What causes "Comparison failed for the following files(s): {}" in dotnet/runtime?

Ensure the process has read access to all common files in srcpath and dstpath.

dotnet/runtime · error · RuntimeError

Comparison failed for the following files(s)

Error message

Comparison failed for the following files(s): {}

What it means

Raised by update_directory() in utilities.py at src/coreclr/scripts/utilities.py:96 when filecmp.cmpfiles reports a non-empty errors list — files present in both source and destination that could not be compared due to an OS-level error (permission denied, file vanished mid-comparison, etc.).

Solutions

Ensure the process has read access to all common files in srcpath and dstpath.
Stop concurrent processes that may lock or delete files during the sync.
Retry — a file may have been transiently locked.
On Windows, exclude the directories from antivirus on-access scanning.

Defensive patterns

Strategy: retry

Validate before calling

import os, stat
for d in (srcpath, dstpath):
    for entry in os.listdir(d):
        p = os.path.join(d, entry)
        if os.path.isfile(p) and not os.access(p, os.R_OK):
            raise SystemExit(f'Unreadable file before sync: {p}')

Try / catch

for attempt in range(3):
    try:
        update_directory(srcpath, dstpath, recursive=True, destructive=True, shallow=False)
        break
    except RuntimeError as e:
        if 'Comparison failed' in str(e) and attempt < 2:
            time.sleep(2)
            continue
        raise

Prevention

Avoid running other processes that touch dstpath during update_directory().
Run syncs with sufficient file permissions and disable AV on-access scanning on the work tree.

When it happens

Trigger: A file exists in both srcpath and dstpath but cmpfiles fails to read/stat it; locked files on Windows; files deleted by another process during the sync.

Common situations: Antivirus locking files on Windows Helix; a concurrent process writing to dstpath; permission mismatch between source and destination.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/64e340a84c1c0cbd. Report an issue: GitHub.

Appendix: source

Thrown at src/coreclr/scripts/utilities.py:96

    """Updates dest directory with files from src directory

    Args:
        destpath (str): The destination path to sync with the source
        srcpath (str): The source path to sync to the destination
        recursive(boolean): If True, descend into and update subdirectories (default: True)
        destructive(boolean): If True, delete files in the destination which do not exist in the source (default: True)
        shallow(boolean): If True, only use os.stat to diff files. Do not examine contents (default: False)
    """
    srcfiles, srcdirs = split_entries(os.listdir(srcpath), srcpath)
    dstfiles, dstdirs = split_entries(os.listdir(dstpath), dstpath)


    # Update files in both src and destination which are different in destination
    commonfiles = srcfiles.intersection(dstfiles)
    _, mismatches, errors = filecmp.cmpfiles(srcpath, dstpath, commonfiles, shallow=shallow)

    if errors:
        raise RuntimeError("Comparison failed for the following files(s): {}".format(errors))

    for mismatch in mismatches:
        shutil.copyfile(os.path.join(srcpath, mismatch), os.path.join(dstpath, mismatch))

    # Copy over files from source which do not exist in the destination
    for missingfile in srcfiles.difference(dstfiles):
        shutil.copyfile(os.path.join(srcpath, missingfile), os.path.join(dstpath, missingfile))

    #If destructive, delete files in destination which do not exist in sourc
    if destructive:
        for deadfile in dstfiles.difference(srcfiles):
            print(deadfile)
            os.remove(os.path.join(dstpath, deadfile))

        for deaddir in dstdirs.difference(srcdirs):
            print(deaddir)
            shutil.rmtree(os.path.join(dstpath, deaddir))

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