Stop the promote script from deleting itself mid-run

promote-to-main.sh checks out main. The tooling lives only on the site branch,
so that checkout removes tools/promote-to-main.sh while the script is still
running - and bash reads a script lazily, by file offset, so it would carry on
executing whatever bytes happen to land there. While manipulating branches.

Not theoretical: main does not have tools/ right now, so the first real use
would have hit it.

Both versions now copy themselves to a temp file and re-exec from there. The
bash copy unlinks itself immediately, since the open handle keeps working. The
Windows copy has the same hazard for the same reason - cmd.exe re-seeks the
batch file after every command - and two extra traps worth naming: the copy runs
from %TEMP%, so the repository root has to travel with it rather than being
derived from %~dp0, and %errorlevel% inside a parenthesised block is expanded
when the block is PARSED, before the call it is meant to report on.

Scope: product
(cherry picked from commit 6d42110a91)

tools/promote-to-main.cmd

@@ -17,7 +17,30 @@ rem the commits keeps the decision explicit and per change.
 rem
 rem It does NOT push. Read what it prints, then run tools\git-push.cmd main.
 
-cd /d "%~dp0.."
+rem RUN FROM A COPY OF OURSELF.
+rem
+rem This checks out main. The tooling may live only on the site branch, in which
+rem case that checkout DELETES this file while cmd.exe is still reading it - and
+rem cmd.exe reads a batch file line by line, by file offset, re-seeking after
+rem every command. It would carry on reading whatever bytes are at that offset
+rem afterwards, while manipulating branches. The bash version has the same
+rem hazard and the same fix.
+if not "%PROMOTE_SELFCOPY%"=="1" (
+  rem The copy runs from %TEMP%, where %~dp0 is the temp directory and not the
+  rem checkout, so the repository root has to travel with it.
+  set "PROMOTE_REPO=%~dp0.."
+  set "PROMOTE_SELFCOPY=1"
+  copy /y "%~f0" "%TEMP%\promote-to-main-copy.cmd" >nul
+  call "%TEMP%\promote-to-main-copy.cmd" %*
+  rem Delayed expansion: a plain %errorlevel% here is expanded when the block is
+  rem parsed, which is BEFORE the call above has run, so it would always report
+  rem the code from whatever ran previously.
+  set "EXITCODE=!errorlevel!"
+  del /q "%TEMP%\promote-to-main-copy.cmd" >nul 2>&1
+  exit /b !EXITCODE!
+)
+
+cd /d "%PROMOTE_REPO%"
 
 set "DRYRUN=0"
 if "%~1"=="--dry-run" (

tools/promote-to-main.sh

@@ -26,6 +26,27 @@
 # main yourself.
 set -euo pipefail
 
+# RUN FROM A COPY OF OURSELF.
+#
+# This script checks out main. The tooling may live only on the site branch, in
+# which case that checkout DELETES this file while bash is still reading it -
+# and bash reads a script lazily, by offset, so it would carry on executing
+# whatever bytes happen to be at that position afterwards. While manipulating
+# branches. This is not theoretical: main did not have tools/ when this was
+# written, so the very first real use would have hit it.
+#
+# Copy to a temp file, re-exec from there, and delete the copy immediately. On
+# Unix the open file handle keeps working after the unlink, so the running
+# script survives whatever git does to the tree.
+if [ "${PROMOTE_SELFCOPY:-}" != "1" ]; then
+  SELF="$(mktemp)"
+  cat "$0" > "$SELF"
+  chmod 700 "$SELF"
+  export PROMOTE_SELFCOPY=1
+  exec bash "$SELF" "$@"
+fi
+rm -f "$0" 2>/dev/null || true
+
 cd "$(git rev-parse --show-toplevel)"
 
 DRYRUN=0