Don't abort upgrade if one formula won't build

Also, unlink previous keg before installing to prevent issues when existing installed brews cause build problems for the newer installed brew.

If the build fails the active keg is relinked before aborting.

Fixes #10341.

Library/Homebrew/cmd/upgrade.rb

@@ -14,9 +14,12 @@ module Homebrew extend self
     outdated = if ARGV.named.empty?
       Homebrew.outdated_brews
     else
-      ARGV.formulae.each do |f|
+      ARGV.formulae.select do |f|
-        raise "#{f} already upgraded" if f.installed?
+        unless f.rack.exist? and not f.rack.children.empty?
-        raise "#{f} not installed" unless f.rack.exist? and not f.rack.children.empty?
+          onoe "#{f} not installed"
+        else
+          true
+        end
       end
     end
 
@@ -35,13 +38,34 @@ module Homebrew extend self
     end
 
     outdated.each do |f|
-      installer = FormulaInstaller.new f
+      upgrade_formula f
-      installer.show_header = false
-      oh1 "Upgrading #{f.name}"
-      installer.install
-      Keg.new(f.linked_keg.realpath).unlink if f.linked_keg.directory?
-      installer.caveats
-      installer.finish # includes link step
     end
   end
+
+  def upgrade_formula f
+    outdated_keg = Keg.new(f.linked_keg.realpath) rescue nil
+
+    installer = FormulaInstaller.new f
+    installer.show_header = false
+
+    oh1 "Upgrading #{f.name}"
+
+    # first we unlink the currently active keg for this formula otherwise it is
+    # possible for the existing build to interfere with the build we are about to
+    # do! Seriously, it happens!
+    outdated_keg.unlink if outdated_keg
+
+    installer.install
+    installer.caveats
+    installer.finish # includes link step
+  rescue CannotInstallFormulaError => e
+    onoe e
+  rescue BuildError => e
+    e.dump
+    puts
+  ensure
+    # restore previous installation state if build failed
+    outdated_keg.link if outdated_keg and not f.linked_keg.directory?
+  end
+
 end