Clarify pure-rust-build cc1 wrapping

This adds comments, and also refactors, to make these steps of the
`pure-rust-build` CI job more readable, and to make clearer what
the wrapping does and how it works:

- "Wrap cc1 (and cc1plus if present) to record calls"
- "Build max-pure with limited dev tools and log cc1"

The logic clarified here was introduced in #1682. The clarification
is mainly through these two changes:

- Document each of the scripts the steps create and use with an
  explanatory comment above the "stanza" of code that creates it.

- Extract the command to create the `~/display` symlink to such a
  script, which also has such a comment to explain the effect of
  writing to that symlink, why it is needed, and why it has to be
  set in the same GitHub Actions step as the related `cargo`
  command.

Two other less significant clarifcations are made, which arguably
are not refactorings in that they could change the behavior (for
the better) in some hypothetical situations, but the goal is
clarity rather than a behavioral change:

- In the scripts that had more than one non-shebang line, take `-e`
  out of the shebangs and use a `set -e` commmand. This makes no
  difference in how the scripts are used, since they are always
  executed directoy. But may make them easier to read, as readers
  need not check that they are only run in this way to verify
  their understanding of what they do.

- Set `noclobber` in the step that uses `>` to create the script
  files in `/usr/local`, so that if they somehow clash with files
  already there, we get an error rather than proceeding and maybe
  having them called in unantiicpated ways. The likelihood this
  would happen on a GHA runner is very low, so the real impact of
  this change is to make immediately clear to readers that the
  scripts and their names do not have a pre-defined meaning but are
  instead simply helpers for these GHA steps.

Author: EliahKagan

.github/workflows/ci.yml

@@ -70,31 +70,56 @@ jobs:
           ! grep -qP '(?<!\blinux-raw)-sys\b' tree.txt
       - name: Wrap cc1 (and cc1plus if present) to record calls
         run: |
+          set -o noclobber  # Catch any collisions with existing entries in /usr/local.
+
+          # Define the wrapper script for a compiler driver (for cc1 or cc1plus). This wrapper
+          # records calls, then delegates to the executable it wraps. When recording calls, writes
+          # to the log are synchronized so fragments of separate log entries aren't interleaved,
+          # even in concurrent runs. This wrapper knows what executable it is wrapping because,
+          # when deployed, this wrapper (or a symlink) replaces that executable, which will itself
+          # have been moved aside by being renamed with a `.orig` suffix, so this can call it.
           cat >/usr/local/bin/wrapper1 <<'EOF'
-          #!/bin/sh -e
+          #!/bin/sh
+          set -e
           printf '%s\n' "$0 $*" |
             flock /run/lock/wrapper1.fbd136bd-9b1b-448d-84a9-e18be53ae63c.lock \
             tee -a -- /var/log/wrapper1.log ~/display >/dev/null  # We'll link ~/display later.
           exec "$0.orig" "$@"
           EOF
 
+          # Define the script that performs the wrapping. This script shall be run once for each
+          # executable to be wrapped, renaming it with a `.orig` suffix and replacing it with a
+          # symlink to the wrapper script, defined above.
           cat >/usr/local/bin/wrap1 <<'EOF'
-          #!/bin/sh -e
+          #!/bin/sh
+          set -e
           dir="$(dirname -- "$1")"
           base="$(basename -- "$1")"
           cd -- "$dir"
           mv -- "$base" "$base.orig"
           ln -s -- /usr/local/bin/wrapper1 "$base"
           EOF
 
-          chmod +x /usr/local/bin/wrap1 /usr/local/bin/wrapper1
+          # Define a script that wires up the `~/display` symlink that `wrapper1` uses to report
+          # calls as GitHub Actions step output (in addition to writing them to a log file). This
+          # is needed because stdout and stderr are both redirected elsewhere when the wrapper
+          # actually runs, and `/dev/tty` is not usable. This script must be run in the same step
+          # as the `cargo` command that causes wrapped executables to be run, because different
+          # steps write to different pipe objects. (As implemented, this also needs the calling
+          # shell to remain running, but that is not the cause of the underlying limitation.)
+          cat >/usr/local/bin/set-display <<'EOF'
+          #!/bin/sh
+          ln -s -- "/proc/$$/fd/1" ~/display
+          EOF
+
+          chmod +x /usr/local/bin/wrapper1 /usr/local/bin/wrap1 /usr/local/bin/set-display
           mkdir /run/lock/wrapper1.fbd136bd-9b1b-448d-84a9-e18be53ae63c.lock
 
           find /usr/lib/gcc \( -name cc1 -o -name cc1plus \) \
             -print -exec /usr/local/bin/wrap1 {} \;
       - name: Build max-pure with limited dev tools and log cc1
         run: |
-          ln -s -- "/proc/$$/fd/1" ~/display  # Bypass `cc1` redirection.
+          /usr/local/bin/set-display
           cargo install --debug --locked --no-default-features --features max-pure --path .
       - name: Show logged C and C++ compilations (should be none)
         run: |