Update PATH in place in env hook

Prior to this change, the default behaviour of `opam init` was to
register a shell hook which ran before each command, and added/moved the
bin dir from the current opam switch to the beginning of the PATH
variable. If a user has a custom bin directory at the beginning of their
PATH containing an executable with the same name as an executable in
their current opam switch, the executable in the opam switch will take
precedence.

This commit changes the behaviour of the shell hook to update the
existing opam switch in PATH in place rather than prepending it, so the
precedence of the opam switch relative to other dirs in PATH is
preserved.

This change doesn't affect the behaviour of running `eval $(opam env)`.

Fixes #6815

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

Author: gridbugs

master_changes.md

@@ -118,6 +118,7 @@ users)
 ## Client
 
 ## Shell
+  * Update PATH in place in env hook [#6859 @gridbugs fix #6815]
 
 ## Internal
   * Improve cache-loading performance when using OCaml >= 5.4 by using `Gc.ramp_up` [#6515 @dra27]

src/state/shellscripts/env_hook.csh

@@ -1 +1 @@
-alias precmd 'eval `opam env --shell=csh --readonly`'
+alias precmd 'eval `opam env --shell=csh --readonly --inplace-path`'

src/state/shellscripts/env_hook.fish

@@ -1,3 +1,3 @@
 function __opam_env_export_eval --on-event fish_prompt
-    eval (opam env --shell=fish --readonly 2> /dev/null)
+    eval (opam env --shell=fish --readonly --inplace-path 2> /dev/null)
 end

src/state/shellscripts/env_hook.sh

@@ -1,6 +1,6 @@
 _opam_env_hook() {
  local previous_exit_status=$?;
- eval $(opam env --shell=bash --readonly 2> /dev/null <&- );
+ eval $(opam env --shell=bash --readonly --inplace-path 2> /dev/null <&- );
  return $previous_exit_status;
 };
 if ! [[ "$PROMPT_COMMAND" =~ _opam_env_hook ]]; then

src/state/shellscripts/env_hook.zsh

@@ -1,5 +1,5 @@
 _opam_env_hook() {
-    eval $(opam env --shell=zsh --readonly 2> /dev/null <&-);
+    eval $(opam env --shell=zsh --readonly --inplace-path 2> /dev/null <&-);
 }
 typeset -ag precmd_functions;
 if [[ -z ${precmd_functions[(r)_opam_env_hook]} ]]; then