harden alias against foo; bar

If the first word of the alias body ends with a semicolon we need to
strip that character, and otherwise escape the extracted command, to
ensure the subsequent function definition is valid.

Fixes #3860

share/functions/alias.fish

@@ -11,6 +11,7 @@ function alias --description 'Creates a function wrapping a command'
     set -l body
     set -l prefix
     set -l first_word
+    set -l wrapped_cmd
     switch (count $argv)
 
         case 0
@@ -43,11 +44,13 @@ function alias --description 'Creates a function wrapping a command'
         return 1
     end
 
-    # Extract the first command from the body
-    # This is supposed to replace all non-escaped (i.e. preceded by an odd number of `\`) spaces with a newline
-    # so it splits on them
+    # Extract the first command from the body. This is supposed to replace all non-escaped (i.e.
+    # preceded by an odd number of `\`) spaces with a newline so it splits on them. See issue #2220
+    # for why the following borderline incomprehensible code exists.
     set -l tmp (string replace -ra "([^\\\ ])((\\\\\\\)*) " '$1\n' $body)
     set first_word (string trim $tmp[1])
+    # If the user does something like `alias x 'foo; bar'` we need to strip the semicolon.
+    set wrapped_cmd (string trim -c ';' $first_word)
     if set -q tmp[2]
         set body $tmp[2..-1]
     else
@@ -56,8 +59,7 @@ function alias --description 'Creates a function wrapping a command'
 
     # Prevent the alias from immediately running into an infinite recursion if
     # $body starts with the same command as $name.
-
-    if test $first_word = $name
+    if test $wrapped_cmd = $name
         if contains $name (builtin --names)
             set prefix builtin
         else
@@ -65,5 +67,7 @@ function alias --description 'Creates a function wrapping a command'
         end
     end
     set -l cmd_string (string escape "alias $argv")
-    echo "function $name --wraps $first_word --description $cmd_string; $prefix $first_word $body \$argv; end" | source
+    set wrapped_cmd (string escape $wrapped_cmd)
+    echo "function $name --wraps $wrapped_cmd --description $cmd_string; $prefix $first_word $body \$argv; end" | source
+    #echo "function $name --wraps $wrapped_cmd --description $cmd_string; $prefix $first_word $body \$argv; end"
 end

tests/alias.in

@@ -0,0 +1,7 @@
+# Avoid regressions of issue #3860 wherein the first word of the alias ends
+# with a semicolon.
+function foo
+    echo ran foo
+end
+alias my_alias "foo; and echo foo ran"
+my_alias

tests/alias.out

@@ -0,0 +1,2 @@
+ran foo
+foo ran