update: add --force argument.

Add a `brew update --force` to side-step all of the clever optimisations
we have to detect if an update is unnecessary. That means if those
optimisations go wrong in future we can tell people just to run this
single command.

This would have been a useful workaround for the issue fixed in 985c672.

Library/Homebrew/cmd/update.sh

@@ -4,6 +4,8 @@
 #:
 #:    If `--merge` is specified then `git merge` is used to include updates
 #:      (rather than `git rebase`).
+#:    If `--force` is specified then always do a slower, full update check even
+#:      if unnecessary.
 
 # Hide shellcheck complaint:
 # shellcheck source=/dev/null
@@ -293,6 +295,7 @@ homebrew-update() {
       --verbose)                      HOMEBREW_VERBOSE=1 ;;
       --debug)                        HOMEBREW_DEBUG=1 ;;
       --merge)                        HOMEBREW_MERGE=1 ;;
+      --force)                        HOMEBREW_UPDATE_FORCE=1 ;;
       --simulate-from-current-branch) HOMEBREW_SIMULATE_FROM_CURRENT_BRANCH=1 ;;
       --preinstall)                   export HOMEBREW_UPDATE_PREINSTALL=1 ;;
       --*)                            ;;
@@ -393,8 +396,11 @@ EOS
     declare UPSTREAM_BRANCH"$TAP_VAR"="$UPSTREAM_BRANCH"
     declare PREFETCH_REVISION"$TAP_VAR"="$(git rev-parse -q --verify refs/remotes/origin/"$UPSTREAM_BRANCH")"
 
-    [[ -n "$SKIP_FETCH_BREW_REPOSITORY" && "$DIR" = "$HOMEBREW_REPOSITORY" ]] && continue
+    if [[ -z "$HOMEBREW_UPDATE_FORCE" ]]
-    [[ -n "$SKIP_FETCH_CORE_REPOSITORY" && "$DIR" = "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-core" ]] && continue
+    then
+      [[ -n "$SKIP_FETCH_BREW_REPOSITORY" && "$DIR" = "$HOMEBREW_REPOSITORY" ]] && continue
+      [[ -n "$SKIP_FETCH_CORE_REPOSITORY" && "$DIR" = "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-core" ]] && continue
+    fi
 
     # The upstream repository's default branch may not be master;
     # check refs/remotes/origin/HEAD to see what the default
@@ -431,7 +437,7 @@ EOS
            "https://api.github.com/repos/$UPSTREAM_REPOSITORY/commits/$UPSTREAM_BRANCH")"
         # Touch FETCH_HEAD to confirm we've checked for an update.
         [[ -f "$DIR/.git/FETCH_HEAD" ]] && touch "$DIR/.git/FETCH_HEAD"
-        [[ "$UPSTREAM_SHA_HTTP_CODE" = "304" ]] && exit
+        [[ -z "$HOMEBREW_UPDATE_FORCE" ]] && [[ "$UPSTREAM_SHA_HTTP_CODE" = "304" ]] && exit
       elif [[ -n "$HOMEBREW_UPDATE_PREINSTALL" ]]
       then
         # Don't try to do a `git fetch` that may take longer than expected.
@@ -484,7 +490,8 @@ EOS
     if [[ -n "$HOMEBREW_SIMULATE_FROM_CURRENT_BRANCH" ]]
     then
       simulate_from_current_branch "$DIR" "$TAP_VAR" "$UPSTREAM_BRANCH" "$CURRENT_REVISION"
-    elif [[ "$PREFETCH_REVISION" = "$POSTFETCH_REVISION" ]] &&
+    elif [[ -z "$HOMEBREW_UPDATE_FORCE" ]] &&
+         [[ "$PREFETCH_REVISION" = "$POSTFETCH_REVISION" ]] &&
          [[ "$CURRENT_REVISION" = "$POSTFETCH_REVISION" ]]
     then
       export HOMEBREW_UPDATE_BEFORE"$TAP_VAR"="$CURRENT_REVISION"
@@ -499,6 +506,7 @@ EOS
 
   if [[ -n "$HOMEBREW_UPDATED" ||
         -n "$HOMEBREW_UPDATE_FAILED" ||
+        -n "$HOMEBREW_UPDATE_FORCE" ||
         (-n "$HOMEBREW_DEVELOPER" && -z "$HOMEBREW_UPDATE_PREINSTALL") ]]
   then
     brew update-report "$@"

share/doc/homebrew/brew.1.html

@@ -440,7 +440,9 @@ source. This is useful for creating patches for the software.</p></dd>
  <code>git</code>(1).</p>
 
 <p>If <code>--merge</code> is specified then <code>git merge</code> is used to include updates
-  (rather than <code>git rebase</code>).</p></dd>
+  (rather than <code>git rebase</code>).
+If <code>--force</code> is specified then always do a slower, full update check even
+  if unnecessary.</p></dd>
 <dt><code>upgrade</code> [<var>install-options</var>] [<code>--cleanup</code>] [<code>--fetch-HEAD</code>] [<var>formulae</var>]</dt><dd><p>Upgrade outdated, unpinned brews.</p>
 
 <p>Options for the <code>install</code> command are also valid here.</p>

share/man/man1/brew.1

@@ -597,7 +597,7 @@ Remove a tapped repository\.
 Fetch the newest version of Homebrew and all formulae from GitHub using \fBgit\fR(1)\.
 .
 .IP
-If \fB\-\-merge\fR is specified then \fBgit merge\fR is used to include updates (rather than \fBgit rebase\fR)\.
+If \fB\-\-merge\fR is specified then \fBgit merge\fR is used to include updates (rather than \fBgit rebase\fR)\. If \fB\-\-force\fR is specified then always do a slower, full update check even if unnecessary\.
 .
 .TP
 \fBupgrade\fR [\fIinstall\-options\fR] [\fB\-\-cleanup\fR] [\fB\-\-fetch\-HEAD\fR] [\fIformulae\fR]