From 53fe6b518fe327b080f9ccc35b8b74a0e6d3f6aa Mon Sep 17 00:00:00 2001
From: Misty De Meo <mistydemeo@gmail.com>
Date: Tue, 12 Jan 2021 23:46:39 -0800
Subject: [PATCH] Keg: allow overwriting same-formula conflicts

This is weird. We're seeing some installs where formulae
have no opt prefix and no linked keg record, *but* still
ended up linked into the prefix. Upgrades will blow up in
this case because the old symlinks won't get deleted and
Homebrew will report a nonsensical error message about
not being able to link over another link from the same
formula!

Since the optpath is either missing or pointing to the
wrong destination at this point, the checks for conflict
linking above will have failed. We should instead be safe
to simply blow away these conflicting symlinks and replace
them with the new targets.

Fixes https://github.com/Homebrew/homebrew-core/issues/68866.
---
 Library/Homebrew/keg.rb | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/Library/Homebrew/keg.rb b/Library/Homebrew/keg.rb
index e2db8254fb..30aca75a1c 100644
--- a/Library/Homebrew/keg.rb
+++ b/Library/Homebrew/keg.rb
@@ -647,6 +647,18 @@ class Keg
     dst.delete if options[:overwrite] && (dst.exist? || dst.symlink?)
     dst.make_relative_symlink(src)
   rescue Errno::EEXIST => e
+    # We're linking a different version of the same formula
+    # Note that the AlreadyLinkedError check above *should*
+    # have caught this, but there are circumstances in which
+    # we end up with symlinks for a formula even though
+    # it seems to be missing an optlink. In that case,
+    # we should be clear to blow those away and replace
+    # them.
+    if dst.symlink? && Keg.for(dst).name == name
+      dst.unlink
+      retry
+    end
+
     raise ConflictError.new(self, src.relative_path_from(path), dst, e) if dst.exist?
 
     if dst.symlink?