Issue Summary

dist/s_fast (which calls dist/s_all -F) is slow and incorrectly scopes its file checks when a branch has multiple merge commits from develop. On a branch with 36 actually-changed files, fast mode was scanning 439 files, negating the purpose of s_fast. A secondary bug caused s_evergreen_validate to prompt for Evergreen credentials on every s_fast run even when no Evergreen yml files were touched. Additionally, s_copyright and s_evergreen had no fast-mode awareness, adding ~17s and credential-check overhead unconditionally.

Root Causes

• The root cause is in last_commit_from_dev() in dist/common_functions.py.
• It uses git rev-list HEAD...develop | tail -n 1 to find the oldest commit in the symmetric difference, then walks one parent back (~).
• On a branch with merge commits from develop, this walks all the way back to the very first commit on the branch rather than the actual merge-base.
• Every script that uses filter_if_fast inherits this wrong base: s_clang_format, s_style, s_string, s_copyright, function.py, and others all scan the inflated file set.
• s_evergreen had no fast-mode awareness at all — it always ran evg_cfg.py check and the pip-version scan.
• s_evergreen_validate had a fast-mode guard but it used last_commit_from_dev, so develop's own changes to evergreen.yml files were mistaken for "your changes", causing evergreen validate to run and prompt for credentials on every invocation.
• s_copyright ran as a sequential (blocking) step in s_all with no fast-mode support, scanning 7,000+ source files unconditionally.

Changes

1. Fix last_commit_from_dev() in dist/common_functions.py

Replace the broken rev-list approach with git merge-base origin/develop HEAD, which correctly returns the most recent common ancestor regardless of how many times develop has been merged in.

def last_commit_from_dev():
    # Find the most recent common ancestor between the current branch and origin/develop.
    # Using git merge-base correctly handles branches that have merged develop multiple
    # times: it returns the latest develop commit reachable from HEAD, so that
    # filter_if_fast only sees files changed by the author's own commits rather than
    # every file touched by develop since the branch was first created.
    return subprocess.run("git merge-base origin/develop HEAD",
        shell=True, capture_output=True, text=True).stdout.strip()

All scripts using filter_if_fast benefit automatically.

2. Add fast-mode guard to dist/s_evergreen

Add check_fast_mode_flag and an early-exit guard that skips the script when no evergreen.*\.yml files are in the changed set.

check_fast_mode_flag

# In fast mode, skip if no evergreen yml files were modified in our commits.
if is_fast_mode; then
    search=`git diff --name-only "$(last_commit_from_dev)" | grep -E 'evergreen.*\.yml$'`
    if test -z "$search"; then
        exit 0
    fi
fi

3. dist/s_evergreen_validate — no change needed

Its existing last_commit_from_dev-based guard works correctly once fix #1 lands. The spurious credential prompts disappear automatically.

4. Add fast-mode support to dist/s_copyright

s_copyright is a sequential (blocking) step in s_all; it was ~17s unconditionally. Its main file scan already uses do_in_parallel, which internally calls filter_if_fast. Adding a single check_fast_mode_flag call is sufficient to enable fast-mode filtering.

. `dirname -- ${BASH_SOURCE[0]}`/common_functions.sh
setup_trap
cd_top
check_fast_mode_flag   # new — enables filter_if_fast inside do_in_parallel

The five one-off checks at the bottom (test/syscall/wt2336_base/base.run and four special_copyright calls) remain unconditional since they are trivially fast single-file checks.

Performance

Measured on a branch with 36 files changed from the merge-base:

Acceptance Criteria

• dist/s_fast on a branch with merge commits from develop scans only the files whose content differs from the merge-base (not every file develop touched since the branch was created).
• Running dist/s_fast does not prompt for Evergreen credentials when no evergreen*.yml files were modified.
• s_copyright completes in under 1s in fast mode when the changed file count is small.
• dist/s_fast on a branch without merge commits behaves identically to before.
• Files changed in early commits (before any merge from develop) are still linted, since their content at HEAD still differs from the merge-base.