Builder logs have a 2MB size limit. Once that limit is reached, all remaining output is silently truncated. For someone reading the log, this truncation can be difficult or impossible to detect.

For example, in https://storage.googleapis.com/go-build-log/9041f75f/linux-amd64-longtest_50b82ee1.log, the truncation can be inferred because the last goroutine stack trace is missing the filename/line number line following sync/atomic_test.TestValueSwapConcurrent.func1(). Alternatively, note that the file is exactly 2MB. If the truncation occurred between two different goroutine stack traces, it would be impossible to tell for sure.

We could make this much more clear by adding a "log truncated" message to the end of the logs.

cc @mknyszek @bcmills