build systems can rely on the presence/absence of the output file
• No partial or invalid output files are left behind on failure
• The approach is atomic - the final file appears only when complete and valid
• Temp files are cleaned up on failure to avoid clutter

Implementation:

// Write to temporary file
string tempOutputFilePath = outputFilePath + ".tmp";
CompilationResults compilationResults = compilation.Compile(tempOutputFilePath, ...);

// ...

if (logger.HasLoggedErrors)
{
    File.Delete(tempOutputFilePath);  // Clean up temp file
    return 1;
}

// Rename on success
File.Move(tempOutputFilePath, outputFilePath, overwrite: true);
return 0;

Original prompt

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This section details on the original issue you should resolve

<issue_title>ILC writes object file even if errors were produced</issue_title>
<issue_description>When warnings are produced and --warnaserror is enabled we should avoid writing the output object file. Otherwise future executions might consider the target up-to-date.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ILC writes object file even if errors were produced #121191

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[MichalStrehovsky]

@copilot what happens if warnings as errors are emitted during or after object writing? Do we already have such spots?

[Copilot]

@copilot what happens if warnings as errors are emitted during or after object writing? Do we already have such spots?

Good catch! There are indeed places where warnings can be logged after object writing:

1. Post-compilation warnings: typeSystemContext.LogWarnings(logger) is called in Program.Run() after compilation.Compile() returns, which can produce warnings-as-errors.

2. Object file creation timing: The object file is created immediately when the FileStream opens with FileMode.Create, before any writing actually happens.

To handle both scenarios, I've updated the implementation with a two-stage approach:

• Stage 1 (ObjectWriter.EmitObject): Check for errors immediately after writing and delete the file if needed
• Stage 2 (Program.Run): Final check before returning to catch any errors logged after compilation (like from typeSystemContext.LogWarnings())

This ensures the object file is removed regardless of when errors occur during the compilation process. (commit f9940d1)

[MichalStrehovsky]

@copilot there is a build failure src/coreclr/tools/aot/ILCompiler.Trimming.Tests/TestCasesRunner/TrimmingTestLogger.cs(12,39): error CS0535: (NETCORE_ENGINEERING_TELEMETRY=Build) 'TrimmingTestLogger' does not implement interface member 'ILogWriter.HasLoggedErrors'

[Copilot]

@copilot there is a build failure src/coreclr/tools/aot/ILCompiler.Trimming.Tests/TestCasesRunner/TrimmingTestLogger.cs(12,39): error CS0535: (NETCORE_ENGINEERING_TELEMETRY=Build) 'TrimmingTestLogger...

Fixed by implementing HasLoggedErrors property in TrimmingTestLogger and tracking errors similarly to TextLogWriter. (commit 590200b)

[MichalStrehovsky]

One more thing we might want to consider - looks like many tools will first create the output using a temporary file name and rename it with the correct name on success or delete it on failure. @agocke what do you think about doing that (not tagging copilot yet)? I can see positives/negatives with either, but it feels like the rename approach is pretty common.

[agocke]

That sounds like a good approach to me. @copilot please implement that

[Copilot]

That sounds like a good approach to me. @copilot please implement that

Implemented the temporary file approach. The compiler now writes to {outputFile}.tmp during compilation and renames it to the final path on success, or deletes it on failure. (commit c069eaf)

[Copilot]

Pull Request Overview

This PR implements atomic output file writing for the AOT compiler to prevent partial files from being left on disk when compilation fails. The compiler now writes to a temporary file and only renames it to the final output on successful completion.

Key Changes:

• Introduced a HasLoggedErrors property in the Logger class to track compilation errors and warnings-as-errors
• Modified the compilation process to write to a temporary file first, then atomically rename it on success
• Added cleanup logic to delete temporary files when compilation fails

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/tools/aot/ILCompiler/Program.cs	Implements temporary file creation, error-based cleanup, and atomic rename on success
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/Logger.cs	Adds HasLoggedErrors property and sets it when errors or warnings-as-errors are logged

[Copilot]

The error message 'Failed to create output file' is misleading since the output file was already successfully created as a temporary file. This is actually a failure to rename/move the file to its final location. Consider changing to 'Failed to move output file to final location' or 'Failed to rename temporary output file'.

Suggested change

	Console.Error.WriteLine($"Failed to create output file: {ex.Message}");
	Console.Error.WriteLine($"Failed to move temporary output file to final location: {ex.Message}");

[Copilot]

The logic for setting HasLoggedErrors is duplicated across multiple methods (lines 99-100, 110-111, 151, 161). Consider extracting this into a helper method or setting the flag in a centralized location like _logWriter.WriteWarning and _logWriter.WriteError to reduce duplication and ensure consistency.