Race condition in mc mirror logging causes incorrect totalSize in log output

[mrluanma]

Expected behavior

When running mc mirror, each log line should show an incrementing or correct totalSize field, representing the cumulative bytes mirrored so far.

Actual behavior

On consecutive log lines, the totalSize field remains the same even as new files are mirrored and the size field changes. This results in log output like:

{"status":"success","source":"...part.155.parquet","target":"...part.155.parquet","size":151084394,"totalCount":773,"totalSize":117613662156,...}
{"status":"success","source":"...part.156.parquet","target":"...part.156.parquet","size":149891402,"totalCount":774,"totalSize":117613662156,...}

Here, totalSize should increase by the size of the file, but it does not.

Steps to reproduce the behavior

1. Run mc mirror on a large set of files (with parallelism enabled, e.g., default or --max-workers > 1).
2. Examine the log output (mc --json mirror ... > mirror.log).
3. Observe that in some consecutive lines, totalSize does not increment, even though the file size does.

Root cause analysis

• The code in cmd/mirror-main.go updates and logs progress (totalSize, totalCount) per file.
• Due to parallel processing (see the use of mj.parallel.queueTask), log messages can be written out of order with respect to the accumulator.
• This causes a race condition where consecutive log lines may have identical totalSize values, even though files are being mirrored sequentially in the log.

Code reference

Example code relevant to logging and parallel execution (from cmd/mirror-main.go):

// Each mirror task is queued in parallel
mj.parallel.queueTask(func() URLs {
    return mj.doMirror(ctx, sURLs, EventInfo{})
}, sURLs.SourceContent.Size)

// Logging of totalSize happens here, but can be out of sync due to race conditions
mj.status.PrintMsg(mirrorMessage{
    Source:     sourcePath,
    Target:     targetPath,
    Size:       length,
    TotalCount: sURLs.TotalCount,
    TotalSize:  sURLs.TotalSize,
    ...
})

mc --version

mc version RELEASE.2025-08-13T08-35-41Z (commit-id=7394ce0dd2a80935aded936b09fa12cbb3cb8096)
Runtime: go1.24.6 linux/amd64
Copyright (c) 2015-2025 MinIO, Inc.
License GNU AGPLv3 <https://www.gnu.org/licenses/agpl-3.0.html>

System information

• Affects all platforms, especially with parallel mirroring (--max-workers > 1).

---

Recommendation

Synchronize accumulator update and log writing so each log entry reflects the true cumulative totals at that moment, avoiding race conditions. Consider using mutex or atomic operations to ensure consistency.

Related files

• cmd/mirror-main.go

[toastsandwich]

Let me try to replicate this issue ...

[klauspost]

I feel like I've seen a fix for this within the last week... Looked for it, but couldn't find it, though.