Conda lock file extraction leaves a stray leading blank line

[pinin4fjords]

Description

Every conda lock file served via /v1alpha1/builds/{buildId}/condalock (and therefore every .conda-lock/*.txt committed by nf-core modules containers create) starts with a stray leading blank line before its real content, e.g.:

version: 6
environments:
default:
...

Confirmed present in every lock file currently in nf-core/modules built through this path (fastqc, multiqc, busco, and the pixi-based bismark bump in nf-core/modules#12356).

Root cause

BuildLogServiceImpl.extractCondaLockFile():

protected static String extractCondaLockFile(String logs) {
        int start = logs.lastIndexOf(CONDA_LOCK_START)
        int end = logs.lastIndexOf(CONDA_LOCK_END)
        if( start >= end ) {
            return null
        }
        return logs.substring(start + CONDA_LOCK_START.length(), end)
                .replaceAll(/#\d+ \d+\.\d+\s*/, '')
}

The Dockerfile/Singularityfile templates wrap the lock file with:

&& echo ">> CONDA_LOCK_START" \
&& cat pixi.lock \
&& echo "<< CONDA_LOCK_END"

In the raw BuildKit log, each line carries a #N T.T prefix, e.g.:

#8 1.234 >> CONDA_LOCK_START
#8 1.235 version: 6
#8 1.236 environments:
...

logs.substring(start + CONDA_LOCK_START.length(), end) starts right after the >> CONDA_LOCK_START text but before that log line's own trailing newline. Every following line's #N T.T prefix gets stripped by the regex, but that first newline was never preceded by a prefix, so it survives untouched as a leading blank line in the extracted content.

Reproduced by feeding a synthetic BuildKit log through the same extraction logic (Python re equivalent of the Groovy regex):

>>> extract(...)
'\nversion: 6\nenvironments:\ndefault:\n'

Note the existing test BuildLogsServiceTest.groovy ('should extract conda lockfile') actually asserts this leading blank line as the expected output today — it was written to match the current (buggy) behavior rather than to catch it.

Suggested fix

Strip the leading whitespace after the existing regex pass, e.g.:

return logs.substring(start + CONDA_LOCK_START.length(), end)
        .replaceAll(/#\d+ \d+\.\d+\s*/, '')
        .stripLeading()

Happy to open a PR with this fix plus an updated/added test if useful.