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dev: sha demo #353

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chore: demo workflow for github.sha vs pull request head sha BM-1850
schmidtnz Aug 13, 2026
7697ae9
chore: fetch enough history to show merge commit parents
schmidtnz Aug 13, 2026
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65 changes: 65 additions & 0 deletions .github/workflows/sha-demo.yml
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name: sha-demo

# Demo only, not for merging. BM-1850 background: on a pull_request event `github.sha` is
# the ephemeral merge commit, not the branch tip. Runs on both events so a push run and a
# pull request run of the same file can be compared line for line.
on:
pull_request:
push:

jobs:
# Which sha is which, and what checkout actually hands you.
values:
runs-on: ubuntu-latest
steps:
- name: Echo the shas
env:
EVENT: ${{ github.event_name }}
GH_SHA: ${{ github.sha }}
PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
HEAD_REF: ${{ github.head_ref }}
REF_NAME: ${{ github.ref_name }}
FALLBACK: ${{ github.event.pull_request.head.sha || github.sha }}
run: |
# Square brackets so an empty value is visible instead of looking like a blank line.
printf '%-26s [%s]\n' \
'github.event_name' "$EVENT" \
'github.sha' "$GH_SHA" \
'pull_request.head.sha' "$PR_HEAD_SHA" \
'github.head_ref' "$HEAD_REF" \
'github.ref_name' "$REF_NAME" \
'head.sha || github.sha' "$FALLBACK"

- uses: actions/checkout@v6
with:
# Default depth 1 leaves HEAD with no parents locally, which is exactly the
# detail worth seeing on a pull request. 2 is enough to resolve both.
fetch-depth: 2

- name: What checkout actually gave us
run: |
# On a pull request this is the merge commit: two parents, "Merge <head> into <base>".
printf '%-26s [%s]\n' \
'git rev-parse HEAD' "$(git rev-parse HEAD)" \
'HEAD subject' "$(git log -1 --format=%s)" \
'HEAD parents' "$(git rev-parse HEAD^@ | tr '\n' ' ')"

# Separate job: if a missing-key expression were a hard error, it fails here on its own
# rather than taking the comparison above down with it.
null-behaviour:
runs-on: ubuntu-latest
steps:
- name: Missing keys and falsiness
env:
MISSING: ${{ github.event.no_such_key }}
DEEP: ${{ github.event.no_such_key.nor.this.one }}
TERNARY: "${{ github.event.pull_request.head.sha && 'truthy' || 'falsy' }}"
EMPTY_FALLS_THROUGH: "${{ '' || 'fell through' }}"
ZERO_FALLS_THROUGH: "${{ 0 || 'fell through' }}"
run: |
printf '%-26s [%s]\n' \
'missing key' "$MISSING" \
'deep null deref' "$DEEP" \
'head.sha truthy?' "$TERNARY" \
"'' || 'fell through'" "$EMPTY_FALLS_THROUGH" \
"0 || 'fell through'" "$ZERO_FALLS_THROUGH"
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